Environmental tobacco smoke is the combination of smoke released from the burning end of a cigarette and the smoke exhaled by a smoker. You may know it better as secondhand smoke. It contains more than 7,000 chemicals, at least 69 of which cause cancer, and it kills over 41,000 nonsmoking adults in the United States every year.
What Makes Up Environmental Tobacco Smoke
Environmental tobacco smoke has two components. The first is sidestream smoke, which drifts from the lit tip of a cigarette, cigar, or pipe. The second is mainstream smoke, which a smoker breathes out and which then mixes with the surrounding air. Of the two, sidestream smoke is the larger contributor to what bystanders actually inhale indoors, because a cigarette smolders between puffs for far longer than a smoker spends exhaling.
Among the 7,000-plus identified chemicals in this smoke are well-known toxins: arsenic, benzene, formaldehyde, and chromium, to name a few. Many of these substances damage DNA, which is the mechanism behind the cancer risk. Others irritate airways or promote inflammation in blood vessels, contributing to heart and lung disease even in people who have never smoked a cigarette themselves.
Health Risks for Adults
Nonsmoking adults regularly exposed to environmental tobacco smoke face a 25 to 30 percent higher risk of coronary heart disease, a 20 to 30 percent higher risk of stroke, and a 20 to 30 percent higher risk of lung cancer compared to adults who avoid exposure. Those numbers may sound modest in percentage terms, but applied across an entire population they translate to enormous harm. The CDC estimates that secondhand smoke causes more than 41,000 deaths among nonsmoking adults each year in the U.S., along with roughly $5.6 billion in lost productivity.
There is no safe threshold. Even brief exposure triggers measurable changes: blood platelets become stickier, blood vessel linings stiffen, and airway inflammation increases. These acute effects explain why heart attack rates in some communities dropped noticeably within months of indoor smoking bans taking effect.
Health Risks for Children
Children are especially vulnerable because they breathe faster than adults relative to their body size, pulling in more contaminated air per pound of body weight. Exposure to environmental tobacco smoke in young children is linked to sudden infant death syndrome (SIDS), more frequent and severe asthma attacks, respiratory infections like bronchitis and pneumonia, and recurring ear infections. Infants whose parents smoke at home face the highest burden, since they spend nearly all of their time indoors and cannot remove themselves from the exposure.
How Exposure Is Measured
Doctors and researchers measure secondhand smoke exposure through cotinine, a substance your body produces when it breaks down nicotine. Cotinine shows up in blood, urine, and saliva, and it lingers long enough to reflect exposure over the previous few days. In population studies, a blood cotinine level between 0.05 and 10 nanograms per milliliter in a nonsmoker signals secondhand smoke exposure.
The good news is that exposure rates have been falling. The percentage of nonsmoking U.S. adults with detectable cotinine levels dropped from 27.7 percent in 2009 to 20.7 percent in 2018, largely thanks to smokefree laws covering workplaces, restaurants, and bars. Still, roughly one in five nonsmoking adults remains exposed, and disparities persist across racial, ethnic, and income groups.
Thirdhand Smoke: The Residue Left Behind
Even after a cigarette is put out and the visible haze clears, tobacco chemicals linger. This residue, called thirdhand smoke, settles on walls, carpeting, furniture, clothing, and vehicle interiors. It contains nicotine, formaldehyde, and naphthalene (a compound also found in mothballs), and it builds up over time with repeated smoking. Thirdhand smoke can persist for months after smoking in a space has stopped.
You encounter it by touching contaminated surfaces or breathing in gases the residue slowly releases. Young children who crawl on floors and put objects in their mouths are at particular risk. The long-term health effects of thirdhand smoke exposure are still being studied, but the known toxicity of the individual chemicals it contains is reason enough to take it seriously.
Why Ventilation Does Not Solve the Problem
A common assumption is that opening a window, running a fan, or using an air purifier can make indoor smoking safe for bystanders. Every major authority on the subject says otherwise. The U.S. Surgeon General, the World Health Organization, and the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) have all concluded that no ventilation system, air cleaning technology, or designated smoking room can reduce secondhand smoke exposure to a safe level.
Conventional air filters can capture large particles but miss the smaller particles and gases in tobacco smoke. Standard heating and air conditioning systems often make things worse by distributing smoke throughout a building. Even separately ventilated, negative-pressure smoking rooms leak smoke into adjacent spaces. The only effective strategy, according to all three organizations, is a completely smokefree indoor environment.
Reducing Your Exposure
If you live with a smoker, the most protective step is for smoking to happen entirely outside, away from doors and windows. Cars should be smokefree as well, since the small enclosed space concentrates pollutants rapidly. For renters, secondhand smoke can drift through shared walls, ventilation ducts, and even electrical outlets in older buildings. Some states and municipalities now include smokefree provisions in multi-unit housing policies.
When moving into a home or buying a used car where smoking previously occurred, be aware that thirdhand smoke residue may still be present on soft surfaces. Deep cleaning, repainting walls, and replacing carpets and upholstered furniture can help, though complete removal is difficult. The simplest path is choosing spaces where smoking has never been allowed.