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Chapter 2 - Cancer 

INTRODUCTION

Neil Pearce, Paolo Boffetta and Manolis Kogevinas

Magnitude of the Problem

The first clear-cut evidence of cancer causation involved an occupational carcinogen (Checkoway, Pearce and Crawford-Brown 1989). Pott (1775) identified soot as the cause of scrotal cancer in London chimney-sweeps, and graphically described the abysmal working conditions, which involved children climbing up narrow chimneys that were still hot. Despite this evidence, reports of the need to prevent fires in chimneys were used to delay legislation on child labour in this industry until 1840 (Waldron 1983). An experimental model of soot carcinogenesis was first demonstrated in the 1920s (Decoufle 1982), 150 years after the original epidemiological observation.

In subsequent years, a number of other occupational causes of cancer have been demonstrated through epidemiological studies (although the association with cancer has usually first been noted by occupational physicians or by workers). These include arsenic, asbestos, benzene, cadmium, chromium, nickel and vinyl chloride. Such occupational carcinogens are very important in public health terms because of the potential for prevention through regulation and improvements in industrial hygiene practices (Pearce and Matos 1994). In most instances, these are hazards which markedly increase the relative risk of a particular type or types of cancer. It is possible that other occupational carcinogens remain undetected because they involve only a small increase in risk or because they simply have not been studied (Doll and Peto 1981). Some key facts about occupational cancer are given in table 2.1.

Table 2.1 Occupational cancer: Key facts

·     Some 20 agents and mixtures are established occupational carcinogens; a similar number of chemicals are highly suspected occupational carcinogens. 

·     In industrialized countries, occupation is causally linked to 2 to 8% of all cancers; among exposed workers, however, this proportion is higher. 

·     No reliable estimates are available on either the burden of occupational cancer or the extent of workplace exposure to carcinogens in developing countries. 

·     The relatively low overall burden of occupational cancer in industrialized countries is the result of strict regulations on several known carcinogens; exposure to other known or highly suspected agents, however, is still allowed. 

·     Although several occupational cancers are listed as occupational diseases in many countries, a very small fraction of cases is actually recognized and compensated. 

·     Occupational cancer is-to a very large extent-a preventable disease.

Occupational causes of cancer have received considerable emphasis in epidemiological studies in the past. However, there has been much controversy regarding the proportion of cancers which are attributable to occupational exposures, with estimates ranging from 4 to 40% (Higginson 1969; Higginson and Muir 1976; Wynder and Gori 1977; Higginson and Muir 1979; Doll and Peto 1981; Hogan and Hoel 1981; Vineis and Simonato 1991; Aitio and Kauppinen 1991). The attributable cancer risk is the total cancer experience in a population that would not have occurred if the effects associated with the occupational exposures of concern were absent. It may be estimated for the exposed population, as well as for a broader population. A summary of existing estimates is shown in table 2.2. Universal application of the International Classification of Diseases is what makes such tabulations possible (see box). 

The International Classification of Diseases 

Human diseases are classified according to the International Classification of Diseases (ICD), a system that was started in 1893 and is regularly updated under the coordination of the World Health Organization. The ICD is used in almost all countries for tasks such as death certification, cancer registration and hospital discharge diagnosis. The Tenth Revision (ICD-10), which was approved in 1989 (World Health Organization 1992), differs considerably from the previous three revisions, which are similar to each other and have been in use since the 1950s. It is therefore likely that the Ninth Revision (ICD-9, World Health Organization 1978), or even earlier revisions, will still be used in many countries during the coming years.

Table 2.2  Estimated proportions of cancer (PAR) attributable to occupations in selected studies

Study

Population

PAR and cancer site

Comments

Higginson 1969

Not stated

1% Oral cancer
1-2% Lung cancer
10% Bladder cancer
2% Skin cancer

No detailed presentation of exposure levels and other assumptions

Higginson and Muir 1976

Not stated

1-3% Total cancer

No detailed presentation of assumptions

Wynder and Gori 1977

Not stated

4% Total cancer in men,
2% for women

Based on one PAR for bladder cancer and two personal communications

Higginson and Muir 1979

West Midland, United Kingdom

6% Total cancer in men,
2% total cancer

Based on 10% of non-tobacco related lung cancer, mesothelioma, bladder cancer (30%), and leukaemia in women (30%)

Doll and Peto 1981

United States early 1980

4% (range 2-8%)
Total cancer

Based on all studied cancer sites; reported as 'tentative' estimate

Hogan and Hoel 1981

United States

3% (range 1.4-4%)
Total cancer

Risk associated with occupational asbestos exposure

Vineis and Simonato 1991

Various

1-5% Lung cancer,
16-24% bladder cancer

Calculations on the basis of data from case-control studies. Percentage for lung cancer considers only exposure to asbestos. In a study with a high proportion of subjects exposed to ionising radiation, a 40% PAR was estimated. Estimates of PAR in a few studies on bladder cancer were between 0 and 3%.

The large variability in the estimates arises from the differences in the data sets used and the assumptions applied. Most of the published estimates on the fraction of cancers attributed to occupational risk factors are based on rather simplified assumptions. Furthermore, although cancer is relatively less common in developing countries due to the younger age structure (Pisani and Parkin 1994), the proportion of cancers due to occupation may be higher in developing countries due to the relatively high exposures which are encountered (Kogevinas, Boffetta and Pearce 1994).

The most generally accepted estimates of cancers attributable to occupations are those presented in a detailed review on the causes of cancer in the population of the United States in 1980 (Doll and Peto 1981). Doll and Peto concluded that about 4% of all the deaths due to cancer may be caused by occupational carcinogens within "acceptable limits" (i.e., still plausible in view of all the evidence at hand) of 2 and 8%. These estimates being proportions, they are dependent on how causes other than occupational exposures contribute to produce cancers. For example, the proportion would be higher in a population of lifetime non-smokers (such as the Seventh-Day Adventists) and lower in a population in which, say, 90% are smokers. Also the estimates do not apply uniformly to both sexes or to different social classes. Furthermore, if one considers not the whole population (to which the estimates refer), but the segments of the adult population in which exposure to occupational carcinogens almost exclusively occurs (manual workers in mining, agriculture and industry, broadly taken, who in the United States numbered 31 million out of a population, aged 20 and over, of 158 million in the late 1980s), the proportion of 4% in the overall population would increase to about 20% among those exposed.

Vineis and Simonato (1991) provided estimates on the number of cases of lung and bladder cancer attributable to occupation. Their estimates were derived from a detailed review of case-control studies, and demonstrate that in specific populations located in industrial areas, the proportion of lung cancer or bladder cancer from occupational exposures may be as high as 40% (these estimates being dependent not only on the local prevailing exposures, but also to some extent on the method of defining and assessing exposure).

Mechanisms and Theories of Carcinogenesis

Studies of occupational cancer are complicated because there are no "complete" carcinogens; that is, occupational exposures increase the risk of developing cancer, but this future development of cancer is by no means certain. Furthermore, it may take 20 to 30 years (and at least five years) between an occupational exposure and the subsequent induction of cancer; it may also take several more years for cancer to become clinically detectable and for death to occur (Moolgavkar et al. 1993). This situation, which also applies to non-occupational carcinogens, is consistent with current theories of cancer causation.

Several mathematical models of cancer causation have been proposed (e.g., Armitage and Doll 1961), but the model which is simplest and most consistent with current biological knowledge is that of Moolgavkar (1978). This assumes that a healthy stem cell occasionally mutates (initiation); if a particular exposure encourages the proliferation of intermediate cells (promotion) then it becomes more likely that at least one cell will undergo one or more further mutations producing a malignant cancer (progression) (Ennever 1993).

Thus, occupational exposures can increase the risk of developing cancer either by causing mutations in DNA or by various "epigenetic" mechanisms of promotion (those not involving damage to DNA), including increased cell proliferation. Most occupational carcinogens which have been discovered to date are mutagens, and therefore appear to be cancer initiators. This explains the long "latency" period which is required for further mutations to occur; in many instances the necessary further mutations may never occur, and cancer may never develop.

In recent years, there has been increasing interest in occupational exposures (e.g., benzene, arsenic, phenoxy herbicides) which do not appear to be mutagens, but which may act as promoters. Promotion may occur relatively late in the carcinogenic process, and the latency period for promoters may therefore be shorter than for initiators. However, the epidemiological evidence for cancer promotion remains very limited at this time (Frumkin and Levy 1988).

Transfer of Hazards

A major concern in recent decades has been the problem of the transfer of hazardous industries to the developing world (Jeyaratnam 1994). Such transfers have occurred in part due to the stringent regulation of carcinogens and increasing labour costs in the industrialized world, and in part from low wages, unemployment and the push for industrialization in the developing world. For example, Canada now exports about half of its asbestos to the developing world, and a number of asbestos-based industries have been transferred to developing countries such as Brazil, India, Pakistan, Indonesia and South Korea (Jeyaratnam 1994). These problems are further compounded by the magnitude of the informal sector, the large numbers of workers who have little support from unions and other worker organizations, the insecure status of workers, the lack of legislative protection and/or the poor enforcement of such protection, the decreasing national control over resources, and the impact of the third world debt and associated structural adjustment programmes (Pearce et al. 1994).

As a result, it cannot be said that the problem of occupational cancer has been reduced in recent years, since in many instances the exposure has simply been transferred from the industrialized to the developing world. In some instances, the total occupational exposure has increased. Nevertheless, the recent history of occupational cancer prevention in industrialized countries has shown that it is possible to use substitutes for carcinogenic compounds in industrial processes without leading industry to ruin, and similar successes would be possible in developing countries if adequate regulation and control of occupational carcinogens were in place.

