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Sidestream Smoke

What is it, What are the Dangers and How is it Different from Mainstream Smoke?


Updated September 03, 2013

Written or reviewed by a board-certified physician. See About.com's Medical Review Board.

We hear a lot about secondhand smoke – also referred to as environmental tobacco smoke (ETS) -- but newer terms such as sidestream smoke and mainstream smoke can make this controversial topic even more confusing. What do these terms mean, and what risks do they carry?


Sidestream smoke (SSM) is defined as the smoke that is released from the end of a burning cigarette, cigar, or pipe. Sidestream smoke is different than another term called mainstream smoke (MSM). Mainstream smoke refers to the smoke that is inhaled by a smoker and then exhaled into the environment. When the terms environmental tobacco smoke or secondhand smoke are used, they include both sidestream and mainstream smoke.

Since roughly 85% of secondhand smoke is sidestream smoke, both people who smoke and non-smokers who are nearby have similar exposures to environmental tobacco smoke. Sidestream smoke is also a danger for a longer period of time. Mainstream smoke exposure ends when someone puts out their cigarette, but sidestream smoke can persist -- affecting both smokers and non-smokers for the remainder of the time spent in a room. There are several things that affect the amount of sidestream smoke a person is exposed to. Some of these include air temperature, humidity, the ventilation of the room, and of course the number of smokers present.

Composition – What is in Sidestream Smoke?

There have been several thousand chemicals identified in tobacco smoke, of which at least 60 are suspected of causing cancer. Some of the chemicals that we know are present in sidestream smoke include:

The amount of these chemicals in the air can differ between sidestream smoke and mainstream smoke. One difference is caused by the incomplete burning of tobacco which results in higher concentrations of the chemicals carbon monoxide, 2-naphthylamine, 4-aminobiphenyl, and N-nitrosodimethylamine than in the mainstream smoke that a smoker exhales.

Effects on the Body

Sidestream smoke affects the body in several ways. In some cases we have been able to evaluate how sidestream smoke affects bodily functions in humans, but much of the research in this area has been done on mice (for ethical reasons, since researchers wouldn’t voluntarily expose people to SSM.) Some of these findings include:
  • SSM affects the "autonomic nervous system" – the part of the nervous system that regulates the heart and influences blood pressure and heart rate.

  • SSM damages the large airways (the bronchi) and the smallest airways (the alveoli) of the lungs.

  • SSM results in the production of a greater number of leukocytes (white blood cells in our immune systems that respond to abnormal substances in the body and fight infections.)Secondhand smoke (the combination of SSM and MSS) results in 150,000 to 300,000 lower respiratory in infants and children less than 18 months, and 7,500 to 15,000 hospitalizations every year.

  • SSM decreases the elasticity (flexibility) of the lungs.

  • In mice, SSM inhibits weight gain in developing animals.

  • SSM increased susceptibility to (and severity of) respiratory infections like the flu and the common cold.

  • SSM promotes atherogenesis – the build-up of plaque in arteries which can result in conditions such as heart attacks and strokes. It's estimated that secondhand smoke (again the combination of SSM and MSM) results in 46,000 heart-related deaths in non-smokers in the U.S. every year.

  • SSM causes sperm mutations in male mice.

  • SSM may predispose babies who are exposed in utero (while in the womb) to early heart disease.

More about the risks of secondhand smoke:

Dangers and Risks

There is no safe level of sidestream smoke exposure – also referred to as "sidestream smoking." In fact, the Environmental Protection Agency (EPA) has classified sidesteam smoking as a class A carcinogen. Carcinogens (cancer causing agents) that are defined as class A are those in which there is enough data to indicate they cause cancer in humans.

SSM is a concern for anyone, but certain people are at greater risk. Pregnant women and young children have an increased risk, due both to these being time periods of rapid cell division, but also because unborn babies and children simply have longer to live with whatever damage occurs. For most cancer causing agents there is a latency period – the period of time from which exposure to a carcinogen occurs and the time a cancer develops. If the average latency period for a chemical is 30 years, this is of much greater concern for a 2 year old than an 80 year old.

