You may never have heard of thirdhand smoke, or THS, but chances are you’ve smelled it. THS is, in the words of The New York Times, “the invisible yet toxic brew of gases and particles clinging to smokers’ hair and clothing, not to mention cushions and carpeting, that lingers long after secondhand smoke [SHS] has cleared from a room.”1 Recent research exploring potential dangers of THS has received a flurry of coverage in the international media2,3,4 and the scientific press.5,6,7 And in the United States, court cases are beginning to appear in which plaintiffs are citing these alleged dangers,8,9 despite a lack of human health studies on the long-term health effects of THS exposure.
So how dangerous might THS really be? The answer, still to be pronounced, will depend on many factors.
THS was a topic of interest long before it received its present name. The seed of the idea that cigarette smoke toxicants might linger on room and car surfaces long after the smoke itself was gone was planted in 1953, when it was reported that smoke condensate painted onto mice caused cancer.10
In 1991 the house dust of smokers’ homes was first found to be contaminated with nicotine.11 Later, in 2004, nicotine was quantified in the dust of nonsmokers’ homes and homes in which mothers smoked in the house over the preceding 3 months.12 In homes with the highest SHS exposure, in which the mothers smoked in areas where their children were present, nicotine in dust averaged 64.0 μg/m2 in living rooms and 15.8 μg/m2 in infants’ bedrooms. Surfaces in living rooms and infants’ bedrooms averaged nicotine coatings of 73.05 μg/m2 and 56.26 μg/m2, respectively. The same study showed the dust and surfaces of homes in which smokers had tried to limit their children’s exposure (for instance, by sometimes smoking outdoors) were also contaminated, although to a lesser degree. However, no nicotine was found in the dust or on the surfaces of homes never exposed to tobacco smoke.12
In 2008 similar findings were reported for cars.13 Nicotine was detected in significantly greater quantities in the dust (mean 19.51 μg/g) and on the dashboards (mean 8.61 μg/m2) of 78 vehicles belonging to people who smoked in their vehicles than in the dust (mean 3.37 μg/g) and on the dashboards (mean 0.06 μg/m2) of 20 vehicles of nonsmokers. Eight smokers had imposed a smoking ban in their vehicles for at least 12 months. Their vehicles nevertheless were contaminated with nicotine (mean 11.61 μg/g in dust and 5.09 μg/m2 on the dashboard). The authors point out, however, that the cars may have been contaminated by smoke that entered the car from outside and that smoking bans may not have been complied with 100% of the time.
A 2010 study showed THS also remains after smokers move out of their homes, even after being vacant for two months and being prepared for new residents, sometimes with new carpeting and paint.14 Meanwhile, other lines of research have confirmed some smoke compounds adsorb onto surfaces and then desorb back into the air over time, providing a source of tobacco toxicants that lingers long after people finish smoking