When it comes to the dangers caused by the smoking of cigarettes, there seems to be a correlation between proximity and risk. Those at highest risk, of course, are those who smoke, and thus inhale first-hand smoke (FHS), as established in the first US Surgeon General's report on smoking in 1964. The case against second-hand smoke (SHS) – both "mainstream smoke" coming directly from the lit end of the cigarette and "side-stream smoke" coming from the lungs of the smoker – seems to have been made in the more than 30 Surgeon General's reports since.
But there is another potential danger from smoking, and that is in the form of third-hand smoke (THS), which is considered to be the residue of nicotine, tars, and other chemicals left on the hair, skin and clothing of smokers and left clinging to walls, furniture, drapes, bedding, carpets, and airborne dust of indoor areas where smoking is permitted. This includes homes that contain smokers, public locations or hotel rooms that allow smoking, and rental cars in which people have smoked.
There is no debate as to whether third-hand smoke exists – anyone with a nose can smell it. Non-smokers can smell it all too well, even days, weeks, or months after someone last smoked in the hotel room or rental car they've just entered. The residue covers the surfaces of such spaces and doesn't go away after a superficial cleaning. Third-hand smoke thus cannot be eliminated by "airing out the room" opening windows, turning on fans or air conditioners, or confining smoking to only certain areas of a building. The only way third-hand smoke can be completely eliminated is by preventing it to form in the first place, by creating completely smoke-free environments and ensuring that they remain smoke-free.
As to whether third-hand smoke carries the same or similar health dangers as FHS and SHS, that is a matter that is still being studied. And one of the biggest concerns of researchers investigating this new field is whether all of the dangers come from the smoke residue itself. They are concerned that these THS compounds may combine with other indoor pollutants such as ozone and nitrous acid to create new compounds. One of these compounds known as NNA has been shown to cause damage to DNA, and to potentially cause cancer. A 2010 study also showed that third-hand smoke caused the formation of known carcinogens.
One of the questions that researchers in this field are dealing with is who the potential victims of third-hand smoke might be. The 2006 Surgeon General's report established that medically there is no safe level of exposure to tobacco smoke, so if the chemicals in smoke or those formed in combination with THS are found to be dangerous, there may be no safe level of exposure to them either. Children of smokers and toddlers have been pinpointed as people most at risk from THS, although people who work in areas where smoking is permitted, such as hotel and restaurant staff, janitorial and cleaning staff, and even the workers who clean returned rental cars may be at high risk from third-hand smoke. The safety of buildings in which smoking has been permitted is also a concern for landlords and the owners of real estate properties or offices. As we mentioned earlier, the only sure way to eliminate third-hand smoke is to prevent it in the first place, so many cities are considering extended anti-smoking bans to do just that, and fight the problem by creating more smoke-free areas.