Cancer risk factors
Any cancer develops as a final result of interrelated effects of numerous risk factors from living and working environment and personal behaviour, from genetic factors and chance. Several factors may act together to cause normal cells to become cancerous over time, so the development of any cancer can not be definitely linked to any single risk factor in a single patient, as the disease is always a final result of influence of many risk, but also protective factors. They result from our own healthy or unhealthy behaviour and from pollution of our living and working environment with several chemical, physical and biological agents; genetics is expressed in different ways, usually as different capability of repair of damages, caused in cell nuclei (genes) by environmental factors.
This is the reason why everybody, exposed to any risk factor, does not develop the disease. In cancer thus the term "cause" is usually avoided and replaced by the term "risk factor" as the research shas shown that certain factors increase the chance that an exposed person will develop cancer. E. g. every smoker does not develop lung cancer, but from epidemiologic studies it is known that smokers have a 20-fold increased risk of lung cancer compared to non-smokers.
To identify the carcinogenic risk to humans, experimental and epidemiological studies are used. Experimental studies include short term tests in cell cultures and bacteria and long-term animal experiments. Analytical epidemiologic studies are used to asses the risk of cancer development in humans. On the basis of predetermined criteria, groups of experts judge, whether an association between an exposure to certain risk factor and cancer may be causal.
In Europe, the classification produced by the Monographs group of the International Agency for Research on Cancer (IARC) is the most widely used. In this program, interdisciplinary working groups of expert scientists review the published studies and evaluate the weight of the evidence that an agent can increase the risk of cancer according to defined principles, procedures, and scientific criteria that guide the evaluations. In the list of this Agency, chemicals, complex mixtures, occupational exposures, physical agents, biological agents, and lifestyle factors are classified in 4 groups.
In the first group (group 1) agents are classified known to be carcinogenic to humans (among them are asbestos, tobacco smoke, alcoholic beverages and solar radiation); in the second group (group 2A and B) are agents with probable and possible carcinogenicity, while in the third group (group 3) agents are not yet classifiable as to their carcinogenicity to humans; the fourth group includes agents probably not carcinogenic to humans. The list is regularly updated and is available on the web.
In 1981, Doll and Peto estimated the proportion of cancer deaths attributable to known risk factors in the USA (Table 1). They judged that more than a third of cancer deaths were attributable to risk factors as consequence of unhealthy lifestyle. The most important are those that are the consequence of western way of life: obesity, diet with too much energy and low in vegetables and fruit, sedentary life style together with smoking and high alcohol consumption. Most frequent cancer sites, associated with these factors, are lung, colorectum and breast.
The list of most important cancer risk factors with estimated proportion of deaths from cancer (Doll R, Peto R, 1981).
| Risk factor | Estimated proportion of cancer deaths |
| Obesity and being overweight, unhealthy diet and physical inactivity | 30 |
| Smoking | 16 |
| Infection | 9 |
| Reproductive factors | 7 |
| Occupation | 4 |
| Environment pollution | 1-4 |
| Alcohol | 3 |
| Radiation | 3 |
