It is the tobacco residue that is left on surfaces, such as walls, furniture, carpets and draperies, clothing and cars. The nicotine in the residue reacts with the air to form carcinogens that can be inhaled, absorbed or ingested. Small children are especially vulnerable when breathing the dust and putting their hands in their mouths after touching contaminated surfaces.
Thirdhand Smoke. http://www.no-smoke.org/learnmore.php?id=671 accessed August 26, 2014.