At least one-third of all cancer cases are preventable. Prevention offers the most cost-effective long-term strategy for the control of cancer.

Tobacco is the single largest preventable cause of cancer in the world today. It causes 80-90% of all lung cancer deaths, and about 30% of all cancer deaths in developing countries, including deaths from cancer of the oral cavity, larynx, oesophagus and stomach. A comprehensive strategy including bans on tobacco advertising and sponsorship, tax increases on tobacco products, and cessation programmes can reduce tobacco consumption in many countries. The WHO Framework Convention on Tobacco Control, adopted in May 2003, aims to curb tobacco-related deaths and disease.

Dietary modification is another important approach to cancer control. There is a link between overweight and obesity to many types of cancer such as oesophagus, colorectum, breast, endometrium and kidney. Diets high in fruits and vegetables may have a protective effect against many cancers. Conversely, excess consumption of red and preserved meat may be associated with an increased risk of colorectal cancer. In addition, healthy eating habits that prevent the development of diet-associated cancers will also lower the risk of cardiovascular disease.

Regular physical activity and the maintenance of a healthy body weight, along with a healthy diet, will considerably reduce cancer risk. National policies and programmes should be implemented to raise awareness and reduce exposure to cancer risk factors, and to ensure that people are provided with the information and support they need to adopt healthy lifestyles.

Infectious agents are responsible for almost 22% of cancer deaths in the developing world and 6% in industrialized countries. Viral hepatitis B and C cause cancer of the liver; human papilloma virus infection causes cervical cancer; the bacterium Helicobacter pylori increases the risk of stomach cancer. In some countries the parasitic infection schistosomiasis increases the risk of bladder cancer and in other countries the liver fluke increases the risk of cholangiocarcinoma of the bile ducts. Preventive measures include vaccination and prevention of infection and infestation.

Exposure to ionizing radiation is also known to cause to certain cancers. Excessive solar ultraviolet radiation increases the risk of all types of cancer of the skin. Avoiding excessive exposure, use of sunscreen and protective clothing are effective preventive measures.

Asbestos can cause lung cancer; aniline dyes have been linked to bladder cancer; and benzene can lead to leukaemia. The prevention of certain occupational and environmental exposure to these and other chemicals is another important element in preventing cancer.