Prevention of Occupational Cancer

Swerdlow (1990) outlined a series of options for the prevention of exposure to occupational causes of cancer. The most successful form of prevention is to avoid the use of recognized human carcinogens in the workplace. This has rarely been an option in industrialized countries, since most occupational carcinogens have been identified by epidemiological studies of populations that were already occupationally exposed. However, at least in theory, developing countries could learn from the experience of industrialized countries and prevent the introduction of chemicals and production processes that have been found to be hazardous to the health of workers.

The next best option for avoiding exposure to established carcinogens is their removal once their carcinogenicity has been established or suspected. Examples include the closure of plants making the bladder carcinogens 2-naphthylamine and benzidine in the United Kingdom (Anon 1965), termination of British gas manufacture involving coal carbonization, closure of Japanese and British mustard gas factories after the end of the Second World War (Swerdlow 1990) and gradual elimination of the use of benzene in the shoe industry in Istanbul (Aksoy 1985).

In many instances, however, complete removal of a carcinogen (without closing down the industry) is either not possible (because alternative agents are not available) or is judged politically or economically unacceptable. Exposure levels must therefore be reduced by changing production processes and through industrial hygiene practices. For example, exposures to recognized carcinogens such as asbestos, nickel, arsenic, benzene, pesticides and ionizing radiation have been progressively reduced in industrialized countries in recent years (Pearce and Matos 1994).

A related approach is to reduce or eliminate the activities that involve the heaviest exposures. For example, after an 1840 act was passed in England and Wales prohibiting chimney-sweeps from being sent up chimneys, the number of cases of scrotal cancer decreased (Waldron 1983). Exposure also can be minimized through the use of protective equipment, such as masks and protective clothing, or by imposing more stringent industrial hygiene measures. 

An effective overall strategy in the control and prevention of exposure to occupational carcinogens generally involves a combination of approaches. One successful example is a Finnish registry which has as its objectives to increase awareness about carcinogens, to evaluate exposure at individual workplaces and to stimulate preventive measures (Kerva and Partanen 1981). It contains information on both workplaces and exposed workers, and all employers are required to maintain and update their files and to supply information to the registry. The system appears to have been at least partially successful in decreasing carcinogenic exposures in the workplace (Ahlo, Kauppinen and Sundquist 1988). 

OCCUPATIONAL CARCINOGENS

Paolo Boffetta, Rodolfo Saracci, Manolis Kogevinas, Julian Wilbourn and Harri Vainio

The control of occupational carcinogens is based on the critical review of scientific investigations both in humans and in experimental systems. There are several review programmes being undertaken in different countries aimed at controlling occupational exposures which could be carcinogenic to humans. The criteria used in different programmes are not entirely consistent, leading occasionally to differences in the control of agents in different countries. For example, 4,4-methylene-bis-2-chloroaniline (MOCA) was classified as an occupational carcinogen in Denmark in 1976 and in the Netherlands in 1988, but only in 1992 has a notation "suspected human carcinogen" been introduced by the American Conference of Governmental Industrial Hygienists in the United States.

The International Agency for Research on Cancer (IARC) has established, within the framework of its Monographs programme, a set of criteria to evaluate the evidence of the carcinogenicity of specific agents. The IARC Monographs programme represents one of the most comprehensive efforts to review systematically and consistently cancer data, is highly regarded in the scientific community and serves as the basis for the information in this article. It also has an important impact on national and international occupational cancer control activities. The evaluation scheme is given in table 2.3. Agents, mixtures and exposure circumstances are evaluated within the IARC Monographs if there is evidence of human exposure and data on carcinogenicity (either in humans or in experimental animals) (for IARC classification groups, see table 2.4).

Table 2.3 Evaluation of evidence of carcinogenicity in the IARC Monographs programme

1. The evidence for the induction of cancer in humans, which obviously plays an important role in the identification of human carcinogens is considered. Three types of epidemiological studies contribute to an assessment of carcinogenicity in humans: cohort studies, case-control studies and correlation (or ecological) studies. Case reports of cancer in humans may also be reviewed. The evidence relevant to carcinogenicity from studies in humans is classified into one of the following categories: 

·     Sufficient evidence of carcinogenicity: A causal relationship has been established between exposure to the agent, mixture or exposure circumstance and human cancer. That is, a positive relationship has been observed between the exposure and cancer in studies in which chance, bias and confounding could be ruled out with reasonable confidence. 

·     Limited evidence of carcinogenicity: A positive association has been observed between exposure to the agent, mixture or exposure circumstance and cancer for which a causal interpretation is considered to be credible, but chance, bias or confounding could not be ruled out with reasonable confidence. 

·     Inadequate evidence of carcinogenicity: The available studies are of insufficient quality, consistency or statistical power to permit a conclusion regarding the presence or absence of a causal association, or no data on cancer in humans are available.

·     Evidence suggesting lack of carcinogenicity: There are several adequate studies covering the full range of levels of exposure that human beings are known to encounter, which are mutually consistent in not showing a positive association between exposure to the agent and the studied cancer at any observed level of exposure.

2. Studies in which experimental animals (mainly rodents) are exposed chronically to potential carcinogens and examined for evidence of cancer are reviewed and the degree of evidence of carcinogenicity is then classified into categories similar to those used for human data.

3. Data on biological effects in humans and experimental animals that are of particular relevance are reviewed. These may include toxicological, kinetic and metabolic considerations and evidence of DNA binding, persistence of DNA lesions or genetic damage in exposed humans. Toxicological information, such as that on cytotoxicity and regeneration, receptor binding and hormonal and immunological effects, and data on structure-activity relationship are used when considered relevant to the possible mechanism of the carcinogenic action of the agent.

4. The body of evidence is considered as a whole, in order to reach an overall evaluation of the carcinogenicity to humans of an agent, mixture or circumstance of exposure (see table 2.4).

Table 2.4 IARC Monograph programme classification groups 

The agent, mixture or exposure circumstance is described according to the wording of one of the following categories: 

Group 1-

The agent (mixture) is carcinogenic to humans. The exposure circumstance entails exposures that are carcinogenic to humans.

Group 2A-

The agent (mixture) is probably carcinogenic to humans. The exposure circumstance entails exposures that are probably carcinogenic to humans.

Group 2B-

The agent (mixture) is possibly carcinogenic to humans. The exposure circumstance entails exposures that are possibly carcinogenic to humans.

Group 3-

The agent (mixture, exposure circumstance) is not classifiable as to its carcinogenicity to humans.

Group 4-

The agent (mixture, exposure circumstance) is probably not carcinogenic to humans.

Known and Suspected Occupational Carcinogens

At present, there are 22 chemicals, groups of chemicals or mixtures for which exposures are mostly occupational, without considering pesticides and drugs, which are established human carcinogens (table 2.5). While some agents such as asbestos, benzene and heavy metals are currently widely used in many countries, other agents have mainly an historical interest (e.g., mustard gas and 2-naphthylamine).

Table 2.5 Chemicals, groups of chemicals or mixtures for which exposures are mostly occupational (excluding pesticides and drugs): Group 1-Chemicals carcinogenic to humans1  

Exposure2

Human target organ(s)

Main industry/use

4-Aminobiphenyl (92-67-1)

Bladder

Rubber manufacture

Arsenic (7440-38-2) and arsenic compounds3

Lung, skin

Glass, metals, pesticides

Asbestos (1332-21-4)

Lung, pleura, peritoneum

Insulation, filter material, textiles

Benzene (71-43-2)

Leukaemia

Solvent, fuel

Benzidine (92-87-5)

Bladder

Dye/pigment manufacture, laboratory agent

Beryllium (7440-41-7) and beryllium compounds

Lung

Aerospace industry/metals

Bis(chloromethyl)ether (542-88-11)

Lung

Chemical intermediate/by-product

Chloromethyl methylether (107-30-2) (technical grade)

Lung

Chemical intermediate/by-product

Cadmium (7440-43-9) and cadmium compounds

Lung

Dye/pigment manufacture

Chromium (VI) compounds

Nasal cavity, lung

Metal plating, dye/pigment manufacture

Coal-tar pitches (65996-93-2)

Skin, lung, bladder

Building material, electrodes

Coal-tars (8007-45-2)

Skin, lung

Fuel

Ethylene oxide (75-21-88)

Leukaemia

Chemical intermediate, sterilant

Mineral oils, untreated and mildly treated

Skin

Lubricants

Mustard gas (sulphur mustard) (505-60-2)

Pharynx, lung

War gas

2-Naphthylamine (91-59-8)

Bladder

Dye/pigment manufacture

Nickel compounds

Nasal cavity, lung

Metallurgy, alloys, catalyst

Shale-oils (68308-34-9)

Skin

Lubricants, fuels

Soots

Skin, lung

Pigments

Talc containing asbestiform fibres

Lung

Paper, paints

Vinyl chloride (75-01-4)

Liver, lung, blood vessels

Plastics, monomer

Wood dust

Nasal cavity

Wood industry

1 Evaluated in the IARC Monographs, Volumes 1-63 (1972-1995) (excluding pesticides and drugs).

2 CAS Registry Nos. appear between parentheses.

3 This evaluation applies to the group of chemicals as a whole and not necessarily to all individual chemicals within the group.

An additional 20 agents are classified as probably carcinogenic to humans (Group 2A); they are listed in table 2.6, and include exposures that are currently prevalent in many countries, such as crystalline silica, formaldehyde and 1,3-butadiene. A large number of agents are classified as possible human carcinogens (Group 2B, table 2.7) - for example, acetaldehyde, dichloromethane and inorganic lead compounds. For the majority of these chemicals the evidence of carcinogenicity comes from studies in experimental animals.

