Secondhand and Thirdhand Smoke

What it is

Secondhand smoke (SHS) is the smoke exhaled by a person smoking plus the smoke coming off the burning end of a cigarette, cigar, pipe, or hookah. It contains more than 7,000 chemicals, of which at least 70 are known carcinogens, including benzene, formaldehyde, 1,3-butadiene, polycyclic aromatic hydrocarbons, and tobacco-specific nitrosamines.

Thirdhand smoke is the residue that secondhand smoke leaves behind. Nicotine and other chemicals settle onto walls, carpets, upholstery, drapes, dust, toys, and skin, then slowly off-gas back into the air for weeks, months, or years after smoking stops. Nicotine on surfaces reacts with indoor nitrous acid to form tobacco-specific nitrosamines (TSNAs), some of the most potent known carcinogens.

Where it comes from

Any combusted tobacco product produces secondhand smoke. Smoke drifts between apartments through shared walls, ventilation, hallways, plumbing chases, and balconies. Studies have measured tobacco markers in non-smoking units of multi-unit buildings even when residents follow strict no-smoking rules. Cannabis smoke and hookah smoke create similar exposure patterns.

Thirdhand smoke builds up in rooms, cars, and on the clothing and skin of smokers. It can persist long after a smoker moves out of a home, and conventional cleaning often does not remove it from porous materials.

Health effects

According to CDC and the Surgeon General, there is no safe level of exposure to secondhand smoke. SHS causes lung cancer, coronary heart disease, and stroke in nonsmoking adults. In infants and children, SHS exposure raises the risk of sudden infant death syndrome (SIDS), low birth weight, more frequent and severe asthma attacks, respiratory infections, and middle ear infections. CDC attributes roughly 41,000 deaths from heart disease and lung cancer in U.S. nonsmoking adults each year to SHS, plus about 400 SIDS deaths in infants.

Thirdhand smoke exposure is harder to quantify, but children are especially vulnerable. Studies have found detectable nicotine on the hands of more than 90 percent of children living with smokers and on a substantial share of children in nonsmoking homes that had previously been smoked in. Hand-to-mouth contact, crawling on contaminated carpets, and dust ingestion all contribute. Researchers have measured carcinogen levels in thirdhand-smoke dust above EPA risk thresholds for young children.

How it's measured

There is no NAAQS for secondhand or thirdhand smoke. Researchers measure exposure using airborne nicotine, particulate matter (PM2.5), or biomarkers like cotinine in saliva, blood, or urine. A typical home with active indoor smoking can sustain PM2.5 levels of 50 to 200 micrograms per cubic meter or more, several times the EPA 24-hour standard of 35. Thirdhand smoke is assessed through surface wipe samples for nicotine, dust analysis, and air sampling for TSNAs.

What you can do

Make your home and car 100 percent smoke-free. Smoking outdoors helps but does not eliminate exposure, because smoke clings to clothing, hair, and skin. Opening a window or running an air filter reduces but does not remove SHS.

If you are moving into a previously smoked-in home or car, expect thirdhand smoke residue. Wash hard surfaces with detergent, launder or replace soft furnishings, shampoo or replace carpets, repaint walls after washing with TSP, and replace HVAC filters. If you live in a building where smoke drifts in, seal gaps around outlets, baseboards, and shared walls, and run a HEPA air purifier.

Sources

• Health Problems Caused by Secondhand Smoke | CDC
• Health Risks of Secondhand Smoke | American Cancer Society
• What are the effects of secondhand and thirdhand tobacco smoke? | NIDA
• Toxic Thirdhand Smoke Residue Reaches Many Children | Cincinnati Children's
• Thirdhand Smoke Resource Center
• Collecting Hand Wipe Samples to Assess Thirdhand Smoke Exposure | PMC