4.1 What is secondhand smoke?

Last updated: March 2021

Suggested citationCampbell, MA, Ford, C, & Winstanley, MH . Ch 4.The health effects of secondhand smoke 4.1 What is secondhand smoke? In Greenhalgh, EM, Scollo, MM and Winstanley, MH [editors]. Tobacco in Australia: Facts and issues. Melbourne: Cancer Council Victoria; 2021. Available from http://www.tobaccoinaustralia.org.au/chapter-4-secondhand/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 refers to the ambient smoke that is a by-product of active smoking. It consists mainly of exhaled mainstream smoke and sidestream smoke mixed with air. In addition 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. Some harmful compounds in sidestream smoke exist at higher concentrations than found in mainstream smoke. This is due to differences in the burning conditions for mainstream and sidestream smoke. The act of drawing on a lit cigarette creates airflow, which makes the tobacco burn at a higher temperature (up to 900 degrees C). Sidestream smoke, produced at the burning end of the cigarette between puffs, is usually formed at a lower temperature (400 degrees C), potentially leading to incomplete combustion.4 This results in a greater concentration of carcinogens in sidestream smoke when compared to mainstream smoke. By 30 minutes after production, sidestream smoke becomes up to three times more toxic than when it was first produced, based on experiments in rodents.5 

The machine-measured levels of compounds in mainstream smoke vary more between cigarette types than levels of compounds in sidestream smoke. Under similar smoking conditions (i.e. same puff volume and puff interval), mainstream smoke compound levels vary according to design features such as whether there is a filter and the number of ventilation holes in the filter. Sidestream smoke compound levels do not vary greatly between different types of cigarettes and instead 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 by or retained in the smoker’s lung before exhalation.

The toxicity of secondhand smoke therefore depends on a variety of factors including the type of tobacco and other compounds in the cigarette, the proportion of burned tobacco producing mainstream and sidestream smoke, the volume and components of the air with which it mixes and the age of secondhand smoke.

Since most tobacco is smoked in the form of cigarettes, cigarettes are the major source of secondhand smoke. All types of smoked tobacco products, including cigars, pipes, waterpipes, kreteks and bidis, produce secondhand smoke.6


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: https://publications.iarc.fr/Book-And-Report-Series/Iarc-Monographs-On-The-Identification-Of-Carcinogenic-Hazards-To-Humans/Tobacco-Smoking-1986

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: https://www.ncbi.nlm.nih.gov/books/NBK53017/.

5. Schick S and Glantz SA. Sidestream cigarette smoke toxicity increases with aging and exposure duration. Tobacco Control, 2006; 15(6):424–9. Available from: http://tc.bmj.com/cgi/content/abstract/15/6/424

6. Daher N, Saleh R, Jaroudi E, Sheheitli H, Badr T, 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