Table 2.6 Chemicals, groups of chemicals or mixtures for which exposures are mostly occupational (excluding pesticides and drugs): Group 2A-Probably carcinogenic to humans1  

Exposure2

Suspected human target organ(s)

Main industry/use

Acrylonitrile (107-13-1)

Lung, prostate, lymphoma

Plastics, rubber, textiles, monomer

Benzidine-based dyes

-

Paper, leather, textile dyes

1,3-Butadiene (106-99-0)

Leukaemia, lymphoma

Plastics, rubber, monomer

p-Chloro-o-toluidine (95-69-2) and its strong acid salts

Bladder

Dye/pigment manufacture, textiles

Creosotes (8001-58-9)

Skin

Wood preservation

Diethyl sulphate (64-67-5)

-

Chemical intermediate

Dimethylcarbamoyl chloride (79-44-7)

-

Chemical intermediate

Dimethyl sulphate (77-78-1)

-

Chemical intermediate

Epichlorohydrin (106-89-8)

-

Plastics/resins monomer

Ethylene dibromide (106-93-4)

-

Chemical intermediate, fumigant, fuels

Formaldehyde (50-00-0)

Nasopharynx

Plastics, textiles, laboratory agent

4,4´-Methylene- bis-2-chloroaniline (MOCA) (101-14-44)

Bladder

Rubber manufacture

Polychlorinated biphenyls (1336-36-3)

Liver, bile ducts, leukaemia, lymphoma

Electrical components

Silica (14808-60-7), crystalline

Lung

Stone cutting, mining, glass, paper

Styrene oxide (96-09-3)

-

Plastics, chemical intermediate

Tetrachloroethylene (127-18-4)

Oesophagus, lymphoma

Solvent, dry cleaning

Trichloroethylene (79-01-6)

Liver, lymphoma

Solvent, dry cleaning, metal

Tris(2,3-dibromopropylphosphate (126-72-7)

-

Plastics, textiles, flame retardant

Vinyl bromide (593-60-2)

-

Plastics, textiles, monomer

Vinyl fluoride (75-02-5)

-

Chemical intermediate

1 Evaluated in the IARC Monographs, Volumes 1-63 (1972-1995) (excluding pesticides and drugs).

2 CAS Registry Nos. appear between parentheses.

Table 2.7 Chemicals, groups of chemicals or mixtures for which exposures are mostly occupational (excluding pesticides and drugs): Group 2B-Possibly carcinogenic to humans1  

Exposure2

Main industry/use

Acetaldehyde (75-07-0)

Plastics manufacture, flavours

Acetamide (60-35-5)

Solvent, chemical intermediate

Acrylamide (79-06-1)

Plastics, grouting agent

p-Aminoazotoluene (60-09-3)

Dye/pigment manufacture

o-Aminoazotoluene (97-56-3)

Dyes/pigments, textiles

o-Anisidine (90-04-0)

Dye/pigment manufacture

Antimony trioxide (1309-64-4)

Flame retardant, glass, pigments

Auramine (492-80-8) (technical-grade)

Dyes/pigments

Benzyl violet 4B (1694-09-3)

Dyes/pigments

Bitumens (8052-42-4), extracts of steam-refined and air-refined

Building material

Bromodichloromethane (75-27-4)

Chemical intermediate

β-Butyrolactone (3068-88-0)

Chemical intermediate

Carbon-black extracts

Printing inks

Carbon tetrachloride (56-23-5)

Solvent

Ceramic fibres

Plastics, textiles, aerospace

Chlorendic acid (115-28-6)

Flame retardant

Chlorinated paraffins of average carbon chain length C12 and average degree of chlorination approximately 60%

Flame retardant

α-Chlorinated toluenes

Dye/pigment manufacture, chemical intermediate

p-Chloroaniline (106-47-8)

Dye/pigment manufacture

Chloroform (67-66-3)

Solvent

4-Chloro-o-phenylenediamine (95-83-9)

Dyes/pigments, hair dyes

CI Acid Red 114 (6459-94-5)

Dyes/pigments, textiles, leather

CI Basic Red 9 (569-61-9)

Dyes/pigments, inks

CI Direct Blue 15 (2429-74-5)

Dyes/pigments, textiles, paper

Cobalt (7440-48-4)and cobalt compounds

Glass, paints, alloys

p-Cresidine (120-71-8)

Dye/pigment manufacture

N,N´-Diacetylbenzidine (613-35-4)

Dye/pigment manufacture

2,4-Diaminoanisole (615-05-4)

Dye/pigment manufacture, hair dyes

4,4´-Diaminodiphenyl ether (101-80-44)

Plastics manufacture

2,4-Diaminotoluene (95-80-7)

Dye/pigment manufacture, hair dyes

p-Dichlorobenzene (106-46-7)

Chemical intermediate

3,3´-Dichlorobenzidine (91-94-1)

Dye/pigment manufacture

3,3´-Dichloro-4,4´-diaminodiphenyl ether (28434-86-8)

Not used

1,2-Dichloroethane (107-06-2)

Solvent, fuels

Dichloromethane (75-09-2)

Solvent

Diepoxybutane (1464-53-5)

Plastics/resins

Diesel fuel, marine

Fuel

Di(2-ethylhexyl)phthalate (117-81-77)

Plastics, textiles

1,2-Diethylhydrazine (1615-80-1)

Laboratory reagent

Diglycidyl resorcinol ether (101-90-6)

Plastics/resins

Diisopropyl sulphate (29973-10-6)

Contaminant

3,3´-Dimethoxybenzidine (o-Dianisidine) (119-90-4)

Dye/pigment manufacture

p-Dimethylaminoazobenzene (60-11-7)

Dyes/pigments

2,6-Dimethylaniline (2,6-Xylidine)(87-62-7)

Chemical intermediate

3,3´-Dimethylbenzidine (o-Tolidine) (119-93-77)

Dye/pigment manufacture

Dimethylformamide (68-12-2)

Solvent

1,1-Dimethylhydrazine (57-14-7)

Rocket fuel

1,2-Dimethylhydrazine (540-73-88)

Research chemical

1,4-Dioxane (123-91-1)

Solvent

Disperse Blue 1 (2475-45-8)

Dyes/pigments, hair dyes

Ethyl acrylate (140-88-5)

Plastics, adhesives, monomer

Ethylene thiourea (96-45-7)

Rubber chemical

Fuel oils, residual (heavy)

Fuel

Furan (110-00-9)

Chemical intermediate

Gasoline

Fuel

Glasswool

Insulation

Glycidaldehyde (765-34-4)

Textile, leather manufacture

HC Blue No. 1 (2784-94-3)

Hair dyes

Hexamethylphosphoramide (680-31-9)

Solvent, plastics

Hydrazine (302-01-22)

Rocket fuel, chemical intermediate

Lead (7439-92-1) and lead compounds, inorganic

Paints, fuels

2-Methylaziridine(75-55-8)

Dyes, paper, plastics manufacture

4,4'-Methylene-bis-2-methylaniline (838-88-0)

Dye/pigment manufacture

4,4'-Methylenedianiline(101-77-9)

Plastics/resins, dye/pigment manufacture

Methylmercury compounds

Pesticide manufacture

2-Methyl-1-nitroanthraquinone (129-15-7) (uncertain purity)

Dye/pigment manufacture

Nickel, metallic (7440-02-0)

Catalyst

Nitrilotriacetic acid (139-13-99) and its salts

Chelating agent, detergent

5-Nitroacenaphthene (602-87-9)

Dye/pigment manufacture

2-Nitropropane (79-46-9)

Solvent

N-Nitrosodiethanolamine (1116-54-7)

Cutting fluids, impurity

Oil Orange SS (2646-17-5)

Dyes/pigments

Phenyl glycidyl ether (122-60-1)

Plastics/adhesives/resins

Polybrominated biphenyls (Firemaster BP-6) (59536-65-1)

Flame retardant

Ponceau MX (3761-53-3)

Dyes/pigments, textiles

Ponceau 3R (3564-09-8)

Dyes/pigments, textiles

1,3-Propane sulphone (1120-71-4)

Dye/pigment manufacture

β-Propiolactone (57-57-857-57-8)

Chemical intermediate; plastics manufacture

Propylene oxide (75-56-9)

Chemical intermediate

Rockwool

Insulation

Slagwool

Insulation

Styrene (100-42-5)

Plastics

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) (1746-01-6)

Contaminant

Thioacetamide (62-55-5)

Textile, paper, leather, rubber manufacture

4,4'-Thiodianiline (139-65-1)

Dye/pigment manufacture

Thiourea (62-56-6)

Textile, rubber ingredient

Toluene diisocyanates (26471-62-5)

Plastics

o-Toluidine (95-53-4)

Dye/pigment manufacture

Trypan blue (72-57-1)

Dyes/pigments

Vinyl acetate (108-05-4)

Chemical intermediate

Welding fumes

Metallurgy

1 Evaluated in the IARC Monographs, Volumes 1-63 (1972-1995) (excluding pesticides and drugs).

2 CAS Registry Nos. appear between parentheses.

Occupational exposures may also occur during the production and use of some pesticides and drugs. Table 2.8 presents an evaluation of the carcinogenicity of pesticides; two of them, captafol and ethylene dibromide, are classified as probable human carcinogens, while a total of 20 others, including DDT, atrazine and chlorophenols, are classified as possible human carcinogens.

Table 2.8 Pesticides evaluated in IARC Monographs, Volumes 1-63 (1972-1995)

IARC Group

Pesticide1

2A-Probably carcinogenic to humans

Captafol (2425-06-1)

Ethylene dibromide (106-93-4)

2B-Possibly carcinogenic to humans

Amitrole (61-82-5)

Aramite (140-57-8)

Atrazine (1912-24-9)

Chlordane (57-74-9)

Chlordecone (Kepone) (143-50-0143-50-0)

Chlorophenols

Chlorophenoxy herbicides

DDT (50-29-3)

1,2-Dibromo-3-chloropropane (96-12-8)

1,3-Dichloropropene (542-75-6) (technical-grade)

Dichlorvos (62-73-7)

Heptachlor (76-44-8)

Hexachlorobenzene (118-74-1)

Hexachlorocyclohexanes (HCH)

Mirex (2385-85-5)

Nitrofen (1836-75-5), technical-grade

Pentachlorophenol (87-86-5)

Sodium o-phenylphenate (132-27-4)

Sulphallate (95-06-7)

Toxaphene (Polychlorinated camphenes) (8001-35-2)

1 CAS Registry Nos. appear between parentheses.

Several drugs are human carcinogens (table 2.9) : they are mainly alkylating agents and hormones; 12 more drugs, including chloramphenicol, cisplatine and phenacetin, are classified as probable human carcinogens (Group 2A). Occupational exposure to these known or suspected carcinogens, used mainly in chemotherapy, can occur in pharmacies and during their administration by nursing staff.

