Epidemiological studies demonstrate that allergy has a protective effect against some types of cancer, or acts as a cancer risk for other cancer types. Most studies indicate that people with a history of allergic disease, particularly with asthma, have an elevated risk of lung cancer. In the case of colon cancer, pancreatic cancer, childhood leukaemia and brain tumours the data indicate a protective effect of allergic conditions. On the other hand, a number of environmental factors have been reported to affect the development of allergy and cancer.