Third-hand smoke is contamination by tobacco smoke that lingers following the extinguishing of a cigarette.

Harmful Affects of Third Hand Smoke
Third-hand smoke is environmental tobacco smoke that has oxidized with environmental nitrous acid to create carcinogens not seen in cigarette ingredients or tobacco smoke. The carcinogens found in third hand smoke are known as tobacco-specific nitrosamines. A 2010 study published in the Proceedings of the National Academy of Sciences journal found that the residue of the nicotine coating smokers as well as, interior car or room surfaces can react with nitrous acid present in the air to create tobacco-specific nitrosamines, carcinogens found in tobacco products. 

Third-hand smoke is thought to potentially cause the greatest harm to infants and young children, because younger children are more likely to put their hands in their mouths, be cuddled up to a smoker with toxins on their skin and clothes. Infants also crawl on the floor and eat from their hands without washing them first, ingesting the toxins into their systems. The 2006 surgeon general's report says there is no risk-free level of tobacco exposure…. 

Third-hand smoke is one of the issues promoting indoor smoking bans!!!