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
Secondhand smoke is a complex mixture of chemical compounds and particulate matter that are damaging to human health.
Secondhand smoke contains more than 4,000 chemical compounds, including at least 250 constituents that are toxic to the central nervous system, immune system, the heart and the liver and cause eye, skin and respiratory problems. Two of these constituents are carbon monoxide, which reduces the oxygen-carrying capacity of blood, and nitrogen oxides, which affect respiratory function.
Secondhand smoke also contains 50 known carcinogens including benzene. Vapour phase constituents affect the central nervous system, the immune system, the heart and the liver, and cause eye, skin and respiratory problems.
The California Environmental Protection Agency has identified other carcinogenic and toxic constituents including acrolein, formaldehyde, carbonyl sulphide, hydrazine, pyridine, styrene and toluene.
In addition to these chemicals, secondhand smoke contains particulate matter which is an independent health hazard.4 Particulate matter refers to microscopic solid and liquid matter suspended in air, which can be inhaled, and if small enough enter your bloodstream, causing serious health problems. The most studied of these include polycyclic aromatic hydrocarbons (PAHs) including benzo(a)pyrene, aromatic amines 2-naphthylamine and 4-aminobiphenyl, and the tobacco-specific nitrosamines N'-nitrosonornicotine (NNN) and 4-(methylnitrosamino-)1-(3,pyridyl)-1-butone (NNK). The metabolites of NNK are detectable in the urine of non-smokers exposed to secondhand smoke.5 PAHs and nitrosamines induce tumours in the respiratory tract and the lung, while the aromatic amines cause bladder cancer. A number of carcinogenic heavy metals, and weaker carcinogens that induce tumours in a variety of organs, are also present in the particulate matter.
Most particulate matter in secondhand smoke is in the submicron (< 1µm diameter) range, meaning that it is classified as fine particulate matter (also referred to as respirable suspended particles). They are smaller than the particles in mainstream smoke and can penetrate deeper into the lungs, resulting in higher toxicity through oxidative stress and inflammation.6 Short and long-term exposure to fine particulate matter contributes to the aggravation of asthma and other respiratory diseases, lung and other cancers, cardiovascular disease and death.6, 7
Secondhand smoke can be detected in the indoor environment well after it has been generated. Research shows that about half the particulate matter from secondhand smoke is still airborne after five to six hours. Many constituents, such as nicotine and some PAHs, exist in both the gaseous and the particulate phase of secondhand smoke. Classified as ‘semi-volatile’, their ability to change form according to environmental conditions means that they remain detectable in the indoor environment for longer periods after active smoking has ceased.
Over time, secondhand smoke changes in nature. Gaseous components can react with other pollutants and sunlight to form new chemicals. Particles can coagulate, expand, condense, settle on surfaces, or evaporate, depending on concentration, ventilation, humidity, sunlight and other conditions (see section 4.3 Thirdhand smoke). Nicotine may react with hydroxyl radicals in ambient air, giving it a half life of approximately one day.
An analysis of experiments funded by a tobacco company during the 1980s has shown that particulate matter in inhaled fresh sidestream smoke is three to four times as toxic per gram compared with mainstream cigarette smoke.8 Further analysis of the same tobacco industry data has shown that toxicity of sidestream smoke increases by a further two to four times as it ages.9 The authors of this study concluded that if aged sidestream smoke is about three times more toxic than fresh sidestream smoke, and fresh sidestream smoke is about four times more toxic than mainstream smoke, then aged sidestream smoke is approximately 12 times more toxic than mainstream smoke. Although non-smokers inhale much less smoke than smokers, by mass secondhand smoke is more toxic than inhaled mainstream smoke and this helps explain the relatively large health effects of secondhand smoke.9
4.2.1 Secondhand smoke from sources other than cigarettes
Secondhand smoke from other types of tobacco products can differ from cigarette smoke. Cigars can be a larger source of carbon monoxide than cigarettes, but have lower emissions of fine particles and polycyclic aromatic hydrocarbons (PAHs). However, as cigars are larger and have a longer smoking time than cigarettes, smoking a single cigar emits more particles and PAHs than smoking a single cigarette.
