If you needed another reason to quit smoking, here’s one for you. “Third-hand smoke,” the residue from nicotine that clings to surfaces, can also cause cancer. Previously an underappreciated health risk, a recent study by Berkeley Labs shows that residue from cigarettes can cling to many indoor surfaces after a cigarette has been extinguished, reacting with nitrous acids in the air to form potent carcinogens.

Cigarette smoking causes the release of nicotine vapor into the air that is absorbed into surfaces such as walls, floors, carpets, curtains, clothing, and even human skin. The residual nicotine reacts with nitrous acid in the ambient air that comes from various air pollutants, including gas appliances and vehicle engines. This reaction causes the formation of some dangerous carcinogens called tobacco-specific nitrosamines, or TSNAs. Some of these carcinogens can already be found in tobacco smoke, while other new and powerful carcinogens are formed in the reaction of nicotine and air pollutants.

Nicotine can persist on surfaces for days, weeks and even months. This “third-hand smoke” poses a risk to humans, who can be exposed to the carcinogens through dust inhalation, or contact with the surfaces or human skin. Those most at risk include babies and toddlers, who can absorb the TSNAs through their skin or inhale it from the clothing and skin of their caregivers. Even smoking outside may not help, since the nicotine stays on human skin and is then carried indoors. According to the study, the amount of TSNAs found in one square meter of third-hand smoke residue on indoor surfaces was found to greatly exceed the amount of TSNAs in one cigarette's worth of smoke. It may be merely a vague indicator of how much TSNA our bodies absorb from indoor surfaces, but it is enough to make us think twice about how the effects of smoking can be compounded.

The study shows that entering a room where someone smoked yesterday can harm you today. Consistent build up of nicotine on surfaces can also raise the levels of toxicity.

What can you do to protect yourself? First of all, the study gives all of us yet another reason to quit smoking. The hazards of first and second-hand smoke have already been well documented, but the knowledge that toxins caused by third-hand smoke can persist for much longer should make us pause before lighting up. Avoiding smoking is particularly important if you are nursing or caring for a baby or small child.

In rooms where furnishings and surfaces have been extensively exposed to cigarette smoke, furniture and carpets can be replaced to reduce exposures. Consider requesting that visitors to your home refrain from smoking in your house or in your car.

Further studies on the formation and nature of TSNAs are ongoing, as well as their effects on our bodies.