Are most cancers caused by exposures to chemicals?
John Higginson investigated the cancer statistics of different countries in the International Agency for Research of Cancer (IARC) in Lyon in 1960s. He noted that the occurrence of the same cancer in different countries was quite different and some of the differences were very large. In Australia, skin cancer was 200 times more common than in India. Oesophageal cancer was 300 times more common in Iran than in Nigeria. Stomach cancer was 25 times more common in Japan than in Uganda.
Heredity or environment?
In principle, there could be two reasons for the different cancer risk in different countries: Australians, Iranians and Japanese could be genetically more sensitive to contract cancer than Indians, Nigerians or Ugandans; alternatively the environment is responsible for the different cancer risks in different countries. It was possible to differentiate between these two possibilities (nature or nurture) by investigating immigrants. It was noted that when Japanese immigrated to America, their children’s stomach cancer risk became reduced to the American level. On the other hand, their children’s breast cancer risk was increased to the American level, five times higher than if they had stayed in Japan.
What does this mean? The differences between countries cannot be attributable exclusively to genes, it is the different environments which are responsible to a major extent for the different risks of cancer. Higginson calculated that as much as 80–90 % of cancer could be accounted for external factors and not genetics.
Possible to prevent?
There is an important implication to this argument. Cancer might be preventable, if the causes are external or environmental. Since chemicals had acquired such a bad reputation for other reasons, not least because of Rachel Carson’s book Silent Spring (1962), the media and even many researchers concluded that 80 to 90 % of all cancers are the fault of chemicals. This is not what Higginson said; he said that cancer depends on external factors, not simply on genes. These factors include many lifestyle factors in addition to environmental chemicals.
Two good examples are breast cancer and the differently behaving cervical cancer of the uterus. A woman has a lower breast cancer risk if she has given birth and breastfed several children. In fact, Bernardino Ramazzini noted the high incidence on breast cancer in nuns already in 1700, and he concluded it to be due to celibacy. On the other hand, cervical cancer is associated with high sexual activity, especially with several partners. It is easy to understand that the high skin cancer incidence among European immigrants to Australia is due to the fact that their fair skins are not designed to cope with excessive sunlight. This situation can be partly avoided by wearing protective clothing and UV-protection. However, if sexual activity increases some cancers and decreases others, we are dealing with more complicated issues in cancer causation than some scientists were anticipating in their wishful thinking.
Prime causes of cancer
British epidemiologists Richard Doll and Richard Peto estimated the most likely causes of cancer in 1981 based on American statistics. According to their classic estimate, one third of all cancers are due to tobacco smoking and alcohol, one third to dietary factors, and one third to all other causes combined. With respect to these other causes, the greatest factors in their estimate were sexual behaviour and occupation, and possibly infectious diseases; the impact of the infections was very uncertain. Occupational exposure was estimated to account for a mere 4% of cancer, and this included both chemical and other factors. Environmental exposure was estimated at 2%.
At the end of 1990s experts in the Nordic cancer registries estimated the percentage of avoidable cancers in the Nordic Countries. Smoking was thought to account for 23% of cancers in men and 5% in women, ((alcohol 2 and 1%, respectively. Occupation was calculated to be responsible for 4% in men and 1% in women. Ionising radiation (both radon and medical uses included) was estimated at 3% and ultraviolet radiation at 3%.))
Food
An unhealthy diet increases the risk of several types of cancer. The higher the energy consumption, the higher is the risk of cancer. A high intake of fat is a risk factor for breast cancer and prostate cancer and according to some studies, also of colon cancer. On the other hand, ample consumption of vegetables and fruit seems to decrease the cancer risk. This means that if one wishes to decrease the cancer risk one should follow a diet which will also reduce the incidences of the diseases of the heart and blood vessels. One should eat only moderately, small amounts of meat and animal fat, not too much salt, but plenty of wholemeal products, fruit, berries and vegetables. The impact of food additives in cancer causation is at its most marginal, in fact, it is actually not sure if they increase or decrease cancer; the best scientific guess would be that they are more likely to decrease the cancer risk by preventing spoilage.
Recent studies have indicated that the most important factor in the environmental causes of cancer is air pollution, especially fine particulate matter that seems to cause lung cancer. The number of lung cancers in Europe associated with fine particulate matter might be around the order of ten to thirty thousand per year. Thus, there may be about as many as those caused by indoor radon. Other important factors at the population level might be some factors present in drinking water. Water disinfection by-products may cause a few cancers as might arsenic in some regions. It is not possible to obtain accurate information on individual chemicals in the environment, because the sensitivity of epidemiological studies is not good enough. One cannot totally exclude the possibility that benzene in petrol might cause a few cases of leukaemia and other solvents may be responsible for a few cancers of other types.
Thus the prediction of Doll and Peto made in 1981 of a couple of percent of all cancer being due to environmental chemicals seems to hold up fairly well. If one wishes to identify the most important risks, we only have to screen our own lifestyle, smoking, alcohol use, and dietary factors. The decline in the popularity of smoking has already decreased lung cancer rates in men in many countries. Unfortunately the female lung cancer rate is still increasing.
Contrary to general beliefs, the major causes of cancer are not due to “chemicalisation” of the world, but to the old dull major factors that should encourage us to change our lifestyles.
One level up: Here a risk, there a risk, everywhere risks, risks!
Previous chapter: Which type of radiation poses the greatest threat to our health?
Next chapter: Is cancer more common today than it was in the past?
