Thirdhand smoke is residual nicotine and other chemicals left on indoor surfaces by tobacco smoke. People are exposed to these chemicals by touching contaminated surfaces or breathing in the off-gassing from these surfaces. This residue is thought to react with common indoor pollutants to create a toxic mix including cancer causing compounds, posing a potential health hazard to nonsmokers — especially children.

Thirdhand smoke clings to clothes, furniture, drapes, walls, bedding, carpets, dust, vehicles and other surfaces long after smoking has stopped. The residue from thirdhand smoke builds up on surfaces over time. To remove the residue, hard surfaces, fabrics and upholstery need to be regularly cleaned or laundered. Thirdhand smoke can't be eliminated by airing out rooms, opening windows, using fans or air conditioners, or confining smoking to only certain areas of a home.

Children and nonsmoking adults might be at risk of tobacco-related health problems when they inhale, swallow or touch substances containing thirdhand smoke. Infants and young children might have increased exposure to thirdhand smoke due to their tendency to mouth objects and touch affected surfaces.

Thirdhand smoke is a relatively new concept, and researchers are still studying its possible dangers. In the meantime, the only way to protect nonsmokers from thirdhand smoke is to create a smoke-free environment.