Thirdhand "smoke" isn't really smoke, but the residue that tobacco smoke leaves in rooms—on furniture, carpets, walls, and clothing, and in dust. It’s the lingering unpleasant odor in the homes of smokers or in hotel rooms where people smoked hours or even days earlier. It penetrates and clings to surfaces and is hard to get rid of even with a fan. The odor means you’re inhaling tobacco-related compounds.
Even when the odor is gone, many of the hundreds of carcinogens and other toxic chemicals in tobacco smoke stay on surfaces and, when disturbed, can become airborne again.
Research on thirdhand smoke—some done by scientists at the Lawrence Berkeley National Laboratory—has found that tobacco substances absorbed by surfaces can react with ozone and other household air pollutants to form additional dangerous compounds (such as nitrosamines) and be discharged into the air as vapor or ultrafine particles or else hitch a ride on dust particles.
Smoke that sticks around
In a study from the Lawrence Berkeley National Laboratory published in 2014, researchers looked at levels of particulate matter and dozens of easily vaporized organic compounds (such as acetaldehyde, benzene, and toluene) for 18 hours after smoking (by machine) had taken place in a room-sized chamber. Testing was also done in a smoker’s home.
Based on the results, the researchers estimated that health harms would be greatest during the first 10 hours after the last cigarette was smoked and then would start to level off (though not disappear even after 18 hours). The most significant damage would come from ultrafine particles, which—like vapor—can be inhaled deep into the lungs. No one knows for certain if thirdhand smoke causes lung cancer in humans, but research in young lab mice showed that those exposed to thirdhand smoke later developed lung tumors.
One complicating factor in trying to quantify the risks is that it’s hard to draw the line between secondhand and thirdhand smoke—there’s a gradual transition as smoke dissipates. Much of the harm—and some of the 50,000 annual deaths in the U.S.—attributed to secondhand smoke are actually caused by thirdhand smoke, according to the Lawrence Berkeley scientists.
As knowledge of the dangers of thirdhand smoke grows, it should certainly encourage even more places to go smoke-free and more smokers to quit.
Originally published March 2015. Updated August 2019.