Table 2.9 Drugs evaluated in IARC Monographs, Volumes 1-63 (1972-1995) 

Drug1

Target organ2

IARC GROUP 1-Carcinogenic to humans

Analgesic mixtures containing phenacetin

Kidney, bladder

Azathioprine (446-86-6)

Lymphoma, hepatobiliary system, skin

N,N-Bis(2-chloroethyl)- β-naphthylamine (Chlornaphazine) (494-03-1)

Bladder

1,4-Butanediol dimethanesulphonate (Myleran) (55-98-1)

Leukaemia

Chlorambucil (305-03-3)

Leukaemia

1-(2-Chloroethyl)-3-(4-methylcyclohexyl)-1-nitrosourea (Methyl-CCNU) (13909-09-6)

Leukaemia

Cyclosporin (79217-60-0)

Lymphoma, skin

Cyclophosphamide (50-18-0) (6055-19-2)

Leukaemia, bladder

Diethylstilboestrol (56-53-1)

Cervix, vagina, breast

Melphalan (148-82-3)

Leukaemia

8-Methoxypsoralen (Methoxsalen) (298-81-7) plus ultraviolet A radiation

Skin

MOPP and other combined chemotherapy including alkylating agents

Leukaemia

Oestrogen replacement therapy

Uterus

Oestrogens, nonsteroidal

Cervix, vagina, breast

Oestrogens, steroidal

Uterus

Oral contraceptives, combined

Liver

Oral contraceptives, sequential

Uterus

Thiotepa (52-24-4)

Leukaemia

Treosulfan (299-75-2)

Leukaemia

IARC GROUP 2A-Probably carcinogenic to humans

Adriamycin (23214-92-8)

-

Androgenic (anabolic) steroids

(Liver)

Azacitidine (320-67-2)

-

Bischloroethyl nitrosourea (BCNU) (154-93-8)

(Leukaemia)

Chloramphenicol (56-75-7)

(Leukaemia)

1-(2-Chloroethyl)-3-cyclohexyl-1-nitrosourea (CCNU) (13010-47-4)

-

Chlorozotocine (54749-90-5)

-

Cisplatin (15663-27-1)

-

5-Methoxypsoralen (484-20-8)

-

Nitrogen mustard (51-75-2)

(Skin)

Phenacetin (62-44-2)

(Kidney, bladder)

Procarbazine hydrochloride (366-70-1)

-

1 CAS Registry Nos. appear between parentheses.

2 Suspected target organs are given in parentheses.

Several environmental agents are known or suspected causes of cancer in humans and are listed in table 2.10 although exposure to such agents is not primarily occupational, there are groups of individuals exposed to them because of their work: examples are uranium miners exposed to radon decay products, hospital workers exposed to hepatitis B virus, food processors exposed to aflatoxins from contaminated foods, outdoor workers exposed to ultraviolet radiation or diesel engine exhaust, and bar staff or waiters exposed to environmental tobacco smoke.

Table 2.10  Environmental agents/exposures known or suspected to cause cancer in humans1  

Agent/exposure

Target organ2

Strength of evidence3

Air pollutants

Erionite

Lung, pleura

1

Asbestos

Lung, pleura

1

Polycyclic aromatic hydrocarbons4

(Lung, bladder)

S

Water pollutants

Arsenic

Skin

1

Chlorination by-products

(Bladder)

S

Nitrate and nitrite

(Oesophagus, stomach)

S

Radiation

Radon and its decay products

Lung

1

Radium, thorium

Bone

E

Other X-irradiation

Leukaemia, breast, thyroid, others

E

Solar radiation

Skin

1

Ultraviolet radiation A

(Skin)

2A

Ultraviolet radiation B

(Skin)

2A

Ultraviolet radiation C

(Skin)

2A

Use of sunlamps and sunbeds

(Skin)

2A

Electric and magnetic fields

(Leukaemia)

S

Biological agents

Chronic infection with hepatitis B virus

Liver

1

Chronic infection with hepatitis C virus

Liver

1

Infection with Helicobacter pylori

Stomach

1

Infection with Opistorchis viverrini

Bile ducts

1

Infection with Chlonorchis sinensis

(Liver)

2A

Human Papilloma virus types 16 and18

Cervix

1

Human Papilloma virus types 31 and 33

(Cervix)

2A

Human Papilloma virus types other than 16, 18, 31 and 33

(Cervix)

2B

Infection with Schistosoma haematobium

Bladder

1

Infection with Schistosoma japonicum

(Liver, colon)

2B

Tobacco, alcohol and related substances

Alcoholic beverages

Mouth, pharynx, oesophagus, liver, larynx

1

Tobacco smoke

Lip, mouth, pharynx, oesophagus, pancreas, larynx, lung, kidney, bladder, (others)

1

Smokeless tobacco products

Mouth

1

Betel quid with tobacco

Mouth

1

Dietary factors

Aflatoxins

Liver

1

Aflatoxin M1

(Liver)

2B

Ochratoxin A

(Kidney)

2B

Toxins derived from Fusarium moniliforme

(Oesophagus)

2B

Chinese style salted fish

Nasopharynx

1

Pickled vegetables (traditional in Asia)

(Oesophagus, stomach)

2B

Bracken fern

(Oesophagus)

2B

Safrole

-

2B

Coffee

(Bladder)

2B

Caffeic acid

-

2B

Hot mate

(Oesophagus)

2A

Fresh fruits and vegetables (protective)

Mouth, oesophagus, stomach, colon, rectum, larynx, lung (others)

E

Fat

(Colon, breast, endometrium)

S

Fibre (protective)

(Colon, rectum)

S

Nitrate and nitrite

(Oesophagus, stomach)

S

Salt

(Stomach)

S

Vitamin A, β-carotene (protective)

(Mouth, oesophagus, lung, others)

S

Vitamin C (protective)

(Oesophagus, stomach)

S

IQ

(Stomach, colon, rectum)

2A

MeIQ

-

2B

MeIQx

-

2B

PhIP

-

2B

Reproductive and sexual behaviour

Late age at first pregnancy

Breast

E

Low parity

Breast, ovary, corpus uteri

E

Early age at first intercourse

Cervix

E

Number of sexual partners

Cervix

E

1 Agents and exposures, as well as medicines, occurring mainly in the occupational setting are excluded.

2 Suspected target organs are given in parentheses.

3 IARC Monograph evaluation reported wherever available (1: human carcinogen; 2A: probable human carcinogen; 2B: possible human carcinogen); otherwise E: established carcinogen; S: suspected carcinogen.

4 Human exposure to polycyclic aromatic hydrocarbons occurs in mixtures, such as engine emissions, combustion fumes and soots. Several mixtures and individual hydrocarbons have been evaluated by IARC. 

The IARC Monograph programme has covered most of the known or suspected causes of cancer; there are, however, some important groups of agents that have not been evaluated by IARC - namely, ionizing radiation and electrical and magnetic fields.

Industries and Occupations

Current understanding of the relationship between occupational exposures and cancer is far from complete; in fact, only 22 individual agents are established occupational carcinogens (table 2.5), and for many more experimental carcinogens no definitive evidence is available based on exposed workers. In many cases, there is considerable evidence of increased risks associated with particular industries and occupations, although no specific agents can be identified as aetiological factors. Table 2.11 and table 2.12 present lists of industries and occupations associated with excess carcinogenic risks, together with the relevant cancer sites and the known (or suspected) causative agent(s).

Table 2.11 Industries, occupations and exposures recognized as presenting a carcinogenic risk 

Industry (ISIC code)

Occupation/process

Cancer site/type

Known or suspected causative agent

Agriculture, forestry and fishing (1)

Vineyard workers using arsenic insecticides

Fishermen

Lung, skin

Skin, lip

Arsenic compounds

Ultraviolet radiation

Mining and quarrying (2)

Arsenic mining

Iron ore (haematite) mining

Asbestos mining

Uranium mining

Talc mining and milling

Lung, skin

Lung

Lung, pleural and peritoneal

mesothelioma

Lung

Lung

Arsenic compounds

Radon decay products

Asbestos

Radon decay products

Talc containing asbestiform fibres

Chemical (35)

Bis(chloromethyl) ether (BCME) and chloromethyl-methyl ether (CMME) production workers and users

Vinyl chloride production

Isopropyl alcohol manufacture (strong-acid process)

Pigment chromate production

Dye manufacturers and users

Auramine manufacture

p-chloro-o-toluidine production

Lung (oat-cell carcinoma)

Liver angiosarcoma

Sinonasal

Lung, sinonasal

Bladder

Bladder

Bladder

BCME, CMME

Vinyl chloride monomer

Not identified

Chromium (VI) compounds

Benzidine, 2-naphthylamine, 4-aminobiphenyl

Auramine and other aromatic amines used in the process

p-chloro-o-toluidine and its strong acid salts

Leather (324)

Boot and shoe manufacture

Sinonasal, leukaemia

Leather dust, benzene

Wood and wood products (33)

Furniture and cabinet makers

Sinonasal

Wood dust

Pesticides and herbicides production (3512)

Arsenical insecticides production and packaging

Lung

Arsenic compounds

Rubber industry (355)

Rubber manufacture

Calendering, tyre curing, tyre building

Millers, mixers

Synthetic latex production, tyre curing, calender operatives, reclaim, cable makers

