Mainstream Smoke Definition and Dangers

How is Mainstream Smoke Different Than Sidestream Smoke?

woman smoking
What is mainstream smoke and what are the dangers?. commons/Faint Sanity

Mainstream smoke?  In recent years, and with all of the interest in the hazards of secondhand smoke exposure, several confusing new terms have been coined. In addition to mainstream smoke which this article addresses, some of these terms include:

  • Environmental tobacco smoke (ETS) 
  • Passive smoking
  • Involuntary smoking
  • Sidestream smoke
  • Thirdhand smoke

And, in addition to these terms, we are learning that not only is secondhand tobacco smoke dangerous, but also secondhand marijuana smoke, secondhand cigar smoke, secondhand pipe smoke, secondhand hookah smoke, and perhaps even secondhand smoke from e-cigarettes.

What is mainstream smoke? How does it differ from sidestream smoke? What percent of secondhand smoke is mainstream smoke? And what do we know about its effect on nearby smokers and non-smokers?

Definition of Mainstream Smoke

Mainstream smoke is defined as the smoke that is exhaled by a smoker after he takes a puff off of a cigarette, cigar, pipe,or joint. It also refers to the smoke that a smoker inhales.

Mainstream smoke is often differentiated from the other primary form of secondhand smoke - sidestream smoke. Sidestream smoke is the smoke that is released from the end of a burning cigarette, cigar, pipe, or marijuana cigarette.

Other Definitions:

  • Environmental tobacco smoke (ETS) - Environmental tobacco smoke is often used interchangeably with secondhand smoke, and refers to the combination of sidestream smoke and mainstream smoke.
  • Sidestream smoke (SSS) - As noted above, sidestream smoke refers to the smoke released at the burning end of a cigarette.
  • Passive smoking - Passive smoking refers to non-smokers who inhale smoke from nearby smokers.
  • Involuntary smoking - Involuntary smoking is the same as passive smoking, and refers to the smoke inhaled by non-smokers who are near someone who smokes.
  • Third-hand smoke - Only recently discussed, third-hand smoke refers to the particles and gases left over after a cigarette is extinguished. These particles can be found on upholstery, clothing, in carpeting, hair, and virtually any surface. Third-hand smoke includes particles deposited by both mainstream and sidestream smoke.

    Chemicals and Composition of Mainstream Smoke

    Mainstream accounts for 15 percent of the secondhand smoke that non-smokers inhale, with sidestream smoke accounting for the remaining 85 percent.

    While the composition of chemicals in mainstream smoke can vary depending on several things, such as air temperature, humidity, cigarette brand, and the number of smokers present in a room, there are several that raise concern for non-smokers. At least 69 chemicals found in secondhand smoke are considered carcinogenic - meaning they can cause cancer in humans. And while many people would believe that those who smoke would be at greatest risk, that's not necessarily the case. Secondhand smoke contains twice as much tar and nicotine than that inhaled from a cigarette.

    The chemicals present in mainstream smoke can differ from those found in sidestream smoke in a few ways. It is felt that the particles inhaled from mainstream smoke are larger than those in sidestream smoke, and for that reason, are less likely to be inhaled and deposited in the small airways of the lungs.

    Mainstream Smoke vs Sidestream Smoke – Which is Worse?

    While both mainstream smoke and sidestream smoke pose dangers for non-smokers, according to the American Lung Association sidestream smoke may be the more dangerous of the two. Some of these reasons include:

    • Sidestream smoke has a higher concentration of cancer-causing chemicals.
    • Sidestream smoke has smaller particles than mainstream smoke, and these particles are easier to inhale.
    • While mainstream smoke exposure ends when someone puts out a cigarette, sidestream smoke can linger as long as someone is present in a room in which smoking took place.
    • It's thought that sidestream smoke is 4 times more toxic than mainstream smoke, and 6 times more likely to cause cancer.

    Why?  There are a few reasons that may account for these differences:

    • Tobacco burns cleaner at higher temperatures as when a smoker inhales directly from a cigarette, whereas sidestream smoke burns at lower temperatures.
    • Sidestream smoke doesn't have the advantage of a filter to filter out some of the dangerous chemicals in cigarettes.

    Dangers of Mainstream Smoke

    Much of what we know about mainstream smoke is from research conducted evaluating secondhand smoke - thus, it has evaluated the combination of mainstream and sidestream smoke.

    According to the Environmental Protection Agency, secondhand smoke is a grade A carcinogen, meaning that there is sufficient evidence that a substance causes cancer. Secondhand smoke is the third leading cause of preventable disability and early death in the United States.

    It is very important to keep in mind that there is absolutely no safe level of exposure to secondhand smoke, whether it is mainstream smoke or sidestream smoke. That said, there are some people who are greater risk from exposure than others, for example, pregnant woman, young children, and those with respiratory diseases such as asthma, COPD, and lung cancer.

    Here are a few statistics that emphasize the risk of secondhand smoke on health:

    • Secondhand smoke causes 7,000 lung cancer deaths each year in the United States.
    • Secondhand smoke is responsible for 42,000 heart disease deaths each year in the United States.
    • Non-smoking women who live with a smoker are 91% more likely to develop heart disease.
    • Secondhand smoke is responsible for between 150,000 and 300,000 respiratory infections in children 18 months and younger each year in the United States.
    • The cost of secondhand smoke exposure is over 10 billion dollars yearly.

    The following articles go into greater depth about secondhand smoke:

    How Potent is Secondhand Smoke?

    While it can be difficult to quantify risk, a common question is, "How potent is secondhand smoke compared to the smoke a smoker inhales?" If a smoker sits in a room with a smoker for over 2 hours, he inhales the equivalent of 4 cigarettes. Yet, while many people would not allow their children to smoke, said children are essentially smoking when they are in a room with a smoker.

    What About Mainstream Smoke From Cigars and Pipes?

    Though there has been an incorrect belief that the smoke from cigars and pipes is less dangerous to a smoker than that from cigarettes, secondhand smoke from cigars and pipes is just as dangerous, if not more dangerous, than secondhand smoke from cigarettes.

    What About Secondhand Marijuana Smoke?

    Research is in its infancy regarding the effect of marijuana smoke on nearby nonusers, but it appears this exposure may be dangerous as well.

    How to Lower Your Exposure to Secondhand Smoke

    There are many ways in which you can reduce your exposure to secondhand smoke; both mainstream and sidestream smoke:

    • Don’t allow smoking in your home or in your car.
    • Avoid public places that allow smoking, and if it is allowed, try to sit as far away as possible from smokers.
    • If your loved ones smoke, encourage them to quit.

    While smoke-free establishments are becoming much more common, we have been surprised when traveling to learn that not all regions are the same when it comes to protecting the lungs of non-smokers. Here are some tips if you are faced with this yourself:

    Next Steps:

    For those who smoke or have a loved one who smokes, check out these articles:


    Ganapathy, V., Ramachandran, I., Rubenstein, D., and L. Queimado. Detection of in vivo DNA damage induced by very low doses of mainstream and sidestream smoke extracts using a novel assay. American Journal of Preventive Medicine. 2015. 48(1 Suppl 1):S102-10.

    Schick, S., and S. Glantz. Philip Morris toxicological experiments with fresh sidestream smoke: more toxic than mainstream smoke. Tobacco Control. 2005. 14(6):396-404.

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