Risk factors for cancer

According to World Health Organization (WHO), common risk factors for cancer include:

• Tobacco use
• Alcohol use
• Overweight and obesity
• Dietary factors, including insufficient fruit and vegetable intake
• Physical inactivity
• Chronic infections from helicobacter pylori, hepatitis B virus (HBV), hepatitis C virus (HCV) and some types of human papilloma virus (HPV)
• Environmental and occupational risks including ionizing and non-ionizing radiation

Cancer-causing agents

Chemical carcinogens
Several chemicals and environmental toxins are responsible for changes in normal cellular DNA. Substances that cause DNA mutations are known as mutagens, and mutagens that cause cancers are known as carcinogens.
Particular substances have been linked to specific types of cancer. Tobacco smoking is associated with many forms of cancer, and causes 90% of lung cancer. Similarly, prolonged exposure to asbestos fibers is associated with mesothelioma.
Tobacco is also related to other cancers such as lung, larynx, head, neck, stomach, bladder, kidney, oesophagus and pancreas as it contains other known carcinogens, including nitrosamines and polycyclic aromatic hydrocarbons.
Ionizing radiations
Radiations due to radon gas and prolonged exposure to ultraviolet radiation from the sun can lead to melanoma and other skin malignancies. Radiation therapy given for one type of cancer may also cause another type of cancer. For example, those who receive chest radiation therapy for lymphomas may later develop breast cancer.
Viral and bacterial infections
Some cancers can be caused by infections with pathogens. Notable among these include liver cancers due to Hepatitic B and C infections; cervical cancer due to infections with Human Papilloma virus (HPV); Epstein Barr virus causing Burkitt’s lymphoma and gastric or stomach cancer due to Helicobacter pylori infection.
Genetic or inherited cancers
Common examples are inherited breast cancer and ovarian cancer genes including BRCA1 and 2. Li-Fraumeni syndrome includes defects in the p53 gene that leads to bone cancers, breast cancers, soft tissue sarcomas, brain cancers etc. Those with Down’s syndrome are known to develop malignancies such as leukemia and testicular cancer.
Hormonal changes
Notable among these are changes in the female hormone levels estrogen. Excess estrogen promotes uterine cancer.
Immune system dysfunction
Impaired immunity including HIV infection leads to several cancers including Kaposi's sarcoma, non-Hodgkin's lymphoma, and HPV-associated malignancies such as anal cancer and cervical cancer.

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