Rubber film production

Leukaemia

Bladder

Leukaemia

Bladder

Bladder

Leukaemia

Benzene

Aromatic amines

Benzene

Aromatic amines

Aromatic amines

Benzene

Asbestos production (3699)

Insulated material production (pipes, sheeting, textile, clothes, masks, asbestos cement products)

Lung, pleural and peritoneal mesothelioma

Asbestos

Metals (37)

Aluminium production

Copper smelting

Chromate production, chromium plating

Iron and steel founding

Nickel refining

Pickling operations

Cadmium production and refining; nickel-cadmium battery manufacture; cadmium pigment manufacture; cadmium alloy production; electroplating; zinc smelters; brazing and polyvinyl chloride compounding

Beryllium refining and machining; production of beryllium-containing products

Lung, bladder

Lung

Lung, sinonasal

Lung

Sinonasal, lung

Larynx, lung

Lung

Lung

Polycyclic aromatic hydrocarbons, tar

Arsenic compounds

Chromium (VI) compounds

Not identified

Nickel compounds

Inorganic acid mists containing sulphuric acid

Cadmium and cadmium compounds

Beryllium and beryllium compounds

Shipbuilding, motor vehicle and railroad equipment manufacture (385)

Shipyard and dockyard, motor vehicle and railroad manufacture workers

Lung, pleural and peritoneal mesothelioma

Asbestos

Gas (4)

Coke plant workers

Gas workers

Gas-retort house workers

Lung

Lung, bladder, scrotum

Bladder

Benzo(a)pyrene

Coal carbonization products, 2-naphthylamine

Aromatic amines

Construction (5)

Insulators and pipe coverers

Roofers, asphalt workers

Lung, pleural and peritoneal mesothelioma

Lung

Asbestos

Polycyclic aromatic hydrocarbons

Other

Medical personnel (9331)

Painters (construction, automotive industry and other users)

Skin, leukaemia

Lung

Ionizing radiation

Not identified

Table 2.12 Industries, occupations and exposures reported to present a cancer excess but for which the assessment of the carcinogenic risk is not definitive 

Industry (ISIC code)

Occupation/process

Cancer site/type

Known (or suspected) causative agent

Agriculture, forestry and fishing (1)

Farmers, farm workers

Lymphatic and haematopoietic system (leukaemia, lymphoma)

Not identified

 

Herbicide application

Malignant lymphomas, soft-tissue sarcomas

Chlorophenoxy herbicides, chlorophenols (presumably contaminated with polychlorinated dibenzodioxins)

 

Insecticide application

Lung, lymphoma

Non-arsenical insecticides

Mining and quarrying (2)

Zinc-lead mining

Lung

Radon decay products

 

Coal

Stomach

Coal dust

 

Metal mining

Lung

Crystalline silica

 

Asbestos mining

Gastrointestinal tract

Asbestos

Food industry (3111)

Butchers and meat workers

Lung

Viruses, PAH1

Beverage industry (3131)

Beer brewers

Upper aero-digestive tract

Alcohol consumption

Textile manufacture (321)

Dyers

Bladder

Dyes

 

Weavers

Bladder, sinonasal, mouth

Dusts from fibres and yarns

Leather (323)

Tanners and processors

Bladder, pancreas, lung

Leather dust, other chemicals, chromium

 

Boot and shoe manufacture and repair

Sinonasal, stomach, bladder

Not identified

Wood and wood products (33), pulp and paper industry (341)

Lumbermen and sawmill workers

Nasal cavity, Hodgkin lymphoma, skin

Wood dust, chlorophenols, creosotes

 

Pulp and papermill workers

Lymphopoietic tissue, lung

Not identified

 

Carpenters, joiners

Nasal cavity, Hodgkin lymphoma

Wood dust, solvents

 

Woodworkers, unspecified

Lymphomas

Not identified

 

Plywood production, particle-board production

Nasopharynx, sinonasal

Formaldehyde

Printing (342)

Rotogravure workers, binders, printing pressmen, machine-room workers and other jobs

Lymphocytic and haemopoietic system, oral, lung, kidney

Oil mist, solvents

Chemical (35)

1,3-Butadiene production

Lymphocytic and haemopoietic system

1,3-Butadiene

 

Acrylonitrile production

Lung, colon

Acrylonitrile

 

Vinylidene chloride production

Lung

Vinylidene chloride (mixed exposure with acrylonitrile)

 

Isopropyl alcohol manufacture (strong-acid process)

Larynx

Not identified

 

Polychloroprene production

Lung

Chloroprene

 

Dimethylsulphate production

Lung

Dimethylsulphate

 

Epichlorohydrin production

Lung, lymphatic and haemopoietic system (leukaemia)

Epichlorohydrin

 

Ethylene oxide production

Lymphatic and haemopoietic system (leukaemia), stomach

Ethylene oxide

 

Ethylene dibromide production

Digestive system

Ethylene dibromide

 

Formaldehyde production

Nasopharynx, sinonasal

Formaldehyde

 

Flame retardant and plasticizer use

Skin (melanoma)

Polychlorinated biphenyls

 

Benzoyl chloride production

Lung

Benzoyl chloride

Herbicides production (3512)

Chlorophenoxy herbicide production

Soft-tissue sarcoma

Chlorophenoxy herbicides, chlorophenols (contaminated with polychlorinated dibenzodioxins)

Petroleum (353)

Petroleum refining

Skin, leukaemia, brain

Benzene, PAH, untreated and mildly treated mineral oils

Rubber (355)

Various occupations in rubber manufacture

Lymphoma, multiple myeloma, stomach, brain, lung

Benzene, MOCA,2 other not identified

 

Styrene-butadiene rubber production

Lymphatic and haematopoietic system

1,3-Butadiene

Ceramic, glass and refractory brick (36)

Ceramic and pottery workers

Lung

Crystalline silica

 

Glass workers (art glass, container and pressed ware)

Lung

Arsenic and other metal oxides, silica, PAH

Asbestos production (3699)

Insulation material production (pipes, sheeting, textiles, clothes, masks, asbestos cement products)

Larynx, gastrointestinal tract

Asbestos

Metals (37, 38)

Lead smelting

Respiratory and digestive systems

Lead compounds

 

Cadmium production and refining; nickel-cadmium battery manufacture; cadmium pigment manufacture; cadmium alloy production; electroplating; zinc smelting; brazing and polyvinyl chloride compounding

Prostate

Cadmium and cadmium compounds

 

Iron and steel founding

Lung

Crystalline silica

Shipbuilding (384)

Shipyard and dockyard workers

Larynx, digestive system

Asbestos

Motor vehicle manufacturing (3843, 9513)

Mechanics, welders, etc.

Lung

PAH, welding fumes, engine exhaust

Electricity (4101, 9512)

Generation, production, distribution, repair

Leukaemia, brain tumours

Extremely low frequency magnetic fields

 

 

Liver, bile ducts

PCBs3

Construction (5)

Insulators and pipe coverers

Larynx, gastrointestinal tract

Asbestos

 

Roofers, asphalt workers

Mouth, pharynx, larynx, oesophagus, stomach

PAH, coal tar, pitch

Transport (7)

Railroad workers, filling station attendants, bus and truck drivers, operators of excavating machines

Lung, bladder

Diesel engine exhaust

 

 

Leukaemia

Extremely low frequency magnetic fields

Other

Service station attendants (6200)

Leukaemia and lymphoma

Benzene

 

Chemists and other laboratory workers (9331)

Leukaemia and lymphoma, pancreas

Not identified (viruses, chemicals)

 

Embalmers, medical personnel (9331)

Sinonasal, nasopharynx

Formaldehyde

 

Health workers (9331)

Liver

Hepatitis B virus

 

Laundry and dry cleaners (9520)

Lung, oesophagus, bladder

Tri- and tetrachloroethylene and carbon tetrachloride

 

Hairdressers (9591)

Bladder, leukaemia and lymphoma

Hair dyes, aromatic amines

 

Radium dial workers

Breast

Radon

1 PAH, polycyclic aromatic hydrocarbon.

2 MOCA, 4,4'-methylene-bis-2-chloroaniline.

3 PCBs, polychlorinated biphenyls.

Table 2.11 presents industries, occupations and exposures in which the presence of a carcinogenic risk is considered to be established, whereas table 2.12 shows industrial processes, occupations and exposures for which an excess cancer risk has been reported but evidence is not considered to be definitive. Also included in table 2.12 are some occupation and industries already listed in table 2.11 for which there is inconclusive evidence of association with cancers other than those mentioned in table 2.11. For example, the asbestos production industry is included in table 2.11 in relation to lung cancer and pleural and peritoneal mesothelioma, whereas the same industry is included in table 2.12 in relation to gastrointestinal neoplasms. A number of industries and occupations listed in tables 2.11 and 2.12 have also been evaluated under the IARC Monographs programme. For example, "occupational exposure to strong inorganic acid mist containing sulphuric acid" was classified in Group 1 (carcinogenic to humans).

Constructing and interpreting such lists of chemical or physical carcinogenic agents and associating them with specific occupations and industries is complicated by a number of factors: (1) information on industrial processes and exposures is frequently poor, not allowing a complete evaluation of the importance of specific carcinogenic exposures in different occupations or industries; (2) exposures to well-known carcinogenic exposures, such as vinyl chloride and benzene, occur at different intensities in different occupational situations; (3) changes in exposure occur over time in a given occupational situation, either because identified carcinogenic agents are substituted by other agents or (more frequently) because new industrial processes or materials are introduced; (4) any list of occupational exposures can refer only to the relatively small number of chemical exposures which have been investigated with respect to the presence of a carcinogenic risk.

All of the above issues emphasize the most critical limitation of a classification of this type, and in particular its generalization to all areas of the world: the presence of a carcinogen in an occupational situation does not necessarily mean that workers are exposed to it and, in contrast, the absence of identified carcinogens does not exclude the presence of yet unidentified causes of cancer.