Waterpipes (also known as hookahs, narghile, shisha or hubble bubble) are generally smoked using charcoal as a burning agent, with smoking sessions typically lasting 30 minutes to an hour.10-13 Research shows that a single person smoking a waterpipe for up to 30 minutes produces, on average, more fine particles than smoking a cigarette.14 A recent study showed that a one-hour waterpipe smoking session can produce secondhand smoke with four times the amount of volatile aldehydes and carcinogenic PAHs and 30 times the carbon monoxide than that produced by a single cigarette.15
Further information on the constituents of tobacco smoke is provided in Chapter 12, including lists of biologically active constituents in secondhand smoke.
Relevant news and research
For recent news items and research on this topic, click here.(Last updated July 2023)
1. International Agency for Research on Cancer Working Group on the Evaluation of Carcinogenic Risks to Humans. Tobacco smoke and involuntary smoking. IARC monographs on the evaluation of carcinogenic risks to humans, Vol. 83.Lyon, France: IARC, 2004. Available from: http://monographs.iarc.fr/ENG/Monographs/PDFs/index.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. Gerber A, Hofen-Hohloch AV, Schulze J, and Groneberg DA. Tobacco smoke particles and indoor air quality (topiq-ii) - a modified study protocol and first results. J Occup Med Toxicol, 2015; 10:5. Available from: http://www.ncbi.nlm.nih.gov/pubmed/25717342
5. Stark M, Rohde K, Maher J, Pizacani B, Dent C, et al. The impact of clean indoor air exemptions and preemption policies on the prevalence of a tobacco-specific lung carcinogen among nonsmoking bar and restaurant workers. American Journal of Public Health, 2007; 97(8):1457–63. Available from: http://www.ajph.org/cgi/reprint/97/8/1457
6. Valavanidis A, Fiotakis K, and Vlachogianni T. Airborne particulate matter and human health: Toxicological assessment and importance of size and composition of particles for oxidative damage and carcinogenic mechanisms. Journal of Environmental Science and Health. Part C, Environmental Carcinogenesis & Ecotoxicology Reviews, 2008; 26(4):339–62. Available from: http://www.ncbi.nlm.nih.gov/pubmed/19034792
7. Brook RD, Rajagopalan S, Pope CA, 3rd, Brook JR, Bhatnagar A, et al. Particulate matter air pollution and cardiovascular disease: An update to the scientific statement from the american heart association. Circulation, 2010; 121(21):2331–78. Available from: https://www.ncbi.nlm.nih.gov/pubmed/20458016
8. Schick S and Glantz S. Philip morris toxicological experiments with fresh sidestream smoke: More toxic than mainstream smoke. Tobacco Control, 2005; 14(6):396–404. Available from: http://tc.bmjjournals.com/cgi/content/abstract/14/6/396
9. 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
10. Maziak W, Rastam S, Ibrahim I, Ward KD, Shihadeh A, et al. Co exposure, puff topography, and subjective effects in waterpipe tobacco smokers. Nicotine & Tobacco Research, 2009; 11(7):806–11. Available from: http://www.ncbi.nlm.nih.gov/pubmed/19420278
11. Shihadeh A, Azar S, Antonios C, and Haddad A. Towards a topographical model of narghile water-pipe cafe smoking: A pilot study in a high socioeconomic status neighborhood of Beirut, Lebanon. Pharmacology, Biochemistry, and Behavior, 2004; 79(1):75–82. Available from: http://www.ncbi.nlm.nih.gov/entrez/pubmed/15388286
12. Shihadeh A, Antonios C, and Azar S. A portable, low-resistance puff topography instrument for pulsating, high-flow smoking devices. Behavioral Research Methods, 2005; 37(1):186–91. Available from: http://www.ncbi.nlm.nih.gov/pubmed/16097360
13. Cobb C, Ward KD, Maziak W, Shihadeh AL, and Eissenberg T. Waterpipe tobacco smoking: An emerging health crisis in the united states. American Journal of Health Behavior, 2009; 34(3):275–85. Available from: http://www.ncbi.nlm.nih.gov/pubmed/20001185
14. Maziak W, Rastam S, Ibrahim I, Ward KD, and Eissenberg T. Waterpipe-associated particulate matter emissions. Nicotine & Tobacco Research, 2008; 10(3):519-23. Available from: http://www.ncbi.nlm.nih.gov/pubmed/18324571
15. 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