Secondhand Smoke

Secondhand smoke is the smoke that non-smokers end up breathing when someone nearby is smoking. It has two parts: the smoke a smoker breathes out, and the "sidestream" smoke that rises straight off the burning tip of a cigarette, cigar, or pipe between puffs. Together they fill a room, a car, or a doorway with the same toxic mixture the smoker inhales — and everyone else breathes it too. You may also hear it called environmental tobacco smoke (ETS), passive smoking, or involuntary smoking, because the people harmed by it usually never chose to smoke at all. This page explains, in plain language, what is actually in secondhand smoke, why public-health experts say there is no safe level of it, how it harms adults and — especially — children, what "thirdhand smoke" is, and the practical steps that genuinely protect the people you love. The tone here is honest and protective: secondhand smoke is one of the clearest and most preventable health hazards we know of, and the good news is that it is almost entirely avoidable.

What Secondhand Smoke Is
What's In Secondhand Smoke
There Is No Safe Level
Health Effects in Adults
Health Effects in Children
Thirdhand Smoke
Who Is Most at Risk
How to Protect Yourself and Your Family
The Honest Bottom Line
Research Papers
Connections
Featured Videos

What Secondhand Smoke Is

Every lit cigarette produces smoke in two ways, and secondhand smoke is a blend of both:

Mainstream smoke — the smoke a smoker draws through the cigarette and then breathes back out into the room.
Sidestream smoke — the smoke that curls up continuously from the smoldering tip in between puffs. This is actually the larger source of the smoke that fills a shared space, and, as the next section explains, it can be more concentrated in some harmful chemicals than what the smoker inhales.

When a non-smoker breathes this mixture, their body absorbs the same nicotine and toxic combustion products a smoker takes in — just at lower, uncontrolled doses that they never agreed to. Researchers can measure this directly: non-smokers exposed to secondhand smoke carry cotinine (a breakdown product of nicotine) in their blood, saliva, and urine, which is proof the exposure is real and not merely unpleasant.

This is not a matter of opinion or of simply disliking the smell. Decades of research by the world's leading health authorities have reached the same conclusion: secondhand smoke is a proven, preventable cause of disease and early death in people who do not smoke. The World Health Organization's cancer agency (IARC) classifies it as a known human carcinogen, and the U.S. Surgeon General has repeatedly reported that it causes serious illness in non-smoking adults and children alike. One global analysis estimated that secondhand smoke was responsible for roughly 600,000 deaths worldwide in a single year, with a striking share of that burden falling on children (Öberg and colleagues, 2011).

What's In Secondhand Smoke

Tobacco smoke is not one chemical — it is a dense, ever-changing mixture of thousands of different compounds. Public-health agencies commonly count around 7,000 chemicals in tobacco smoke, of which at least about 70 are known to cause cancer, alongside many more that are toxic, irritating, or damaging to the heart and lungs. Some of the better-known culprits include:

Benzene — a solvent and known cause of leukemia. Tobacco smoke is one of the largest sources of benzene exposure in the general population.
Formaldehyde — the same pungent chemical used to preserve tissue; an irritant and carcinogen.
Polonium-210 — a radioactive element that rides along in tobacco smoke and delivers a small dose of radiation to the lungs.
Tobacco-specific nitrosamines (such as NNK and NNN) — a family of powerful carcinogens found essentially only in tobacco.
Toxic metals such as arsenic, cadmium, and chromium.
Carbon monoxide, hydrogen cyanide, and ammonia — poisonous gases that interfere with breathing and oxygen delivery.
Fine and ultrafine particles — microscopic specks small enough to travel deep into the lungs and pass into the bloodstream, driving much of the harm to the heart and blood vessels.

A crucial and under-appreciated point is that sidestream smoke is often more concentrated than the smoke the smoker inhales. Because the smoldering tip burns at a lower temperature and its smoke is never filtered through the cigarette or the smoker's lungs, sidestream smoke can carry higher amounts of many toxic and cancer-causing chemicals per unit of smoke. In other words, the haze drifting across a room is not a watered-down version of smoking — in some respects it is a dirtier one.

There Is No Safe Level

If there is one sentence to remember from this entire page, it is this: the U.S. Surgeon General has concluded that there is no risk-free level of exposure to secondhand smoke (Surgeon General's Report, 2006). This is the settled, mainstream, expert view — not a cautious guess. Even short, occasional exposure is not harmless.

Why can't a "little" be safe? Because some of the damage begins almost immediately:

Blood becomes stickier. Within minutes to hours of exposure, platelets (the cells that form clots) become more prone to clumping, which is a step on the path to heart attacks and strokes.
Blood-vessel linings are injured. Secondhand smoke damages the delicate endothelium that lines arteries, reducing their ability to relax and open normally.
The airways react. Eyes water, noses run, throats sting, and people with asthma can start to wheeze — sometimes after just a brief exposure.

