It is almost always impossible to prove exactly what caused a cancer in any individual, because most cancers have multiple possible causes. For example, if a person who uses tobacco heavily develops lung cancer, then it was very probably caused by the tobacco use, but since everyone has a small chance of developing lung cancer as a result of air pollution or radiation, then there is a tiny chance that the smoker's lung cancer actually developed because of air pollution or radiation.

Cancers are primarily an environmental disease with 90-95% of cases attributed to environmental factors and 5-10% due to genetics.^[1] Environmental, as used by cancer researchers, means any cause that is not genetic, not merely pollution. Common environmental factors that contribute to cancer death include tobacco (25-30%), diet andobesity (30-35%), infections (15-20%), radiation (both ionizing and non-ionizing, up to 10%), stress, lack of physical activity, and environmental pollutants.^[1]