TOBACCO, through its various forms of exposure, constitutes the main cause of cancer-related deaths worldwide among men, and increasingly among women. Forms of exposure include active smoking, breathing secondhand tobacco smoke (passive or involuntary smoking) and smokeless tobacco. Tobacco causes a variety of cancer types, such as lung, oesophageal, laryngeal, oral, bladder, kidney, stomach, cervical and colorectal. The total death toll in 2005 from tobacco use was estimated at 5.4 million people (Mathers & Loncar, 2006), including about 1.5 million cancer deaths. If present usage patterns continue, the overall number of tobacco-related deaths is projected to rise to about 6.4 million in 2015, including 2.1 million cancer deaths. In 2030, the projected overall death toll will amount to 8.3 million. In low- and middle-income countries, tobacco attributable deaths have been projected to double between 2002 and 2030.

PHYSICAL INACTIVITY, DIETARY FACTORS, OBESITY AND BEING OVERWEIGHT play an important role as causes of cancer. These factors are affected by gender norms. Because all these factors are intimately interconnected at the individual and contextual levels, estimating the specific contribution of each of these risk factors is difficult and might underestimate the cumulative potential risk.

Overweight and obesity are causally associated with several common cancer types, including cancers of the oesophagus, colorectum, breast in postmenopausal women, endometrium and kidney (WHO, 2003a).

Physical inactivity is a major contributor to the rise in rates of overweight and obesity in many parts of the world, and independently increases the risk of some cancers. Taken together, raised body mass index and physical inactivity account for an attributable fraction of 19% of breast cancer mortality, and 26% of colorectal cancer mortality (Danaei et al., 2005). Overweight and obesity alone account for 40% of endometrial (uterus) cancer. Overweight, obesity and physical inactivity collectively account for an estimated 159 000 colon and rectum cancer deaths per year, and 88 000 breast cancer deaths per year.

ALCOHOL USE is a risk factor for many cancer types including cancer of the oral cavity, pharynx, larynx, oesophagus, liver, colorectum and breast. Risk of cancer increases with the amount of alcohol consumed. The risk from heavy drinking for several cancer types (e.g. oral cavity, pharynx, larynx and oesophagus) substantially increases if the person is also a heavy smoker. Attributable fractions vary between men and women for certain types of alcohol-related cancer, mainly because of differences in average levels of consumption. For example, 22% of mouth and oropharynx cancers in men are attributable to alcohol whereas in women the attributable burden drops to 9%. A similar sex difference exists for oesophageal and liver cancers (Rehm et al., 2004).

Chronic HEPATITIS B VIRUS (HBV) infection (chronic hepatitis) causes about 52% of the world's hepatocellular carcinomas, resulting in nearly 340 000 deaths per year (Perz et al 2006). Another 20% of hepatocellular cancers (124 000 deaths) are caused by hepatitis C virus (HCV) infection. HBV infections interact with exposure to aflatoxin (through consumption of contaminated food) in increasing the risk of liver cancer. Both HBV infections and exposure to aflatoxin are particularly common in sub-Saharan Africa and some parts of south-east Asia, and are believed to be the cause of up to 80% of liver cancer cases that occur in these regions (IARC/WHO, 2003).

HUMAN PAPILLOMA VIRUS (HPV) is the world's most common sexually transmitted viral infection of the reproductive tract, infecting an estimated 660 million people per year. It is also estimated to cause almost all cases of cervical cancer, 90% of anal cancers and 40% of cancers of the external genitalia. HPV also causes cancer of the oral cavity and the oropharynx. Of the many HPV genotypes, types 16, 18 and more than 10 other types are causal for cervical cancer. The most common high-risk genotypes, 16 and 18, account for about 70% of cervical cancer cases worldwide. There is, however, some regional variation, mainly resulting from differences in prevalence of HPV type 18 (WHO, 2006a).

ENVIRONMENTAL POLLUTION of air, water and soil with carcinogenic chemicals accounts for 1–4% of all cancers (IARC/WHO, 2003). Exposure to carcinogenic chemicals in the environment can occur through drinking water or pollution of indoor and ambient air. In Bangladesh, 5–10% of all cancer deaths in an arsenic-contaminated region were attributable to arsenic exposure (Smith, Lingas & Rahman, 2000). Exposure to carcinogens also occurs via the contamination of food by chemicals, such as aflatoxins or dioxins. Indoor air pollution from coal fires doubles the risk of lung cancer, particularly among non-smoking women (Smith, Mehta & Feuz, 2004). Worldwide, indoor air pollution from domestic coal fires is responsible for approximately 1.5% of all lung cancer deaths. Coal use in households is particularly widespread in Asia.

More than 40 agents, mixtures and exposure circumstances in the working environment are carcinogenic to humans and are classified as OCCUPATIONAL CARCINOGENS (Siemiatycki et al., 2004). That occupational carcinogens are causally related to cancer of the lung, bladder, larynx and skin, leukaemia and nasopharyngeal cancer is well documented. Mesothelioma (cancer of the outer lining of the lung or chest cavity) is to a large extent caused by work-related exposure to asbestos.

Occupational cancers are concentrated among specific groups of the working population, for whom the risk of developing a particular form of cancer may be much higher than for the general population. About 20–30% of the male and 5–20% of the female working-age population (people aged 15–64 years) may have been exposed to lung carcinogens during their working lives, accounting for about 10% of lung cancers worldwide. About 2% of leukaemia cases worldwide are attributable to occupational exposures.

RADIATION is energy emitted in the form of waves or rays. Ionizing radiation removes electrons from material (called ionization) when passing through cells and tissue, leading to cell or tissue injury. Medical X-rays and radiation emitted from natural sources, such as radon gas and radioactive materials, are examples of ionizing radiation.

Ionizing radiation can cause almost any type of cancer, but particularly leukaemia, lung, thyroid and breast cancer. Exposure to natural radiation is largely a result of radon gas in homes, which increases the risk of lung cancer (Darby et al., 2005).

Non-ionizing radiation comprises electromagnetic fields like those emitted by mobile phones or power lines and ultraviolet radiation (mainly from the sun), the latter causing chromosomal damages. Ultraviolet radiation is a recognized cause of skin cancer including malignant melanomas.

While REPRODUCTIVE FACTORS, such as mother's age when she first gives birth, and number of births, affect cancer risk, they are not considered in this module. Decisions on childbirth are usually made in a complex context of societal, familial, and individual perspectives and are not primarily driven by the desire to reduce cancer risk.

The longer women breastfeed the more they are protected against breast cancer (Collaborative Group on Hormonal Factors in Breast Cancer, 2002). WHO is promoting breastfeeding by means of the global strategy for infant and young child feeding (http://www.who.int/nutrition/publications/infantfeeding/en/index.html <http://www.who.int/nutrition/publications/infantfeeding/en/index.html>).

Combined hormonal contraception modifies slightly the risk of some cancers. However, recent reviews have shown that for most healthy women the health benefit clearly exceed the health risk. Some combined hormonal menopausal regimens have been shown to increase cancer risk (http://www.who.int/reproductive-health/family_planning/cocs_hrt.html <http://www.who.int/reproductive-health/family_planning/cocs_hrt.html>).