Most people know secondhand smoke is harmful — but thirdhand smoke? That’s the toxic residue tobacco leaves behind on surfaces like carpets, furniture, wall boards, and clothing, where it can linger for years.
Researchers from UC San Francisco leading the California Thirdhand Smoke Research Consortium, which includes San Diego State University and Lawrence Berkeley Lab, have been studying its health risks. Their findings helped drive California’s Assembly Bill 455, the world’s first law requiring disclosure of thirdhand smoke in real estate deals. It takes effect Jan. 1, 2026.
We spoke with Neal Benowitz, MD, UC San Francisco professor emeritus of medicine and a leading voice behind the new law. He also co-authored a recent JAMA paper on why this groundbreaking legislation matters for public health.
Can you tell us where thirdhand smoke is found and how long it remains in the environment?
Thirdhand smoke is found in places where people have smoked, for example in homes, hotel rooms, and cars. Chemicals from secondhand smoke penetrate fabrics, rugs, and even wall boards. They accumulate and can be released back into the air, ingested with household dust, and even absorbed through the skin.
Why is it a concern and who is most at risk?
The chemicals in thirdhand smoke are similar to those in secondhand smoke, which are known to cause cancer and heart disease. Twenty-six chemicals identified in thirdhand smoke are classified by the State of California as causes of cancer, birth defects, and reproductive harm. Most at risk are children, who crawl on the floor, can put objects contaminated with thirdhand smoke into their mouths, and can absorb it through the skin.
Also at risk are people with allergies and asthma, whose symptoms can be worsened by thirdhand smoke emissions, people who are immunocompromised, and the elderly. Of particular concern are people who live in multi-unit low-cost housing, where thirdhand smoke contamination is ubiquitous.
How do the risks of third hand smoke compare to first and secondhand smoke?
It is difficult to distinguish risks of thirdhand smoke from secondhand smoke since most people exposed to secondhand smoke are also exposed to thirdhand smoke.
In mice, researchers have found that thirdhand smoke exposure causes DNA damage, can cause or promote cancer, cause immune dysfunction and behavior disturbances. Studies of people exposed to thirdhand smoke found changes in blood proteins that have been associated with inflammation and heart disease.
How does this new California law address thirdhand smoke?
AB455 defines thirdhand smoke as an environmental hazard, similar to lead, radon, or asbestos. The bill mandates disclosure if the seller has personal knowledge of smoking or vaping in the home. The bill also mandates consumer education and adds thirdhand smoke to the homeowner’s guide to environmental hazards.
What needs to happen next to address the problem?
Hopefully, future legislation will address thirdhand smoke concerns in multi-unit housing, where in many cases, a smoker moves out of an apartment and a nonsmoker moves in unaware of the risk.
We also hope that other states in the U.S., as well as other countries, will adopt laws similar to AB455 that advance smokefree indoor policies and, ultimately, the tobacco endgame.
How can we mitigate the risk of thirdhand smoke?
Depending on the extent of contamination, one can begin with washing surfaces and deep cleaning of carpets and upholstered furniture.
Where contamination is heavier, remove carpets and furniture. And where there’s more extreme contamination, you may need to remodel — remove walls, insulation, and heating and air conditioning systems.
Can I find out whether I’m at risk, if I don’t know the smoking history of my home?
Over time, the risk gradually decreases. For an older home where no one has smoked for a while, I imagine the levels would be quite low, but probably not zero on the wallboard where it can stay for a long time.
If you have concerns, it is possible to have your home checked with indoor surface sampling. The Thirdhand Smoke Resource Center at San Diego State University offers FAQs and free sampling kits to qualifying residents of California.