The dangers of smoking tobacco have been well documented since the 1960s. For years, health experts have also warned about the dangers of second-hand smoke - breathing in other people's cigarette smoke. Now, some researchers are sounding the alarm over what they call "third-hand smoke."

Even after a cigarette is extinguished, toxin-containing tobacco residue lingers on carpets, furniture and clothing

Third-hand smoke is the tobacco contamination that remains after a cigarette is extinguished. Dr. Jonathan Winickoff of Harvard University has been studying it.

"What happens is, the smoker goes outside to smoke their cigarette. Then they come back inside. They're still contaminated with smoke," he says. "They offgas [give off] the smoke toxins from their clothes. They also exhale residual particulates into their home space so that there is still contamination inside the home that is detectable."

Winickoff says this third-hand smoke - the often-smelly residue that's left on carpets, furniture, clothing and so on - contains all the nasty toxic chemical material that's in cigarette smoke.

He says it includes "some of the most toxic compounds known to humankind."

"And we're talking about hydrogen cyanide, which is used in chemical weapons; carbon monoxide, car exhaust; ammonia, those are household cleaners; arsenic, which is used as a pesticide to kill mammals; lead; chromium, which is used to make steel; and, believe it or not, polonium 210, which is a highly radioactive and deadly compound."

Third-hand smoke poses the most risk to children, who are more susceptible than adults

Winickoff says that third-hand smoke can be harmful to anyone, but he says children are most at risk.

"Children have a higher metabolic rate than adults. They breathe faster. And they have less body mass. So for the same exposure, they'll get more of it into them, and they are more susceptible. Their susceptibility really stems from the fact that their tissues are developing. And their brains, especially, are developing."

In the January issue of the journal Pediatrics, Winickoff reports on a survey that found parents are more likely to ban smoking at home if they believe third-hand smoke is going to harm their children. As a result, he says it's important to get the word out.

"If the parent smells the smoke, then it's important for the parents to realize that it's really there. So where a parent smells that exposure, it's important that they remove the child from that space. For smokers, I think the message is really clear also: If you really want to protect non-smokers and children, then the best thing to do is continue on your process of quitting smoking."

Winickoff is also a practicing pediatrician. He says he got interested in the subject of third-hand smoke about 10 years ago when he was treating children for asthma and smelled tobacco smoke on their parents.