As in any other disease, in cancer also, both the environmental as well as the genetic factors, play a role in the causation of the disease. Over the last few decades, it has been found that the environment plays a prominent role in the causation of most cancers. 80 to 90 per cent of all cancers are said to be dependant directly or indirectly on environmental factors. Cancer problem in the world and in India is fast increasing with the increasing industrialisation and the presence of innumerable chemicals and fumes in the environment, changes in life-style including smoking and eating habits.
Among the environmental factors that can cause cancer, the following are the well-recognised ones.
Physical Reagents or Reactive Substances
In the England of the eighteenth century, naked boys used to crawl down chimneys to sweep them before the beginning of winter. Many of these boys, as they grew up, were noticed to develop cancer of the scrotum, i.e. the sac that holds the testes and the skin over the abdomen. The cause of this cancer was found to be the deposition and rubbing of soot, coal-ckist and coal tar on their skin over many years. Later on, scientists isolated from coal tar, substances such as (dibenzanthracene, which could cause skin cancers in mice when it was painted daily on their skins for a period of a few months.
Benzapyrene, a substance akin to dibenzanthracene has been seen in a number of edibles and in the environments, including places of work. The distribution of benz(a) pyrene in nature is very wide, as smoke contains the substance along with other polycyclic aromatic hydrocarbons, and during rains these are brought down to the earth, where in turn, these are taken up by the vegetables and plants.
Numerous other chemical agents have either been identified or come under suspicion. The list is a long one: tars, dyes, aromatic amines, urethane, various metals, e.g. nickle, beryllium, arsenic, chromium, asbestos, hormones, and more recently anatoxin.
A number of chemicals utilized as medicinal compounds can cause cancer. Diethyl stilbesterol (female sex hormone) is probably the best example having been shown to cross the placenta and cause vaginal cancer (adenocarcinoma) in offspring of women who have taken large doses during their pregnancy. The use of estrogens (female sex hormones) in menopausal women has also been found to be associated with a very significant increase in risk of uterine cancer.
Alkylating agents and other compounds employed in the treatment of cancer are carcinogenic, i.e. can cause cancer, in animals and their potential risk as well as risk benefit considerations must be carefully evaluated.
Mesothelioma, a cancer of the pleura, a covering layer over the lungs, is a rare tumour, occuring in individuals exposed to asbestos. In recent years, it has been found that asbestos workers also have a significantly increased risk for lung cancer. Here again, however, this risk is associated with smoking which has a dramatic synergistic effect. The asbestos worker who is also a heavy smoker has a 20-fold increase in risk of lung cancer. Cancer of the urinary bladder has been found to occur frequendy in workers in aniline dye (synthetic colour) factories.
Thus, we see that there are many cancer-causing chemicals in our environment, exposure to which for longer or shorter duration, can make one liable to getting lung and other cancers.