4.1 What is secondhand smoke?

Unless otherwise noted, the following section is compiled from reviews published by the International Agency for Research on Cancer (2004),1 California Environmental Protection Agency (2005),2 and the Office of the US Surgeon General (2006)3 and 2010.4

Burning tobacco produces a complex mixture that comprises about 7000 chemical substances in the form of gases, liquid vapours and particulate matter.4 The process of smoking a cigarette produces three different types of tobacco smoke. The first is mainstream smoke, the smoke directly inhaled into the smoker's lungs through the burning cigarette. The second is exhaled mainstream smoke, the smoke breathed out by the smoker from their lungs. The third is sidestream smoke, the smoke that drifts from the smouldering tip of the cigarette.

Secondhand smoke is the term commonly used to describe the ambient smoke that is a by-product of active smoking. It consists mainly of exhaled mainstream smoke and sidestream smoke. Small amounts of mainstream smoke also escape through the cigarette mouthpiece, and vapour compounds diffuse through the cigarette wrapper. Secondhand smoke has also commonly been referred to as environmental tobacco smoke. Breathing in secondhand smoke is also called passive smoking or involuntary smoking.

Mainstream smoke and sidestream smoke contain a similar range of chemicals, but they differ in the relative proportions and amounts of these chemicals produced per cigarette. For example, the levels of some compounds in sidestream smoke are more than 10 times that found in mainstream smoke. In general, smokers take only several puffs from a cigarette, which may smoulder for some minutes, so sidestream smoke comprises the greater share of secondhand smoke. Differences between mainstream and sidestream smoke are also due to the burning conditions under which each is produced. The act of drawing on a lit cigarette creates airflow, which makes the tobacco burn at a higher temperature (up to 850 degrees C). Sidestream smoke, produced at the burning end of the cigarette between puffs, is usually formed at a lower temperature (600 degrees C), leading to incomplete combustion. This also results in a greater quantity of compounds being released into sidestream smoke than mainstream smoke, per cigarette.

The machine-measured yields of constituents in mainstream smoke are subject to more variation between cigarette types than sidestream smoke yields. Under similar smoking conditions (i.e. same puff volume and puff interval), mainstream smoke yields will vary according to design features such as whether there is a filter and the number of ventilation holes in the filter. Sidestream smoke yields do not vary greatly between different types of cigarettes, because they reflect the weight of the tobacco burned during smouldering.

Mainstream smoke differs from exhaled mainstream smoke as it undergoes some changes after being inhaled. Some constituents of the smoke are absorbed through or retained in the smoker's lung tissue before exhalation.

Since most tobacco is smoked in the form of cigarettes, cigarettes are the major source of secondhand smoke. Other smoked tobacco products, such as cigars, pipes, waterpipes, kreteks and bidis, also produce secondhand smoke.5


1. International Agency for Research on Cancer. Tobacco smoking. IARC Monographs on the evaluation of carcinogen risk of chemicals to humans, Vol. 38. Lyon, France: IARC, 1986. Available from: http://monographs.iarc.fr/ENG/Monographs/allmonos47.php

2. Office of Environmental Health Hazard Assessment and California Air Resources Board. Health effects of exposure to environmental tobacco smoke: final report, approved at the Panel's June 24, 2005 meeting. Sacramento: California Environmental Protection Agency, 2005. Available from: http://www.oehha.ca.gov/air/environmental_tobacco/2005etsfinal.html

3. US Department of Health and Human Services. The health consequences of involuntary exposure to tobacco smoke: a report of the Surgeon General. Atlanta, Georgia: US Department of Health and Human Services, Centers for Disease Control and Prevention, Coordinating Center for Health Promotion, National Center for Chronic Disease Prevention and Health Promotion, Office on Smoking and Health, 2006. Available from: http://www.cdc.gov/tobacco/data_statistics/sgr/sgr_2006/index.htm

4. US Department of Health and Human Services. How tobacco smoke causes disease: the biology and behavioral basis for smoking-attributable disease. A report of the Surgeon General. Atlanta, Georgia: U.S. Department of Health and Human Services, Centers for Disease Control and Prevention, National Center for Chronic Disease Prevention and Health Promotion, Office on Smoking and Health, 2010. Available from: http://www.surgeongeneral.gov/library/tobaccosmoke/report/index.html

5. Daher N, Saleh R, Jaroudi E, Sheheitli H, Badr T, Sepetdjian E, et al. Comparison of carcinogen, carbon monoxide, and ultrafine particle emissions from narghile waterpipe and cigarette smoking: sidestream smoke measurements and assessment of second-hand smoke emission factors. Atmospheric Environment 2010;44(1):8–14. Available from: http://www.ncbi.nlm.nih.gov/pubmed/20161525

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