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.
Neal Benowitz, MD
UC San Francisco professor emeritus of medicine
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.