This is also why a common "solution" does not actually work. Ventilation, air conditioning, fans, and separate smoking sections cannot eliminate secondhand smoke. Diluting the air lowers the smell and some of the visible haze, but it does not remove the gases and fine particles to a safe level, and it does not undo the biological effects. The Surgeon General was explicit on this point: the only way to fully protect non-smokers indoors is to eliminate smoking in the shared space entirely. Everything short of that is a partial measure.

Health Effects in Adults

In non-smoking adults, secondhand smoke is a cause — not merely a suspect — of several of the leading killers. Three links are firmly established as causal by the Surgeon General and IARC:

Lung cancer

Non-smokers can get lung cancer from breathing other people's smoke. A classic pooled analysis found that a lifelong non-smoker who lives with a smoker has roughly a 20–30% higher risk of lung cancer than one who does not (Hackshaw and colleagues, 1997), and later analyses confirmed the association (Taylor and colleagues, 2007). It is one reason lung cancer sometimes strikes people who never smoked a day in their lives.

Coronary heart disease

Secondhand smoke harms the heart and arteries. Non-smokers regularly exposed to it have about a 25–30% higher risk of coronary heart disease (He and colleagues, 1999) — a surprisingly large effect for a "secondhand" exposure, driven by the fine particles and their impact on clotting, inflammation, and blood vessels. This is also why communities that adopted smoke-free laws have seen measurable drops in heart-attack hospitalizations.

Stroke

The 2014 Surgeon General's report concluded that secondhand smoke causes stroke in non-smokers. A meta-analysis estimated roughly a 25% increase in stroke risk among those exposed (Oono and colleagues, 2011).

Everyday irritation and worsened lung disease

Short of these major diseases, secondhand smoke routinely causes eye, nose, and throat irritation, coughing, and headaches, and it worsens existing conditions such as asthma, COPD, and allergies. For someone who already has heart or lung disease, even a modest exposure can trigger symptoms.

Health Effects in Children

Children deserve to be first in this discussion, because they carry the heaviest burden and have the least ability to protect themselves. A small child cannot leave the room, roll down a window, or ask an adult to step outside. Children also breathe faster than adults, so they take in proportionally more of any smoke in the air, and their lungs, brains, and immune systems are still developing. The evidence that secondhand smoke harms children is deep and consistent:

Sudden infant death syndrome (SIDS). Exposure to tobacco smoke — both during pregnancy and after birth — substantially raises a baby's risk of dying suddenly and unexpectedly in their sleep (Zhang and Wang, 2013). Keeping smoke entirely away from infants is one of the recognized ways to lower SIDS risk.
Asthma — more of it, and worse. Secondhand smoke both helps cause new asthma and wheezing illness in children and makes existing asthma more frequent and more severe, with more attacks, more medication, and more emergency visits (Burke and colleagues, 2012).
Respiratory infections. Children in smoking households get more bronchitis, bronchiolitis, and pneumonia, and tend to have more severe cases (Jones and colleagues, 2011).
Middle-ear infections. Secondhand smoke increases the risk of the painful, recurrent ear infections (otitis media) that plague early childhood and can affect hearing (Jones and colleagues, 2012).
Impaired lung development. Growing up around smoke is linked to reduced lung growth and lower lung function — a disadvantage that can follow a child into adulthood.

The harm can begin before birth. When a pregnant woman who does not smoke is exposed to secondhand smoke, her baby is more likely to be born with low birth weight (Leonardi-Bee and colleagues, 2008), which is itself a risk factor for a range of newborn problems. Protecting a pregnancy from other people's smoke is protecting the baby.

Thirdhand Smoke

There is a newer, more subtle concern beyond the smoke you can see and smell. Thirdhand smoke is the invisible residue that secondhand smoke leaves behind. As smoke drifts through a room, its chemicals — nicotine chief among them — settle onto and soak into surfaces: walls, carpets, curtains, upholstery, bedding, dust, toys, clothing, hair, and the interior of a car (Matt and colleagues, 2011).

What makes thirdhand smoke different from a lingering odor is that it persists and even keeps changing for weeks to months. The residue slowly re-releases chemicals back into the air, and, more troublingly, it can react with ordinary indoor pollutants to form brand-new toxins. In particular, nicotine on surfaces reacts with nitrous acid in the air to form tobacco-specific nitrosamines — potent carcinogens that were not present in the original smoke (Sleiman and colleagues, 2010). Airing out a room or opening a window does not remove this residue, and normal cleaning only partly reduces it (Jacob and colleagues, 2017).

