In 1915, two Japanese scientists named Katsusaburo Yamagiwa and Koichi Ishikawa induced cancer in rabbits by painting coal tar on their ears. Later, in 1925, Ernest L. Kennaway of England isolated certain chemicals from coal tar and demonstrated their carcinogenic effect. In another experiment, butter yellow (a dye used to give yellow colour to butter) was fed to rats for a few weeks and it was found that these rats developed cancer in their liver. It was also observed that the workers of aniline dye factories were having much higher incidence of bladder cancer. Followed by such observations, certain programmes were initiated all over the world to identify cancer-causing chemicals. Carcinogenic activity of a chemical can be measured by its ability to cause cancer in the test animals. Specific strains of mice are used in such experiments. The chemical is either painted over skin or injected below the skin in test animals according to a predetermined schedule and they are kept under observation. Over a period of time, many chemical carcinogens have been identified.
Tobacco is considered as the most recognised source of chemical carcinogens. Tobacco smoke generates more than 2000 chemical compounds, most of which are carcinogens. The level of Carbon monoxide in tobacco smoke is eight times higher than the maximum permissible limits. Tobacco smoke also contains Nicotine, which is one of the major cancer promoters. Tobacco tar contains hydrocarbons such as Nitrosamines, Benzene, Benzopyrenes and other carcinogenic compounds. Tobacco chewing & smoking can cause many cancers including those of the lung, mouth, larynx, stomach and bladder. India has the highest rate of oral cancers in the world due to widespread habit of tobacco chewing. Tobacco related cancers account for more than one third of all the cancer.
The polluted environment is another major source of chemical carcinogens. Fumes emitted by vehicles contain many toxic chemical compounds, most of which are carcinogens such as Carbon monoxide, Lead, Nitrous oxide, Benzene and many other toxic volatile compounds. The list of chemical carcinogens also includes salts of heavy metals and complex organic chemicals.
Most of the industrial toxic chemicals and heavy metals such as Lead, Arsenic, Mercury, Aluminium, Nickel and Cadmium having carcinogenic activity may find their way into the human body. The bioaccumulation of these industrial carcinogens in different tissues & organs of the human body can cause cancer by damaging the DNA.Farmers and agricultural workers are directly exposed to pesticides such as Carbaryl, Chlordane, Diazinon, Dichlorvos, Dichloro-diphenyl-trichloroethane (DDT), Lindane, Malathion and Toxaphene, which are carcinogens.
Chlorine is commonly used to purify the drinking water. Chlorine can form cancer-causing compounds in the drinking water such as Chloroform and Trichloroethylene.
Food containing the residues of pesticides & herbicides is a major source of chemical carcinogens. Most of the food additives like preservatives, sweeteners and colours can cause cancer. Butylated hydroxytoluene used as a preservative can cause liver cancer. Saccharin and cyclamates used as artificial sweeteners can cause cancer of the bladder. Another artificial sweetener called aspartame, which is used in many food products can cause brain tumours. Other food additives, which are known to cause cancer, include Blue Dye No 2, Red Dye No. 3, Propyl gallate, Gentian violet, Nitro furans and Aldicarb.