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Second-Hand Smoke

What is second-hand smoke?

Second-hand smoke contains over 4000 chemicals and is a mix of mainstream smoke exhaled by smokers and sidestream smoke emitted from the tips of burning cigarettes. Second-hand smoke is also known as passive smoke or environmental tobacco smoke (ETS). Most public health authorities will use the term second-hand smoke as opposed to ETS because the latter infers a relationship between tobacco smoke and the environment in general resulting in confusion about its exact meaning.

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Exposure to second-hand smoke

Involuntary exposure to second-hand smoke results from non-smokers breathing air containing second-hand smoke. Involuntary smoking involves inhaling carcinogens, as well as other toxic components, that are present in both mainstream and sidestream smoke. Carcinogens that occur in second-hand smoke include benzene, 1,3-butadiene, benzo[a]pyrene, 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone and many others (IARC Monograph, Volume 83. Tobacco Smoke and Involuntary Smoking. 2002).

The International Agency for Research on Cancer (IARC) concluded in its 2002 Monograph on tobacco smoke and second-hand smoke that that "there is sufficient evidence that involuntary smoking (exposure to second-hand or 'environmental' tobacco smoke) causes lung cancer in humans” and makes the overall evaluation that “Involuntary smoking (exposure to secondhand or 'environmental' tobacco smoke) is carcinogenic to humans (Group 1)."

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Carcinogens in Tobacco Smoke

Every time a person breathes in second-hand smoke, he/she consumes over 100 harmful chemical agents - carcinogens and toxins. Involuntary smoking involves exposure to the same numerous carcinogens and toxic substances that are present in mainstream tobacco smoke. There are 69 identified carcinogens in tobacco smoke (IARC Monographs on the Evaluation of Carcinogenic Risks. Volume 1 and Supplements 1-8, 1972-1999. (1) Human carcinogens; (2A) Probably carcinogenic in humans; (2B) Possibly carcinogenic to humans; (3) Not classifiable as to their carcinogenicity to humans.).

Of the 69 cancer-causing chemical agents, 11 are human carcinogens, 7 are probably carcinogenic in humans, and 49 of animal carcinogens are possibly also carcinogenic to humans. To view IARC’s list of the 69 carcinogens in tobacco smoke, please refer to the National Cancer Institute Monograph 13, Risks Associated with Smoking Cigarettes with Low-Machine Measured Yields of Tar and Nicotine. (U.S. Department of Health Human Services. October 2001: pages 160-165).

While this is a list for mainstream smoke, the same chemicals are also found in sidestream smoke, which together make second-hand smoke a very lethal by-product. The carcinogens found in sidestream smoke often contain higher concentrations of particular carcinogens than found in mainstream smoke because it is produced at a lower temperature, resulting in incomplete combustion of the tobacco (IARC Monograph, volume 83. Tobacco Smoke and Involuntary Smoking. 2002).


In Ontario, the Occupational Health and Safety Act lists fifteen “known toxic agents for which exposure values have not been established, and to which any exposure should be avoided” in the workplace. This is found in Part 10 of the Schedule to the Control of Exposure to Biological or Chemical Agents Regulations, adopted pursuant to the Occupational Health and Safety Act. These fifteen chemicals include fourteen individual agents and one class of n-nitrosamines. Of the fourteen chemical agents, five are in tobacco smoke, in addition to twelve n-nitrosamines. In total, there are seventeen chemicals found in tobacco smoke for which the Occupational Health and Safety Act has not set exposure levels. As noted above, the Act recommends that exposure should be avoided. Furthermore, seven of these seventeen chemicals that are present in tobacco smoke are present both in mainstream and sidestream smoke of all 33 cigarette brands reported to the British Columbia government. A regulatory requirement of B.C.’s Tobacco Sales Amendment Act is that tobacco companies must report 44 chemicals in tobacco smoke. The 33 brands in B.C. account for 69% of cigarettes smoked in Canada. OCAT recommends the following sources on the contents of tobacco smoke and second-hand smoke:

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Health effects of second-hand smoke

The hazardous health effects of exposure to second-hand smoke are now well-documented and established in various independent research studies and numerous international reports. The body of scientific evidence is overwhelming: there is no doubt within the international scientific community that second-hand smoke causes heart disease, lung cancer, nasal sinus cancer, sudden infant death syndrome (SIDS), asthma and middle ear infections in children and various other respiratory illnesses. Second-hand smoke exposure is also causally associated with stroke, low birthweight, spontaneous abortion, negative effects on the development of cognition and behaviour, exacerbation of cystic fibrosis and cervical cancer.

During the past two decades, numerous scientific bodies have concluded that second-hand smoke is dangerous to health:

Since 1986, more than 100 major studies have examined the health effects of exposure to second-hand smoke, and most (about 63%), have found evidence of harm, from respiratory problems through to lung cancer (Why Review Articles on the Health Effects of Passive Smoking Reach Different Conclusions. Barnes Deborah E. and Bero Lisa A. Journal of the American Medical Association 20 May 1998; 279:1566-1570). The following are the seven most comprehensive reviews of the health effects of exposure to second-hand smoke to be published in the last six years:

Passive Smoking Study by Enstrom and Kabat- MAY 2003

This study, published in the British Medical Journal, reports the dangers of second-hand smoke may not be as significant as previously thought. It has been admitted, however, by the authors that the study was largely funded by the tobacco industry. And there have since been several rebuttals and responses to the study, which point to the various flaws with both the study design and its conclusions.

Below are links to some of the comments:

Letters to the Editor. British Medical Journal. December 2003; 237: E266-E267. (See response from Michael Thun on Page 2)

Letters to the Editor. British Medical Journal. August 2003; 327:501-505.

More misleading science from the tobacco industry: Delaying clean air laws through disinformation. Thun, M.J. British Medical Journal. July 2003; 3: 352-353.

British Medical Association Response to BMJ Paper on ‘Effect of Passive Smoking on Health’ May 2003.

American Cancer Society condemns Tobacco Industry Study for Inaccurate Use of Data. May 15, 2003.

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