Basic concepts in hazardous chemicals: Carcinogenicity

By Noor Hasani Hashim
The Sun, April 20, 1996

Cancer is a world-wide problem in that it affects people regardless of race or age, rich or poor.

Each year, it is estimated that about one-fifth of the total number of deaths worldwide is due to cancer. The World Health Organisation has forecast that in another 20 years, there will be five million new cancer cases annually in developed countries and 10 million in developing countries.

There is no single cause of cancer just as there is no single cause of deaths on the road. Cancer is thought to be caused by a complex interaction between harmful agents in our general environment and a person's own susceptibility or predisposition to the disease. The term "general environment" used here refers to everything which people are exposed to arising from outside their bodies, for example, the air we breathe, the food and drink we consume as well as any specific exposure which occurs as a result of our occupations or personal habits such as smoking.

The environment that is of particular relevance is the workplace. It is here that many people are exposed to a wide variety of potentially carcinogenic (cancer-inducing) agents.

The effects of exposure to these agents may not be seen for many years, by which time it is too late, not only for those particular workers, but also for those who're in the same occupation.

Carcinogens fall into two broad categories - those which occur naturally and those produced as a result of industrial processes. There is no sharp dividing line between them. Indeed, many industrial carcinogens are also naturally occurring. Both categories are present in the workplace.

Example of natural carcinogens include ultra-violet light in sunlight, which is potentially carcinogenic to fair skinned people. Aflatoxin, the most potent natural carcinogen, comes from mouldy foodstuffs. Viruses have also been linked to cancer occurring in animals although there is no evidence at present that any human cancer is caused by viruses, despite an enormous amount of research in the matter. However, certain viruses are known to contribute to the disease.

Industrial carcinogens include physical agents such as ionising radiations. X-rays are known to cause skin cancers in people who are not suitably protected and subjected to high doses over a period of time.

Chemical carcinogens are thought to be the most important factor in causing many occupational cancers. However, of roughly 70,000 chemicals now used in commercial production, most are assumed to be either completely safe or at least non-carcinogenic.

We simply do not know the effects of many of these chemicals. At most, only 1% to 2% have been studied for carcinogenic activity using currently accepted tests. The policy at present seems to be that a chemical is innocent or 'safe' until proven otherwise, but the way we prove it otherwise is by observing its effects on people, which by then may be too late. Most hazardous compounds known today have been identified in this manner, at the cost of human lives.

The induction of cancer by chemicals is called chemical carcinogenesis. Compounds or materials that can promote carcinogenesis tend to have the ability to irritate cells and increase the tendency of cells to multiply. In addition, promoters of cancer may interfere with the processes which serve to repair mistakes in the genetic material in a cell.

Occupational chemical carcinogenesis is thus a potential health hazard in the workplace. As the induction of cancer by chemicals often has a long latent period (10 to 40 years), this toxic effect generally appears a long time after the original exposure to the carcinogen.

Once the carcinogen has entered the body (by inhalation, absorption or occasionally ingestion), it may exert its effects in two major ways. The chemical itself causes the development of a tumour, or, the chemical that entered the body is converted into a carcinogenic compound by action of the body on the chemical.

Many carcinogens belong to this group. Present day awareness about chemical carcinogenesis developed from observations that certain types of human cancer were associated with employment in particular industrial activities.

The continuing progress of industrialisation through the 20th century has carried along with it a toll of deaths from occupational cancer among workers in the dye, rubber, asbestos, heavy chemical and metal industries.

A brief history of the detection of occupational cancer
1775 Scrotal skin cancer in chimney sweeps
1822 Scrotal cancer in Cornish copper and tin smelters
1875 Skin cancer in coal tar worker
1876 "Paraffin cancer" due to shale oils
1890 "Aniline cancer*" in workers involved in the manufacture of magenta
1950 Lung and nasal sinus cancers in workers exposed to nickel and chromium compounds
1954 Bladder cancer in workers in the dye, rubber and chemical manufacturing industries
1960 Mesothelioma in workers exposed to asbestos
1970 Angiosarcoma in workers exposed to vinyl chloride in polymer manufacturing
*Aniline is not carcinogenic, this cancer was in fact caused by 4 - amino biphenyl, a compound present as in impurity.

The continuing progress of industrialisation through the 20th century has carried along with it a toll of deaths from occupational cancer among workers in the dye, rubber, asbestos, heavy chemical and metal industries.

In many of these cases, a specific chemical has eventually been shown to be the ultimate initiator of the disease process. The sad aspect of these occupational diseases is that they have been identified at the expense of thousands of individual human tragedies.

This is the inevitable consequence of the epidemiological approach to the detection of occupational cancer because the method is essentially based on "body count". By the year 2000, who knows what new carcinogens will have been discovered or what this body count will be.

Hence, we should aim to minimise the exposure of all workers to all chemicals whether or not they are known to be carcinogenic (or toxic in any other way). We should assume all new chemicals entering the workplace are potentially hazardous (that is carcinogenic), unless conclusive proof of long-term safety is clearly available.

We should attempt to monitor the appearance of adverse effects (or overt illness) in workers in relation to the specific activities and/or chemical in which they have been employed (or exposed to).

The writer is a Science Officer at the National Poison Centre, Universiti Sains Malaysia, Penang.

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