Although researchers are still unaware on how carcinogens cause cancer but one theory is that exposure contact with carcinogens when combined with aging, causes and increase in chemicals in the body called free radicals. Smoking is associated with cancer in the lungs, eosophagus, respiratory tract, bladder, pancreas and even cancer of the stomach liver and kidneys. The risk of cancer increases depending on the number of cigarettes smoked per day, the cigarettes tar content and how many years a person smokes. Exposure to the smoke of others cigarettes is called second hand smoke or environmental tabacco smoke. This can also cause cancer. 3,000 non smoking adults in the US have died from this. Cigars, pipes and smokeless tobacco have also been implicated in increased risk for cancer. Snuff users are 50 times more likely to develop cancers of the cheek or gum than non users. Another fact is that obesity increases the risk of developing cancer in the breast, colon, kidney and gall bladder. Another major cause is the exposure to electromagnetic radiation, invisible, high energy light waves such as sunlight and x-rays these are the cause for most skin cancers. Air pollution water pollution and pollutants in the soil also cause cancer particularly in the bladder and lungs. .
The genetic and family factors associated with cancer risk and causation is recognised but not well understood. There are two approaches in the study of hereditary factors. The epidemiologic approach examines the frequency of the disease among relatives. The genetic approach studies the pattern of disease expression among relatives. These approaches help to provide evidence of familial aggregation but do not answer why a particular cancer express itself. The greatest known cancer risk exists when there is a primary relative of a patient with an autosomal dominant inherited cancer. A thorough and complete family history can be invaluable in identifying relative risks and traits within a family aggregate.