Thirdhand smoke is an emerging area of science — less thoroughly studied than secondhand smoke — but the concern is especially real for crawling babies and toddlers. Young children spend their days on the floor, put their hands and objects in their mouths, and swallow far more household dust per pound of body weight than adults do. That means a home or a car where smoking once happened — even if no one is smoking now — can still expose the smallest, most vulnerable people to tobacco toxins. It is one more reason the goal is a completely smoke-free environment, not just "not right now."

Who Is Most at Risk

Anyone who breathes secondhand smoke is affected, but some groups carry far more of the burden:

Infants and children — the most vulnerable of all, for every reason described above: developing bodies, faster breathing, hand-to-mouth behavior, and no ability to remove themselves.
Pregnant people and their unborn babies — exposure raises the risk of low birth weight and other pregnancy complications.
People who live with a smoker — the home is where most non-smokers get the largest share of their exposure, and a car is even more concentrated because the space is so small.
People with existing heart or lung disease — those with asthma, COPD, or heart disease can have symptoms triggered by exposures that a healthy person might barely notice.
Hospitality and service workers — historically, bartenders, restaurant and casino workers, and others in indoor venues faced some of the heaviest occupational secondhand-smoke exposure of anyone. Smoke-free workplace laws were, in large part, written to protect them, and they have sharply reduced that exposure where adopted.

How to Protect Yourself and Your Family

The encouraging part of this topic is that secondhand smoke is almost entirely preventable. These steps, roughly in order of impact, genuinely work:

Make your home 100% smoke-free. Not "only in the garage," not "only by an open window," not "only when the baby is asleep." A truly smoke-free home is the single most protective step, and it also makes quitting easier for anyone who does smoke.
Make your car 100% smoke-free. A car is a tiny sealed box; smoking in it produces extremely high concentrations, and cracking a window does not fix that. Keep it smoke-free with the windows up or down.
Understand what does not work. Ventilation, air conditioning, fans, opening windows, and even air purifiers and filters do not eliminate secondhand smoke. They may cut the smell or some particles, but they cannot make a smoke-filled space safe. Only keeping the smoke out of the air in the first place protects people.
Choose smoke-free spaces. Favor smoke-free workplaces, restaurants, hotels, and rentals, and support smoke-free policies in the places you and your family spend time.
Support a smoker to quit — with kindness. The best possible outcome for everyone, including the smoker, is quitting. Nicotine addiction is genuinely powerful and hard to beat, so blame and shame rarely help; encouragement and practical support do. Free, confidential help exists in most countries — telephone quitlines (in the U.S., 1-800-QUIT-NOW), counseling, and proven medications can several-fold increase the odds of success. Offering to help someone find that support is often more useful than asking them to "just stop."
Don't assume vaping is a safe alternative for bystanders. The aerosol exhaled from e-cigarettes is not harmless water vapor. It contains nicotine, ultrafine particles, and other chemicals, and it settles onto surfaces much like tobacco residue does. Secondhand e-cigarette aerosol is likely less toxic than secondhand cigarette smoke, but it is not clean air and is not risk-free — particularly around children, pregnant people, and anyone with heart or lung conditions. Treat indoor vaping around others with the same caution you would tobacco smoke.

The Honest Bottom Line

Secondhand smoke is a proven, preventable cause of cancer, heart disease, stroke, and serious childhood illness — and there is no safe level of it. Children bear the heaviest burden: SIDS, asthma, infections, ear problems, and impaired lung growth all trace back, in part, to the smoke in the air around them. The residue those chemicals leave behind, thirdhand smoke, can linger and even turn more toxic for months, which is why "not right now" is not the same as safe.

The fix is simple to state, even when it is hard to live: keep smoke out of the air that other people — especially children — have to breathe. A 100% smoke-free home and car is the most protective single decision a family can make. And the kindest, most effective long-term move is to support any smoker in your life toward quitting, with patience rather than blame. Protecting the people you love from secondhand smoke is one of the clearest good-news stories in public health: the hazard is real, but it is one we can almost entirely prevent.

