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The residue from cigarette smoke could linger for months, potentially reacting with particles in the air to produce cancer-causing compounds. Getty Images


- Residual tobacco smoke reacts with pollution in the air to produce potent carcinogens.

- This tobacco residue is known as "thirdhand smoke."

- To avoid the many risks of cigarette smoke, people -- especially children -- might want to avoid places where people have smoked.

When tobacco smoke seeps into carpets, clothes and furniture, it leaves behind more than just a telltale smell.

According to a new study, the residue also reacts with particles in the air to produce cancer-causing compounds that linger on surfaces for months.

The study didn't directly consider whether people are at risk from smoke-tainted objects. However, it's conceivable that carcinogenic byproducts could seep through our skin on contact. They could also turn into dust that we breathe in. Babies and small children would be at greater risk, since they are most likely to touch floors and other surfaces.

"Many parents that smoke do it in ways that they think are protecting their children by, for example, smoking at certain times when their children are not present," said Hugo Destaillats, a physical chemist at the Lawrence Berkeley National Laboratory in Berkeley, Calif. "But the fact that these contaminants remain on some places in the absence of active smoking might be another route of exposure that people are not aware of."

It's usually easy to tell whether someone has been smoking in a room or car. Nicotine from tobacco smoke seeps into rugs and other surfaces, and stays there for long periods of time.

In addition to the firsthand smoke that smokers inhale directly and the secondhand smoke that bystanders breathe in, scientists have recently become concerned about this type of "thirdhand smoke."

To see what happens to nicotine after it settles in, Destaillats and colleagues focused on the compound's interaction with nitrous acid, a type of air pollution produced by gas-burning appliances and vehicle engines. Nitrous acid is abundant inside most homes and cars.

In their lab, the scientists used a type of cellulose-containing paper as a model for carpets, drapes, wallboard and other common surfaces that also usually contain cellulose. They allowed the paper to absorb nicotine from cigarette smoke before putting it in a special chamber that contained nitrous acid. The scientists also took samples from the inside of an old pickup truck, in which the driver frequently smoked.

In both the lab and in the truck, the reaction between nicotine and nitrous acid produced substantial amounts of three types of toxic compounds, the scientists report today in the Proceedings of the National Academies of Science. All three compounds belonged to a group called nitrosamines, which are known to prompt tumor growth.

The chemical reactions happened quickly. After just three hours in the lab, the number of nitrosamines on the paper's surface jumped by a factor of more than 10.

"Tobacco smoking is bad for people who smoke and people who don't smoke, and here's another way it might be bad for people who don't smoke," said Jonathan Samet, a pulmonary physician and epidemiologist at the University of Southern California in Los Angeles.

"There are so many reasons why people should not smoke," he added. "This suggests that there may be residual contamination to surfaces relevant to the health of nonsmokers. It's a potential risk beyond inhalation."

It's too soon to know whether sitting on a couch that smells of tobacco smoke could cause cancer, the researchers cautioned. Future studies will have to look at how many cigarettes or cigars it would take to have an effect, how many nitrosamines could actually get into people through physical contact and how that exposure would affect their health.

If nitrosamine-production pans out as another route of exposure to the dangers of tobacco smoke, small children would be most at risk. They're the ones who spend the most time playing on the floor, touching objects, and putting everything in their mouths.

"We want to be very cautious about not making claims about toxicity and health," Destaillats warned. "We did not address that. What we're saying is that this is a reaction that takes place and has been overlooked. Nobody has looked at this before."