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Most cancers are related to environmental, lifestyle, or behavioral exposures. The term "environmental", as used by cancer researchers, refers to everything outside the body that interacts with humans. In this sense, the environment is not limited to the biophysical environment (e.g. exposure to factors such as air pollution or sunlight, encountered outdoors or indoors, at home or in the workplace), but also includes lifestyle, economic and behavioral factors. Common environmental factors that contribute to cancer death include tobacco (according to one estimate, accounting for 25–30% of deaths), diet and obesity (30–35%), infections (15–20%), radiation (both ionizing and non-ionizing, up to 10%), stress, lack of physical activity, and environmental pollutants.〔 It is nearly impossible to prove what caused a cancer in any individual, because most cancers have multiple possible causes. For example, if a person who uses tobacco heavily develops lung cancer, then it was probably caused by the tobacco use, but since everyone has a small chance of developing lung cancer as a result of air pollution or radiation, then there is a small chance that the cancer developed because of air pollution or radiation. Cancer is generally not contagious in humans, though it can be caused by oncoviruses and cancer bacteria. It should be noted that aging has been repeatedly and consistently regarded as an important aspect to consider when evaluating the risk factors for the development of particular cancers; aging is considered a risk factor and this is explained by the observation that many molecular and cellular changes are involved in the development of cancer, so it is very likely that these changes accumulate during the aging process (that is, molecular and cellular changes collectively leading to cancer accumulate throughout the years), eventually manifesting themselves as cancer. Over 30% of cancers are potentially avoidable by reducing key risk factors, of which much the significant is tobacco use, which is the cause of about 22% of cancer deaths. Another 10% is due to obesity, a poor diet, lack of physical activity, and drinking alcohol.〔 Other factors include certain infections, exposure to ionizing radiation, and environmental pollutants. In the developing world nearly 20% of cancers are due to infections such as hepatitis B, hepatitis C, and human papillomavirus.〔 These factors act, at least partly, by changing the genes of a cell. Typically many such genetic changes are required before cancer develops.〔 Approximately 5–10% of cancers are due to genetic defects inherited from a person's parents.〔(【引用サイトリンク】American Cancer Society "> Heredity and Cancer )〕 ==Chemicals== 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. Daily long-term vaping with a high voltage (5.0 V) electronic cigarette may generate formaldehyde-forming chemicals at a greater level than smoking, which was determined to be a lifetime cancer risk of approximately 5 to 15 times greater than smoking. Many mutagens are also carcinogens, but some carcinogens are not mutagens. Alcohol is an example of a chemical carcinogen that is not a mutagen. In Western Europe 10% of cancers in males and 3% of cancers in females are attributed to alcohol. Decades of research has demonstrated the link between tobacco use and cancer in the lung, larynx, head, neck, stomach, bladder, kidney, esophagus and pancreas. Tobacco smoke contains over fifty known carcinogens, including nitrosamines and polycyclic aromatic hydrocarbons.〔 Tobacco is responsible for about one in three of all cancer deaths in the developed world,〔 and about one in five worldwide. Lung cancer death rates in the United States have mirrored smoking patterns, with increases in smoking followed by dramatic increases in lung cancer death rates and, more recently, decreases in smoking rates since the 1950s followed by decreases in lung cancer death rates in men since 1990. However, the numbers of smokers worldwide is still rising, leading to what some organizations have described as the ''tobacco epidemic''. Cancer related to one's occupation is believed to represent between 2–20% of all cases. Every year, at least 200,000 people die worldwide from cancer related to their workplace. Most cancer deaths caused by occupational risk factors occur in the developed world.〔 It is estimated that approximately 20,000 cancer deaths and 40,000 new cases of cancer each year in the U.S. are attributable to occupation.〔(【引用サイトリンク】title=National Institute for Occupational Safety and Health- Occupational Cancer )〕 Millions of workers run the risk of developing cancers such as lung cancer and mesothelioma from inhaling asbestos fibers and tobacco smoke, or leukemia from exposure to benzene at their workplaces.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Causes of cancer」の詳細全文を読む スポンサード リンク
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