Chemical Carcinogens

In 1915, two Japanese scientists named Katsusaburo Yamagiwa and Koichi Ishikawa induced cancer in rabbits by using coal tar. Later, in 1925, Ernest L. Kennaway of England isolated some chemicals from coal tar and demonstrated the carcinogenic effect of these chemicals. In another experiment, butter yellow (dye used to give yellow colour to butter) was fed to rats that led to genesis of the liver cancer in these test animals. Similarly, a study revealed that there was a much higher incidence of bladder cancer among the workers of aniline dye factories. Followed by such observations, certain programmes were initiated all over the world to identify various cancer-causing chemicals. Many chemical carcinogens have been identified so far. The carcinogenic activity of a chemical is measured by its ability to cause cancer in the test animals, for which specific strains of mice are used. The chemical is either painted over the skin or injected in the subcutaneous tissue of the test animals according to a pre-determined schedule, after which the animals are kept under observation.

Tobacco is the most recognised source of chemical carcinogens. Tobacco related cancers account for one third of the human cancers. Tobacco smoke generates more than 2000 chemical compounds, most of which are carcinogenic. Carbon monoxide level in the tobacco smoke is eight times higher than the maximum permissible limit. Tobacco smoke contains nicotine, which is one of the major cancer promoters. Tobacco tar contains certain hydrocarbons such as nitrosamines, benzene, benzopyrenes and many other carcinogenic compounds. The chewing or smoking of tobacco may lead to many cancers including those of the mouth, larynx, lung, stomach and the bladder. India has highest incidence of the oral cavity cancers in the world due to wide spread habit of tobacco-chewing.

Polluted environment is another major source of chemical carcinogens. Fumes emitted by vehicles contain many toxic compounds, most of which are carcinogenic. These include carbon monoxide, lead, nitrous oxide, benzene and many other toxic volatile compounds. The list of chemical carcinogens further includes complex organic chemicals and salts of heavy metals.

Industrial toxic chemicals and heavy metals such as lead, arsenic, mercury, aluminium, nickel and cadmium having carcinogenic activity may enter and get accumulated in the human body. The bioaccumulation of these industrial carcinogens in different tissues and organs of the body may lead to genesis of cancer by damaging the DNA.

Chlorine, used to purify drinking water, may produce certain carcinogenic compounds in the water including chloroform and trichloroethylene, which have carcinogenic activity.

Farmers and agricultural workers are directly exposed to pesticides, insecticides and herbicides such as Carbaryl, Chlordane, Diazinon, Dichlorvos, Dichloro-diphenyl-trichloroethane (DDT), Lindane, Malathion and Toxaphene, most of which are having carcinogenic activity. The food containing residues of pesticides, insecticides and herbicides is one of the three major causes of cancer.

Most of the food additives including preservatives, sweeteners and colorants have carcinogenic activity. Butylated hydroxytoluene, used as a preservative, may lead to the liver cancer. Saccharin and cyclamates, used as artificial sweeteners, may cause cancer of the bladder. Another artificial sweetener, known as Aspartame, may cause the brain tumours. Other food additives, known to cause cancer, include Blue Dye No. 2, Red Dye No. 3, Propyl gallate, Gentian violet, Nitrofurans and Aldicarb.