Third-hand smoke is a term used to describe the residual contamination from tobacco smoke that lingers following the extinguishing of a cigarette, cigar, or other combustible tobacco product. It is made up of the pollutants that settle indoors when tobacco is smoked, and the chemicals in third-hand smoke include nicotine, carcinogens, and heavy metals like arsenic, lead, and cyanide. Third-hand smoke residue can remain for weeks, months, or even years, and it can cling to walls, ceilings, carpets, rugs, clothes, bedsheets, and even toys. Children have increased exposure to third-hand smoke due to their tendency to mouth objects and touch affected surfaces, and pets can also be affected by third-hand smoke.

Research has shown that third-hand smoke can cause health problems, including DNA strand breaks, inhibition of proliferation, and cell death. It can also cause the formation of carcinogens, and the nicotine in tobacco smoke reacts with nitrous acid to form hazardous carcinogens. Although no research has been conducted yet on the effects of third-hand smoke on humans, a study in mice showed that third-hand smoke exposure has several behavioral and physical health impacts, including hyperactivity and adverse effects on the liver and lungs.

To protect oneself from third-hand smoke, it is recommended to not allow smoking indoors, help tobacco users quit, and implement smoke-free policies that help enforce a smoke-free environment.