Sidestream vs. Mainstream Smoke: Definition and Effects

Is One Worse Than the Other?

Sidestream smoke (SSM) is the smoke that is released from the end of a burning cigarette, cigar, or pipe.

Mainstream smoke (MSM) is 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.

This article will go over sidestream smoke and mainstream smoke. You will learn about how sidestream smoke is different from mainstream smoke and how they affect the body.

Hand holding burning cigarette
Thana Prasongsin / Getty Images

Sidestream vs. Mainstream Smoke: Which Is Worse?

There has been debate over whether sidestream smoke is more dangerous than mainstream smoke. One summary evaluating unpublished research by the Philip Morris Company found that:

  • Sidestream smoke was 4 times more toxic in total particulate matter
  • Sidestream smoke was 3 times more toxic per gram (by weight)
  • Sidestream smoke was 2 to 6 times more tumorigenic (cancer-causing)

There are two main reasons sidestream smoke may be more dangerous than mainstream smoke:

  • The concentration of certain chemicals is higher (e.g. nicotine, formaldehyde, ammonia).
  • It produces smaller particles that may more easily enter and penetrate the tissues in our bodies.

Characteristics of Sidestream Smoke

Since roughly 85% of secondhand smoke is sidestream smoke, both people who smoke and non-smokers nearby have similar exposures to environmental tobacco smoke.

Sidestream smoke is also a danger for a longer 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
  • Ventilation
  • The number of cigarettes smoked in a specific timeframe

Composition of Sidestream Smoke

There have been several thousand chemicals, metals, and gases identified in tobacco smoke, of which nearly 70 are suspected of causing cancer. A few of the most toxic known to be present in sidestream smoke include:

  • Benzene: A carcinogen (cancer-causing agent) that is thought to cause leukemias and lymphomas. It can also damage the immune system, raising the risk of infections.
  • Phenol: Polyphenolic compounds such as catechol and hydroquinone are known to promote tumor growth and metastasis in lung cancer, and are toxic to the cardiovascular system.
  • Styrene: The outcome of a large Danish study showed the risk of acute myeloid leukemia, a rare form of leukemia, is doubled with exposure to styrene. The central nervous system can also be affected.
  • Hydrogen cyanide: Cyanide is most toxic when it is breathed in, and it causes the cells in human and animal tissues to stop using oxygen, leading to cell death and organ damage. The heart and brain are the most vulnerable to cyanide poisoning.
  • Formaldehyde: This is linked to both nasopharyngeal carcinoma and myeloid leukemias. Formaldehyde can also paralyze the cilia, the small hair-like structures that line the respiratory tract to catch toxins and push them back into the mouth to be swallowed. This can result in other substances in smoke gaining access to the deeper regions of the lungs where they can do damage.
  • Nicotine: Nicotine is the addictive component of tobacco. Alone, it does not appear to cause cancer but may work along with other toxins to result in the changes which create cancer. Nicotine may also aid in the progression and spread of cancer.
  • Carbon monoxide: A poisonous gas present in tobacco smoke that enters the bloodstream and reduces oxygen supply to the body's tissues and organs. It adversely affects the central nervous system and the cardiovascular system.

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.

Sidestream Smoke Effects on the Body

Much of the research in this area has been done on rats and mice, but the implications for humans are plenty alarming.

Sidestream smoke affects the autonomic nervous system, the part of the nervous system that regulates the heart and influences blood pressure. It also damages the large airways (the bronchi) and the smallest airways (the alveoli) of the lungs.

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

Sidestream smoke has also been found to decrease the elasticity (flexibility) of the lungs, inhibit weight gain in developing animals, and increase susceptibility to (and severity of) asthma and respiratory infections like the flu and the common cold.

The long-term damage from sidestream smoke includes the promotion of atherogenesis, and 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 combining SSM and MSM) results in 46,000 heart-related deaths in non-smokers in the U.S. every year. It may even predispose babies who are exposed in utero (while in the womb) to early heart disease.

Dangers and Risks of Sidestream Smoke

There is no safe level of sidestream smoke exposure. In fact, the Environmental Protection Agency (EPA) has classified sidestream smoking as a class A carcinogen, meaning there is enough data to indicate they cause cancer in humans.

Sidestream smoke 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 cancer develops. If the average latency period for a chemical is 30 years, this is of 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 7,300 cases of lung cancer in the United States each year are related to this exposure.

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 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.

Is Sidestream From Cigars Safer?

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. For those who smoke cigars, it's important to learn about the research that has looked specifically at cigar smoking and lung cancer.

How Long Does Sidestream Smoke Linger?

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) 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.


Sidestream smoke and mainstream smoke both have negative effects on your health. The difference between sidestream and mainstream smoke is that sidestream smoke that comes from the end of a burning cigarette, while mainstream smoke is what a smoker exhales.

Smoke exposure does not end when a person puts out a cigarette; the people around them can still be exposed to smoke that lingers in the air. Over time, being exposed to smoke can cause health problems, including increasing a person's risk for cancer and heart disease.

Frequently Asked Questions

  • What are the three types of smoke?

    People often refer to exposure to cigarettes or other sources of smoke as "first-hand," "second-hand," and "third-hand." First-hand smoke is what a person inhales when they smoke. Second-hand smoke is what they exhale. Third-hand smoke is inhaling the lingering smoke that's left behind after a person puts out a cigarette. The terms "sidestream smoke," "mainstream smoke," and "environmental smoke" are also used to describe smoke exposure.

  • What is sidestream smoking and how is it different from mainstream smoking?

    Sidestream smoke comes from the end of a lit cigarette or pipe. Mainstream smoke is the smoke a person exhales while they're smoking.

  • Is second-hand smoke worse than mainstream smoke?

    All smoke has dangerous chemicals in it but second-hand smoke usually has higher concentrations of these chemicals than mainstream smoke, making it more dangerous.

  • What are the effects of sidestream smoke?

    Sidestream smoke exposure has been linked to many negative health effects, including an increased risk for asthma, COPD, cancer, and heart disease.

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Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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By Lynne Eldridge, MD
 Lynne Eldrige, MD, is a lung cancer physician, patient advocate, and award-winning author of "Avoiding Cancer One Day at a Time."