Another group of people at increased risk are those with medical conditions, especially heart and lung related diseases such as asthma, COPD, lung cancer, and coronary artery disease.

Cancer risks related to secondary smoke including SSM have only recently been studied intensively, but we know a few things:

  • Exposure to secondhand smoke increases the risk of lung cancer, and roughly 3,000 cases of lung cancer in the United States each year are related to this exposure. One study found that children who grew up in households with smoking parents (and were thus exposed to both sidestream and mainstream smoke) were 50% more likely to develop lung cancer in their lifetime.

  • Sidestream smoke may also increase the risk of breast cancer. In one study it was found that exposure to sidestream smoke was just as important as active smoking (being a smoker) when it came to breast cancer risk. When looking at women who had a life-long exposure to secondhand smoke, their risk of developing premenopausal breast cancer was around twice as likely as those who were not exposed to secondhand smoke.

Further information on secondhand smoke and medical conditions:

Sidestream Smoke vs Mainstream Smoke – Which is Worse?

There has been debate over whether sidestream smoke may be even more dangerous than mainstream smoke. One summary (an evaluation of unpublished research by the Philip Morris Company,) found that sidestream smoke:
  • Was 4 times more toxic in total particulate matter.
  • Was 3 times more times toxic per gram (by weight.)
  • Was 2 to 6 times more tumorigenic (cancer causing.)

According to the American Lung Association, sidestream smoke may be more dangerous for 2 reasons: One because the concentration of chemicals is higher (since they are burning at a lower temperature,) and secondly, because it produces smaller particles which may more easily enter and penetrate the tissues in our bodies.

Sidestream Cigar Smoke

While some people may think of cigar smoking as less dangerous it may be even more dangerous to the non-smoker lurking nearby. Since cigars typically burn longer, they give off greater amounts of secondhand smoke than cigarettes.

When the Smoke Clears

After sidestream smoke disappears visually and dissipates into the environment, is the risk gone? For example, if you enter a room in which someone had been smoking days or weeks earlier, is there any danger? Nobody is certain exactly how much of a problem it is, but what has now been coined "thirdhand smoke" has many researchers concerned.

Several of the toxic particles present in sidestream smoke (such as arsenic and cyanide for starters) settle as particles in the area where someone has been smoking and remain on surfaces for an extended period of time. This can pose a problem in a few ways. The toxins may be absorbed through the skin (such as when a toddler is crawling around) or particles may be released back into the air as gases (in a process called off-gassing.) It’s likely that thirdhand smoke is much less dangerous than sidestream smoke, but until we know more, avoiding thirdhand smoke as well as sidestream smoke may not be a bad idea.


American Cancer Society. Cigar Smoking. What about secondhand cigar smoke? Updated 11/18/12. http://www.cancer.org/cancer/cancercauses/tobaccocancer/cigarsmoking/cigar-smoking-secondhand-smoke

American Lung Association. What is Secondhand Smoke (SHS)? Accessed 07/25/13. http://www.lung.org/associations/states/colorado/tobacco/secondhand-smoke.html

Canadian Centre for Occupational Health and Safety. Environmental Tobacco Smoke (ETS): General Information and Health Effects. Updated 03/01/11. http://www.ccohs.ca/oshanswers/psychosocial/ets_health.html

Environmental Protection Agency. Technology Transfer Network Air Toxics Website. Risk Assessment for Carcinogens. Updated 01/04/12. http://www.epa.gov/ttnatw01/toxsource/carcinogens.html

Gairola, C., Drawdy, M., Block, A., and A. Daugherty. Sidestream cigarette smoke accelerates atherogenesis in apolipoprotein E-/-mice. Atherosclerosis. 2001. 156(2):49-55.

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Marchetti, F. et al. Sidestream Smoke is a Male Germ Cell Mutagen. Proceedings of the National Academy of Sciences of the United States. 2011. 108(31):12811-4.

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Villablanca A., Pinkerton, K., and J. Rutledge. Maternal and neonatal exposure to environmental tobacco smoke targets pro-inflammatory genes in neonatal arteries. Journal of Cardiovascular Translational Research. 2010. 3(6):696-703.

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