A particular problem in developing countries is that much of the industrial activity is fragmented and takes place in local settings. These small industries are often characterized by old machinery, unsafe buildings, employees with limited training and education, and employers with limited financial resources. Protective clothing, respirators, gloves and other safety equipment are seldom available or used. The small companies tend to be geographically scattered and inaccessible to inspections by health and safety enforcement agencies.

ENVIRONMENTAL CANCER

Bruce K. Armstrong and Paolo Boffetta

Cancer is a common disease in all countries of the world. The probability that a person will develop cancer by the age of 70 years, given survival to that age, varies between about 10 and 40% in both sexes. On average, in developed countries, about one person in five will die from cancer. This proportion is about one in 15 in developing countries. In this article, environmental cancer is defined as cancer caused (or prevented) by non-genetic factors, including human behaviour, habits, lifestyle and external factors over which the individual has no control. A stricter definition of environmental cancer is sometimes used, comprising only the effect of factors such as air and water pollution, and industrial waste.

Geographical Variation

Variation between geographical areas in the rates of particular types of cancer can be much greater than that for cancer as a whole. Known variation in the incidence of the more common cancers is summarized in table 2.13. The incidence of nasopharyngeal carcinoma, for example, varies some 500-fold between South East Asia and Europe. This wide variation in frequency of the various cancers has led to the view that much of human cancer is caused by factors in the environment. In particular, it has been argued that the lowest rate of a cancer observed in any population is indicative of the minimum, possibly spontaneous, rate occurring in the absence of causative factors. Thus the difference between the rate of a cancer in a given population and the minimum rate observed in any population is an estimate of the rate of the cancer in the first population which is attributable to environmental factors. On this basis it has been estimated, very approximately, that some 80 to 90% of all human cancers are environmentally determined (International Agency for Research on Cancer 1990).

Table 2.13 Variation between populations covered by cancer registration in the incidence of common cancers1  

Cancer (ICD9 code)

High-incidence area

CR2

Low-incidence area

CR2

Range of variation

Mouth (143-5)

France, Bas Rhin

2

Singapore (Malay)

0.02

80

Nasopharynx (147)

Hong Kong

3

Poland, Warsaw (rural)

0.01

300

Oesophagus (150)

France, Calvados

3

Israel (Israeli-born Jews)

0.02

160

Stomach (151)

Japan, Yamagata

11

USA, Los Angeles (Filipinos)

0.3

30

Colon (153)

USA, Hawaii (Japanese)

5

India, Madras

0.2

30

Rectum (154)

USA, Los Angeles (Japanese)

3

Kuwait (non-Kuwaiti)

0.1

20

Liver (155)

Thailand, Khon Khaen

11

Paraguay, Asuncion

0.1

110

Pancreas (157)

USA, Alameda County (Calif.) (Blacks)

2

India, Ahmedabad

0.1

20

Lung (162)

New Zealand (Maori)

16

Mali, Bamako

0.5

30

Melanoma of skin (172)

Australia, Capital Terr.

3

USA, Bay Area (Calif.)(Blacks)

0.01

300

Other skin cancers (173)

Australia, Tasmania

25

Spain, Basque Country

0.05

500

Breast (174)

USA, Hawaii (Hawaiian)

12

China, Qidong

1.0

10

Cervix uteri (180)

Peru, Trujillo

6

USA, Hawaii (Chinese)

0.3

20

Corpus uteri (182)

USA, Alameda County (Calif.) (Whites)

3

China, Qidong

0.05

60

Ovary (183)

Iceland

2

Mali, Bamako

0.09

20

Prostate (185)

USA, Atlanta (Blacks)

12

China, Qidong

0.09

140

Bladder (188)

Italy, Florence

4

India, Madras

0.2

20

Kidney (189)

France, Bas Rhin

2

China, Qidong

0.08

20

1 Data from cancer registries included in IARC 1992. Only cancer sites with cumulative rate larger or equal to 2% in the high-incidence area are included. Rates refer to males except for breast, cervix uteri, corpus uteri and ovary cancers.

2 Cumulative rate % between 0 and 74 years of age.

Source: International Agency for Research on Cancer 1992.

There are, of course, other explanations for geographical variation in cancer rates. Under-registration of cancer in some populations may exaggerate the range of variation, but certainly cannot explain differences of the size shown in table 2.13. Genetic factors also may be important. It has been observed, however, that when populations migrate along a gradient of cancer incidence they often acquire a rate of cancer which is intermediate between that of their home country and that of the host country. This suggests that a change in environment, without genetic change, has changed the cancer incidence. For example, when Japanese migrate to the United States their rates of colon and breast cancer, which are low in Japan, rise, and their rate of stomach cancer, which is high in Japan, falls, both tending more closely towards United States' rates. These changes may be delayed until the first post-migration generation but they still occur without genetic change. For some cancers, change with migration does not occur. For example, the Southern Chinese retain their high rate of cancer of the nasopharynx wherever they live, thus suggesting that genetic factors, or some cultural habit which changes little with migration, are responsible for this disease.

Time Trends

Further evidence of the role of environmental factors in cancer incidence has come from the observation of time trends. The most dramatic and well-known change has been the rise in lung cancer rates in males and females in parallel with but occurring some 20 to 30 years after the adoption of cigarette use, which has been seen in many regions of the world; more recently in a few countries, such as the United States, there has been the suggestion of a fall in rates among males following a reduction in tobacco smoking. Less well understood are the substantial falls in incidence of cancers including those of the stomach, oesophagus and cervix which have paralleled economic development in many countries. It would be difficult to explain these falls, however, except in terms of reduction in exposure to causal factors in the environment or, perhaps, increasing exposure to protective factors-again environmental.

Main Environmental Carcinogenic Agents

The importance of environmental factors as causes of human cancer has been further demonstrated by epidemiological studies relating particular agents to particular cancers. The main agents which have been identified are summarized in table 2.10. This table does not contain the drugs for which a causal link with human cancer has been established (such as diethylstilboestrol and several alkylating agents) or suspected (such as cyclophosphamide) (see also table 2.9). In the case of these agents, the risk of cancer has to be balanced with the benefits of the treatment. Similarly, table 2,10 does not contain agents that occur primarily in the occupational setting, such as chromium, nickel and aromatic amines. For a detailed discussion of these agents see the previous article "Occupational Carcinogens." The relative importance of the agents listed in table 2.10 varies widely, depending on the potency of the agent and the number of people involved. The evidence of carcinogenicity of several environmental agents has been evaluated within the IARC Monographs programme (International Agency for Research on Cancer 1995) (see again "Occupational Carcinogens" for a discussion of the Monographs programme); table 2.10 is based mainly on the IARC Monograph evaluations. The most important agents among those listed in table 2.10 are those to  which a substantial proportion of the population is exposed in relatively large amounts. They  include particularly: ultraviolet (solar) radiation; tobacco smoking; alcohol drinking; betel quid chewing; hepatitis B; hepatitis C and human papilloma viruses;  aflatoxins; possibly dietary fat, and dietary fibre and vitamin A and C deficiency; reproductive delay; and asbestos.

Attempts have been made to estimate numerically the relative contributions of these factors to the 80 or 90% of cancers which might be attributed to environmental factors. The pattern varies, of course, from population to population according to differences in exposures and possibly in the genetic susceptibility to various cancers. In many industrialized countries, however, tobacco smoking and dietary factors are likely to be responsible each for roughly one-third of environmentally determined cancers (Doll and Peto 1981); while in developing countries the role of biological agents is likely to be large and that of tobacco relatively small (but increasing, following the recent increase in the consumption of tobacco in these populations).

Interactions between Carcinogens

An additional aspect to consider is the presence of interactions between carcinogens. Thus for example, in the case of alcohol and tobacco, and cancer of the oesophagus, it has been shown that an increasing consumption of alcohol multiplies manyfold the rate of cancer produced by a given level of tobacco consumption. Alcohol by itself may facilitate transport of tobacco carcinogens, or others, into the cells of susceptible tissues. Multiplicative interaction may also be seen between initiating carcinogens, as between radon and its decay products and tobacco smoking in miners of uranium. Some environmental agents may act by promoting cancers which have been initiated by another agent-this is the most likely mechanism for an effect of dietary fat on the development of breast cancer (probably through increased production of the hormones which stimulate the breast). The reverse may also occur, as, for example, in the case of vitamin A, which probably has an anti-promoting effect on lung and possibly other cancers initiated by tobacco. Similar interactions may also occur between environmental and constitutional factors. In particular, genetic polymorphism to enzymes implicated in the metabolism of carcinogenic agents or DNA repair is probably an important requirement of individual susceptibility to the effect of environmental carcinogens. 

The significance of interactions between carcinogens, from the point of view of cancer control, is that withdrawal of exposure to one of two (or more) interacting factors may give rise to a greater reduction in cancer incidence than would be predicted from consideration of the effect of the agent when acting alone. Thus, for example, withdrawal of cigarettes may eliminate almost entirely the excess rate of lung cancer in asbestos workers (although rates of mesothelioma would be unaffected).

Implications for Prevention

The realization that environmental factors are responsible for a large proportion of human cancers has laid the foundation for primary prevention of cancer by modification of exposure to the factors identified. Such modification may comprise: removal of a single major carcinogen; reduction, as discussed above, in exposure to one of several interacting carcinogens; increasing exposure to protective agents; or combinations of these approaches. While some of this may be achieved by community-wide regulation of the environment through, for example, environmental legislation, the apparent importance of lifestyle factors suggests that much of primary prevention will remain the responsibility of individuals. Governments, however, may still create a climate in which individuals find it easier to take the right decision.

PREVENTION

Per Gustavsson

Occupational exposures account for only a minor proportion of the total number of cancers in the entire population. It has been estimated that 4% of all cancers can be attributed to occupational exposures, based on data from the United States, with a range of uncertainty from 2 to 8%. This implies that even total prevention of occupationally induced cancers would result in only a marginal reduction in national cancer rates.

