Many nonsmokers are exposed to tobacco smoke, and environmental tobacco smoke has been classified as a known human carcinogen in the USA since 1992.20 Although the risks are, naturally, smaller, the number of people affected is large. One study estimated that breathing other people's smoke increases a person's risk of ischaemic heart disease by a quarter.21
Smoke drawn through the tobacco and taken in by the smoker is known as mainstream smoke. Smoke which arises from smouldering tobacco and passes directly into the surrounding air, whence it may be inhaled by smokers and nonsmokers alike, is known as sidestream smoke. Mainstream and sidestream smoke differ in composition, partly because of the different temperatures at which they are produced. Substances found in greater concentrations in undiluted sidestream smoke than in undiluted mainstream smoke include: nicotine (x 2.7), carbon monoxide (x 2.5), ammonia (x 73),
20 Environmental Protection Agency (EPA 1992 A/600/6-90/006F).
21 Law M R, Morris J K, Wald N J 1997 Environmental tobacco smoke exposure and ischaemic heart disease: an evaluation of the evidence. British Medical Journal 315: 973-988.
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