"Third-hand" smoke — which lingers in cars, on furniture and on smokers themselves after a cigarette is extinguished — leaves toxic chemicals that crawling children can ingest, say pediatricians.

In the January issue of the journal Pediatrics, Dr. Jonathan Winickoff of Harvard Medical School and his colleagues said parents sometimes try to shield their children from second-hand smoke by rolling down the car window or smoking in the kitchen with the fan on, but the risks of third-hand smoke still exist.

The researchers surveyed 1,500 U.S. households to learn about parents' attitudes toward third-hand smoke. They found 65 per cent of non-smokers and 43 per cent of smokers surveyed agreed that third-hand smoke can harm the health of children.

"When you smoke — any place — toxic particulate matter from tobacco smoke gets into your hair and clothing," said Winickoff, assistant director of the MassGeneral Hospital for Children Center for Child and Adolescent Health Policy.

"When you come into contact with your baby, even if you're not smoking at the time, she comes in contact with those toxins. And if you breastfeed, the toxins will transfer to your baby in your breast milk."

Taking steps to protect children

Children pick up the residue from dust when crawling, and then may ingest it by sucking on their hands, said study co-author Joan Friebely of MassGeneral Hospital for Children. Off-gassing from walls, furniture and the skin of smokers are other sources of the contaminants.

Infants are also more susceptible because they are smaller and have faster breathing rates, which means they are exposed to higher concentrations than older children, she added.

Third-hand smoke does not pose more of a health risk than second-hand smoke, Friebely said, but people may not know that they are exposed to it.

"What we know from the 2006 [U.S.] surgeon general's report that discusses the health consequences of … involuntary exposure to second-hand smoke is that there is no safe level of second-hand smoke exposure," Friebely told CBC Newsworld on Tuesday.

"We haven't been able to separate them because where there's second-hand smoke, there will be third-hand smoke."

But studies suggest that young children who live in homes with non-smokers show much lower levels of nicotine in their systems than those who live with adults who smoke — whether the puffs were taken inside or outside.

The U.S. National Toxicology Program said particulate matter from tobacco smoke includes 250 poisonous chemicals, including at least 10 cancer-causing agents and:

• Hydrogen cyanide (used in chemical weapons).
• Carbon monoxide.
• Butane.
• Ammonia.
• Toluene (found in paint thinners).
• Arsenic.
• Lead.
• Chromium (used to make steel).
• Cadmium (used to make batteries).
• Polonium-210 (a highly radioactive carcinogen).

Friebely said parents who smoke and wish to protect their children should:

• Smoke away from children at all times.
• Use a smoking jacket outside.
• Wash their hands when they come inside after smoking.
• Use all available aids to quit, such as nicotine gum.

The researchers found higher support for home smoking bans among people who believed that third-hand smoke is dangerous.

The term third-hand smoking alone could help behaviour, said Paul Thomey of the Canadian Lung Association in St. John's.

"I think this will help people start to think about smoking in their homes a lot more, and putting restrictions on it because that's where the real big impact is now," said Thomey.