However, for several reasons, this should not discourage efforts to prevent occupationally induced cancers. First, the estimate of 4% is an average figure for the entire population, including unexposed persons. Among people actually exposed to occupational carcinogens, the proportion of tumours attributable to occupation is much larger. Second, occupational exposures are avoidable hazards to which individuals are involuntarily exposed. An individual should not have to accept an increased risk of cancer in any occupation, especially if the cause is known. Third, occupationally induced cancers can be prevented by regulation, in contrast to cancers associated with lifestyle factors.

Prevention of occupationally induced cancer involves at least two stages: first, identification of a specific compound or occupational environment as carcinogenic; and second, imposing appropriate regulatory control. The principles and practice of regulatory control of known or suspected cancer hazards in the work environment vary considerably, not only among different parts of the developed and developing world, but also among countries of similar socio-economic development.

The International Agency for Research on Cancer (IARC) in Lyon, France, systematically compiles and evaluates epidemiological and experimental data on suspected or known carcinogens. The evaluations are presented in a series of monographs, which provide a basis for decisions on national regulations on the production and use of carcinogenic compounds (see "Occupational Carcinogens").

Historical Background

The history of occupational cancer dates back to at least 1775, when Sir Percivall Pott published his classical report on scrotal cancer in chimney-sweeps, linking exposure to soot to the incidence of cancer. The finding had some immediate impact in that sweeps in some countries were granted the right to bathe at the end of the working day. Current studies of sweeps indicate that scrotal and skin cancer are now under control, although sweeps are still at increased risk for several other cancers.

In the 1890s, a cluster of bladder cancer was reported at a German dye factory by a surgeon at a nearby hospital. The causative compounds were later identified as aromatic amines, and these now appear in lists of carcinogenic substances in most countries. Later examples include skin cancer in radium-dial painters, nose and sinus cancer among woodworkers caused by inhalation of wood dust, and "mule-spinner's disease"-that is, scrotal cancer among cotton industry workers caused by mineral oil mist. Leukaemia induced by exposure to benzene in the shoe repair and manufacturing industry also represents a hazard that has been reduced after the identification of carcinogens in the workplace.

In the case of linking asbestos exposure to cancer, this history illustrates a situation with a considerable time-lag between risk identification and regulatory action. Epidemiological results indicating that exposure to asbestos was associated with an increased risk of lung cancer were already starting to accumulate by the 1930s. More convincing evidence appeared around 1955, but it was not until the mid-1970s that effective steps for regulatory action began. 

The identification of the hazards associated with vinyl chloride represents a different history, where prompt regulatory action followed identification of the carcinogen. In the 1960s, most countries had adopted an exposure limit value for vinyl chloride of 500 parts per million (ppm). In 1974, the first reports of an increased frequency of the rare tumour liver angiosarcoma among vinyl chloride workers were soon followed by positive animal experimental studies. After vinyl chloride was identified as carcinogenic, regulatory actions were taken for a prompt reduction of the exposure to the current limit of 1 to 5 ppm.

Methods Used for the Identification of Occupational Carcinogens

The methods in the historical examples cited above range from observations of clusters of disease by astute clinicians to more formal epidemiological studies-that is, investigations of the disease rate (cancer rate) among human beings. Results from epidemiological studies are of high relevance for evaluations of the risk to humans. A major drawback of cancer epidemiological studies is that a long time period, usually at least 15 years, is necessary to demonstrate and evaluate the effects of an exposure to a potential carcinogen. This is unsatisfactory for surveillance purposes, and other methods must be applied for a quicker evaluation of recently introduced substances. Since the beginning of this century, animal carcinogenicity studies have been used for this purpose. However, the extrapolation from animals to humans introduces considerable uncertainty. The methods also have limitations in that a large number of animals must be followed for several years.

The need for methods with a more rapid response was partly met in 1971, when the short-term mutagenicity test (Ames test) was introduced. This test uses bacteria to measure the mutagenic activity of a substance (its ability to cause irreparable changes in the cellular genetic material, DNA). A problem in the interpretation of the results of bacterial tests is that not all substances causing human cancers are mutagenic, and not all bacterial mutagens are considered to be cancer hazards for human beings. However, the finding that a substance is mutagenic is usually taken as an indication that the substance might represent a cancer hazard for humans.

New genetic and molecular biology methods have been developed during the last 15 years, with the aim of detecting human cancer hazards. This discipline is termed "molecular epidemiology." Genetic and molecular events are studied in order to clarify the process of cancer formation and thus develop methods for early detection of cancer, or indications of increased risk of the development of cancer. These methods include analysis of damage to the genetic material and the formation of chemical linkages (adducts) between pollutants and the genetic material. The presence of chromosomal aberrations clearly indicates effects on the genetic material which may be associated with cancer development. However, the role of molecular epidemiological findings in human cancer risk assessment remains to be settled, and research is under way to indicate more clearly exactly how results of these analyses should be interpreted.

Surveillance and Screening

The strategies for prevention of occupationally induced cancers differ from those applied for control of cancer associated with lifestyle or other environmental exposures. In the occupational field, the main strategy for cancer control has been reduction or total elimination of exposure to cancer-causing agents. Methods based on early detection by screening programmes, such as those applied for cervical cancer or breast cancer, have been of very limited importance in occupational health. 

Surveillance

Information from population records on cancer rates and occupation may be used for surveillance of cancer frequencies in various occupations. Several methods to obtain such information have been applied, depending on the registries available. The limitations and possibilities depend largely on the quality of the information in the registries. Information on disease rate (cancer frequency) is typically obtained from local or national cancer registries (see below), or from death certificate data, while information on the age-composition and size of occupational groups is obtained from population registries.

The classical example of this type of information is the "Decennial supplements on occupational mortality," published in the UK since the end of the nineteenth century. These publications use death certificate information on cause of death and on occupation, together with census data on frequencies of occupations in the entire population, to calculate cause-specific death rates in different occupations. This type of statistic is a useful tool to monitor the cancer frequency in occupations with known risks, but its ability to detect previously unknown risks is limited. This type of approach may also suffer from problems associated with systematic differences in the coding of occupations on the death certificates and in the census data.

The use of personal identification numbers in the Nordic countries has offered a special opportunity to link individual census data on occupations with cancer registration data, and to directly calculate cancer rates in different occupations. In Sweden, a permanent linkage of the censuses of 1960 and 1970 and the cancer incidence during subsequent years have been made available for researchers and have been used for a large number of studies. This Swedish Cancer-Environment Registry has been used for a general survey of certain cancers tabulated by occupation. The survey was initiated by a governmental committee investigating hazards in the work environment. Similar linkages have been performed in the other Nordic countries.

Generally, statistics based on routinely collected cancer incidence and census data have the advantage of ease in providing large amounts of information. The method gives information on the cancer frequencies regarding occupation only, not in relation to certain exposures. This introduces a considerable dilution of the associations, since exposure may differ considerably among individuals in the same occupation. Epidemiological studies of the cohort type (where the cancer experience among a group of exposed workers is compared with that in unexposed workers matched for age, sex and other factors) or the case-control type (where the exposure experience of a group of persons with cancer is compared to that in a sample of the general population) give better opportunities for detailed exposure description, and thus better opportunities for investigation of the consistency of any observed risk increase, for example by examining the data for any exposure-response trends.

The possibility of obtaining more refined exposure data together with routinely collected cancer notifications was investigated in a prospective Canadian case-control study. The study was set up in the Montreal metropolitan area in 1979. Occupational histories were obtained from males as they were added to the local cancer registry, and the histories were subsequently coded for exposure to a number of chemicals by occupational hygienists. Later, the cancer risks in relation to a number of substances were calculated and published (Siemiatycki 1991).

In conclusion, the continuous production of surveillance data based on recorded information provides an effective and comparatively easy way to monitor cancer frequency by occupation. While the main purpose achieved is surveillance of known risk factors, the possibilities for the identification of new risks are limited. Registry-based studies should not be used for conclusions regarding the absence of risk in an occupation unless the proportion of individuals significantly exposed is more precisely known. It is quite common that only a relatively small percentage of members of an occupation actually are exposed; for these individuals the substance may represent a substantial hazard, but this will not be observable (i.e., will be statistically diluted) when the entire occupational group is analysed as a single group.

Screening

Screening for occupational cancer in exposed populations for purposes of early diagnosis is rarely applied, but has been tested in some settings where exposure has been difficult to eliminate. For example, much interest has focused on methods for early detection of lung cancer among people exposed to asbestos. With asbestos exposures, an increased risk persists for a long time, even after cessation of exposure. Thus, continuous evaluation of the health status of exposed individuals is justified. Chest x rays and cytological investigation of sputum have been used. Unfortunately, when tested under comparable conditions neither of these methods reduces the mortality significantly, even if some cases may be detected earlier. One of the reasons for this negative result is that the prognosis of lung cancer is little affected by early diagnosis. Another problem is that the x rays themselves represent a cancer hazard which, while small for the individual, may be significant when applied to a large number of individuals (i.e., all those screened).

Screening also has been proposed for bladder cancer in certain occupations, such as the rubber industry. Investigations of cellular changes in, or mutagenicity of, workers' urine have been reported. However, the value of following cytological changes for population screening has been questioned, and the value of the mutagenicity tests awaits further scientific evaluation, since the prognostic value of having increased mutagenic activity in the urine is not known.

Judgements on the value of screening also depend on the intensity of the exposure, and thus the size of the expected cancer risk. Screening might be more justified in small groups exposed to high levels of carcinogens than among large groups exposed to low levels.

To summarize, no routine screening methods for occupational cancers can be recommended on the basis of present knowledge. The development of new molecular epidemiological techniques may improve the prospects for early cancer detection, but more information is needed before conclusions can be drawn.

