Table 1 Causes of and risk factors for cancer
GeneticsMutations associated with carcinogenesis may accumulate during DNA replication over time as we age or be inherited (germline mutations)
SmokingTobacco smoke contains more than 7000 chemicals, at least 60 of which cause cancer. Examples include benzene, formaldehyde and polycyclic aromatic hydrocarbons
ObesityA high body mass index (BMI), a useful measure of obesity, is strongly correlated with an increased risk of various cancers
AlcoholDrinking too much alcohol is well established as a cancer risk factor
Ionising radiationX-rays and γ-rays can damage DNA directly or react with water to produce damaging intermediates (reactive oxygen species)
UV radiationUV radiation from the sun is carcinogenic. UV-B is the most effective carcinogen and causes pyrimidine (thymine and cytosine) dimers in DNA leading to mutations
ChemicalsMany chemicals in the environment may cause cancer. Some chemicals may act directly on DNA while others are metabolised in the liver to yield the ultimate carcinogen. Many dietary components may increase or decrease cancer risk; however, with a few exceptions direct evidence demonstrating carcinogenic or protective effects in humans has not been obtained
Infectious agentsBoth viruses and bacteria are recognised as causative factors in various cancers: e.g. human papilloma virus – cervical cancer, hepatitis B virus – liver cancer; Helicobacter pylori (H. pylori) – gastric cancer
Reproductive lifeBreast cancer risk in women is influenced by reproductive history: e.g. not having children, age at giving birth for the first time, and hormonal contraceptive and hormonal replacement therapy