Research Papers

U.S. Department of Health and Human Services. The Health Consequences of Involuntary Exposure to Tobacco Smoke: A Report of the Surgeon General. Atlanta, GA: CDC; 2006. PubMed topic search — The landmark report whose central conclusion is that there is no risk-free level of secondhand-smoke exposure and that ventilation cannot eliminate it.
U.S. Department of Health and Human Services. The Health Consequences of Smoking—50 Years of Progress: A Report of the Surgeon General. Atlanta, GA: CDC; 2014. PubMed topic search — The 50-year review that formally added stroke to the list of diseases causally linked to secondhand smoke.
International Agency for Research on Cancer (IARC). Tobacco Smoke and Involuntary Smoking. IARC Monographs on the Evaluation of Carcinogenic Risks to Humans, Volume 83. Lyon, France: IARC; 2004. PubMed topic search — The WHO cancer agency's review classifying secondhand (involuntary) smoke as a Group 1 known human carcinogen.
Öberg M, Jaakkola MS, Woodward A, Peruga A, Prüss-Ustün A. Worldwide burden of disease from exposure to second-hand smoke: a retrospective analysis of data from 192 countries. The Lancet. 2011;377(9760):139–146. doi:10.1016/S0140-6736(10)61388-8 — Estimated roughly 600,000 deaths worldwide in one year from secondhand smoke, with a large share of the burden falling on children.
Hackshaw AK, Law MR, Wald NJ. The accumulated evidence on lung cancer and environmental tobacco smoke. BMJ. 1997;315(7114):980–988. doi:10.1136/bmj.315.7114.980 — A pooled analysis finding non-smokers living with a smoker have roughly a 20–30% higher risk of lung cancer.
Taylor R, Najafi F, Dobson A. Meta-analysis of studies of passive smoking and lung cancer: effects of study type and continent. International Journal of Epidemiology. 2007;36(5):1048–1059. doi:10.1093/ije/dym158 — A large meta-analysis confirming the excess lung-cancer risk from secondhand smoke across study types and regions.
He J, Vupputuri S, Allen K, Prerost MR, Hughes J, Whelton PK. Passive smoking and the risk of coronary heart disease—a meta-analysis of epidemiologic studies. New England Journal of Medicine. 1999;340(12):920–926. doi:10.1056/NEJM199903253401204 — Found about a 25% higher risk of coronary heart disease in non-smokers exposed to secondhand smoke.
Oono IP, Mackay DF, Pell JP. Meta-analysis of the association between secondhand smoke exposure and stroke. Journal of Public Health (Oxford). 2011;33(4):496–502. doi:10.1093/pubmed/fdr025 — Pooled evidence showing roughly a 25% increase in stroke risk from secondhand-smoke exposure.
Zhang K, Wang X. Maternal smoking and increased risk of sudden infant death syndrome: a meta-analysis. Legal Medicine (Tokyo). 2013;15(3):115–121. doi:10.1016/j.legalmed.2012.10.007 — Confirmed that tobacco-smoke exposure substantially raises the risk of sudden infant death syndrome.
Burke H, Leonardi-Bee J, Hashim A, et al. Prenatal and passive smoke exposure and incidence of asthma and wheeze: systematic review and meta-analysis. Pediatrics. 2012;129(4):735–744. doi:10.1542/peds.2011-2196 — Found that secondhand smoke increases the incidence of asthma and wheezing illness in children.
Jones LL, Hashim A, McKeever T, Cook DG, Britton J, Leonardi-Bee J. Parental and household smoking and the increased risk of bronchitis, bronchiolitis and other lower respiratory infections in infancy: systematic review and meta-analysis. Respiratory Research. 2011;12(1):5. doi:10.1186/1465-9921-12-5 — Household smoking markedly raises infants' risk of lower respiratory infections such as bronchitis and pneumonia.
Jones LL, Hassanien A, Cook DG, Britton J, Leonardi-Bee J. Parental smoking and the risk of middle ear disease in children: a systematic review and meta-analysis. Archives of Pediatrics & Adolescent Medicine. 2012;166(1):18–27. doi:10.1001/archpediatrics.2011.158 — Parental smoking significantly increases a child's risk of recurrent middle-ear (otitis media) infections.
Leonardi-Bee J, Smyth A, Britton J, Coleman T. Environmental tobacco smoke and fetal health: systematic review and meta-analysis. Archives of Disease in Childhood — Fetal and Neonatal Edition. 2008;93(5):F351–F361. doi:10.1136/adc.2007.133553 — Secondhand-smoke exposure in non-smoking pregnant women increases the risk of low birth weight.
Matt GE, Quintana PJE, Destaillats H, et al. Thirdhand tobacco smoke: emerging evidence and arguments for a multidisciplinary research agenda. Environmental Health Perspectives. 2011;119(9):1218–1226. doi:10.1289/ehp.1103500 — A foundational review defining thirdhand smoke as the toxic residue left on surfaces, dust, and clothing after smoking.
Sleiman M, Gundel LA, Pankow JF, Jacob P 3rd, Singer BC, Destaillats H. Formation of carcinogens indoors by surface-mediated reactions of nicotine with nitrous acid, leading to potential thirdhand smoke hazards. Proceedings of the National Academy of Sciences. 2010;107(15):6576–6581. doi:10.1073/pnas.0912820107 — Showed that surface nicotine reacts with indoor air to form new tobacco-specific nitrosamine carcinogens.
Jacob P 3rd, Benowitz NL, Destaillats H, et al. Thirdhand smoke: new evidence, challenges, and future directions. Chemical Research in Toxicology. 2017;30(1):270–294. doi:10.1021/acs.chemrestox.6b00343 — A comprehensive review of thirdhand-smoke chemistry, persistence, and the special concern for young children.

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