Cancer Registration

During this century, cancer registries have been set up at several locations throughout the world. The International Agency for Research on Cancer (IARC) (1992) has compiled data on cancer incidence in different parts of the world in a series of publications, "Cancer Incidence in Five Continents." Volume 6 of this publication lists 131 cancer registries in 48 countries.

Two main features determine the potential usefulness of a cancer registry: a well-defined catchment area (defining the geographical area involved), and the quality and completeness of the recorded information. Many of those registries that were set up early do not cover a geographically well-defined area, but rather are confined to the catchment area of a hospital.

There are several potential uses of cancer registries in the prevention of occupational cancer. A complete registry with nationwide coverage and a high quality of recorded information can result in excellent opportunities for monitoring the cancer incidence in the population. This requires access to population data to calculate age-standardized cancer rates. Some registries also contain data on occupation, which therefore facilitates the monitoring of cancer risk in different occupations.

Registries also may serve as a source for the identification of cases for epidemiological studies of both the cohort and case-control types. In the cohort study, personal identification data of the cohort is matched to the registry to obtain information on the type of cancer (i.e., as in record linkage studies). This assumes that a reliable identifying system exists (for example, personal identification numbers in the Nordic countries) and that confidentiality laws do not prohibit use of the registry in this way. For case-control studies, the registry may be used as a source for cases, although some practical problems arise. First, the cancer registries cannot, for methodological reasons, be quite up to date regarding recently diagnosed cases. The reporting system, and necessary checks and corrections of the obtained information, results in some lag time. For concurrent or prospective case-control studies, where it is desirable to contact the individuals themselves soon after a cancer diagnosis, it usually is necessary to set up an alternative way of identifying cases, for example via hospital records. Second, in some countries, confidentiality laws prohibit the identification of potential study participants who are to be contacted personally.

Registries also provide an excellent source for calculating background cancer rates to use for comparison of the cancer frequency in cohort studies of certain occupations or industries.

In studying cancer, cancer registries have several advantages over mortality registries commonly found in many countries. The accuracy of the cancer diagnoses is often better in cancer registries than in mortality registries, which are usually based on death certificate data. Another advantage is that the cancer registry often holds information on histological tumour type, and also permits the study of living persons with cancer, and is not limited to deceased persons. Above all, registries hold cancer morbidity data, permitting the study of cancers that are not rapidly fatal and/or not fatal at all.

Environmental Control

There are three main strategies for reducing workplace exposures to known or suspected carcinogens: elimination of the substance, reduced exposure by reduced emission or improved ventilation, and personal protection of the workers.

It has long been debated whether a true threshold for carcinogen exposure exists, below which no risk is present. It is often assumed that the risk should be extrapolated linearly down to zero risk at zero exposure. If this is the case, then no exposure limit, no matter how low, would be considered entirely risk-free. Despite this, many countries have defined exposure limits for some carcinogenic substances, while, for others, no exposure limit value has been assigned.

Elimination of a compound may give rise to problems when replacement substances are introduced and when the toxicity of the replacement substance must be lower than that of the substance replaced.

Reducing the exposure at the source may be relatively easily accomplished for process chemicals by encapsulation of the process and ventilation. For example, when the carcinogenic properties of vinyl chloride were discovered, the exposure limit value for vinyl chloride was lowered by a factor of one hundred or more in several countries. Although this standard was at first considered impossible to achieve by industry, later techniques allowed compliance with the new limit. Reduction of exposure at the source may be difficult to apply to substances that are used under less controlled conditions, or are formed during the work operation (e.g., motor exhausts). The compliance with exposure limits requires regular monitoring of workroom air levels.

When exposure cannot be controlled either by elimination or by reduced emissions, the use of personal protection devices is the only remaining way to minimize the exposure. These devices range from filter masks to air-supplied helmets and protective clothing. The main route of exposure must be considered in deciding appropriate protection. However, many personal protection devices cause discomfort to the user, and filter masks introduce an increased respiratory resistance which may be very significant in physically demanding jobs. The protective effect of respirators is generally unpredictable and depends on several factors, including how well the mask is fitted to the face and how often filters are changed. Personal protection must be considered as a last resort, to be attempted only when more effective ways of reducing exposure fail.

Research Approaches

It is striking how little research has been done to evaluate the impact of programmes or strategies to reduce the risk to workers of known occupational cancer hazards. With the possible exception of asbestos, few such evaluations have been conducted. Developing better methods for control of occupational cancer should include an evaluation of how present knowledge is actually put to use.

Improved control of occupational carcinogens in the workplace requires the development of a number of different areas of occupational safety and health. The process of identification of risks is a basic prerequisite for reducing exposure to carcinogens in the workplace. Risk identification in the future must solve certain methodological problems. More refined epidemiological methods are required if smaller risks are to be detected. More precise data on exposure for both the substance under study and possible confounding exposures will be necessary. More refined methods for description of the exact dose of the carcinogen delivered to the specific target organ also will increase the power of exposure-response calculations. Today, it is not uncommon that very crude substitutes are used for the actual measurement of target organ dose, such as the number of years employed in the industry. It is quite clear that such estimates of dose are considerably misclassified when used as a surrogate for dose. The presence of an exposure-response relationship is usually taken as strong evidence of an aetiological relationship. However, the reverse, lack of demonstration of an exposure-response relationship, is not necessarily evidence that no risk is involved, especially when crude measures of target organ dose are used. If target organ dose could be determined, then actual dose-response trends would carry even more weight as evidence for causation. 

Molecular epidemiology is a rapidly growing area of research. Further insight into the mechanisms of cancer development can be expected, and the possibility of the early detection of carcinogenic effects will lead to earlier treatment. In addition, indicators of carcinogenic exposure will lead to improved identification of new risks.

Development of methods for supervision and regulatory control of the work environment are as necessary as methods for the identification of risks. Methods for regulatory control differ considerably even among western countries. The systems for regulation used in each country depend largely on socio-political factors and the status of labour rights. The regulation of toxic exposures is obviously a political decision. However, objective research into the effects of different types of regulatory systems could serve as a guide for politicians and decision-makers.

A  number  of  specific  research  questions  also  need  to  be addressed. Methods to describe the expected effect of withdrawal of a carcinogenic substance or reduction of exposure to the substance need to be developed (i.e., the impact of interventions must be assessed). The calculation of the preventive effect of risk reduction  raises  certain problems  when  interacting substances are studied (e.g., asbestos and tobacco smoke). The preventive effect of removing one of two interacting substances is comparatively greater than when the two have only a simple additive effect.

The implications of the multistage theory of carcinogenesis for the expected effect of withdrawal of a carcinogen also adds a further complication. This theory states that the development of cancer is a process involving several cellular events (stages). Carcinogenic substances may act either in early or late stages, or both. For example, ionizing radiation is believed to affect mainly early stages in inducing certain cancer types, while arsenic acts mainly at late stages in lung cancer development. Tobacco smoke affects both early and late stages in the carcinogenic process. The effect of withdrawing a substance involved in an early stage would not be reflected in a reduced cancer rate in the population for a long time, while the removal of a "late-acting" carcinogen would be reflected in a reduced cancer rate within a few years. This is an important consideration when evaluating the effects of risk-reduction intervention programmes.

Finally,  the  effects  of  new  preventive  factors  have  recently attracted considerable interest. During the last five years, a large number of reports have been published on the preventive effect on lung cancer of consuming fruits and vegetables. The effect seems to be very consistent and strong. For example, the risk of lung cancer has been reported as double among those with a low consumption  of  fruits  and  vegetables  versus  those  with  high intake. Thus, future studies of occupational lung cancer would have greater precision and validity if individual data on fruit and vegetable consumption can be included in the analysis.

In conclusion, improved prevention of occupational cancer involves both improved methods for risk identification and more research on the effects of regulatory control. For risk identification, developments in epidemiology should mainly be directed toward better exposure information, while in the experimental field, validation of the results of molecular epidemiological methods regarding cancer risk are needed.

REFERENCES 

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OTHER RELEVANT READINGS 

Alderson, M. 1986. Occupational Cancer. London: Butterworths. 

Drever, F. (ed.). 1995. Occupational Health, The Registrar General's Decennial Supplement for England and Wales. London: Her Majesty's Stationary Office. 

Flamm, WG and RJ Scheuplein. 1988. Use of short-term test data in risk analysis of chemical carcinogens. In Carcinogen Risk Assessment, edited by CC Travis. London: Plenum Press. 

Hakama, M, V Beral, JW Cullen, and DM Parkin. 1990. Evaluating Effectiveness of Primary Prevention of Cancer. IARC Scientific Publications, No. 103. Lyon: International Agency for Research on Cancer (IARC). 

Muir, C, J Waterhouse, T Mack, J Powell, and S Whelan. 1987. Cancer Incidence in Five Continents. Vol. V. IARC Scientific Publications, No. 88. Lyon: International Agency for Research on Cancer (IARC). 

Office of Population Censuses and Surveys. 1982. Cancer Mortality by Occupation and Social Class 1851-1971. IARC Scientific Publications, No. 36. London: Her Majesty's Stationery Office. 

-. 1986. Occupational Mortality 1979-1980, 1982-1983. Decennial suppl. London: Her Majesty's Stationery Office. 

Parkin, DM, G Wagner, and C Muir. 1985. The Role of the Registry in Cancer Control. IARC Scientific Publications, No. 66. Lyon: International Agency for Research on Cancer (IARC). 

Pearce, NE. 1988. Multistage modeling of lung cancer mortality in asbestos textile workers. Int J Epidemiol 17:747-752.  

Pearce, NE, E Matos, H Vainio, P Boffetta and M Kogevinas. 1994. Occupational Cancer in Developing Countries. Lyon: International Agency for Research on Cancer (IARC). 

Schottenfeld, D and JF Fraumeni. 1982. Cancer Epidemiology and Prevention. London: WB Saunders. 

Vainio, H and J Wilbourn. 1992. Identification of carcinogens within the IARC monograph programme. Scand J Work Environ Health 18 Suppl. 1:64-73.