Parents have long been warned against exposing their children to secondhand smoke. So they smoke outside, or smoke when their children are not around.

This isn't enough, however. Wherever people smoke, the toxins in cigarette smoke persist, even if no one is actively smoking. A recent study examined the impact of thirdhand smoke on children and measured their exposure to cigarette-related toxins from thirdhand smoke.

Thirdhand smoke is the residue from cigarettes, cigars and other tobacco products left behind after smoking ceases. It contains contaminants and carcinogens that threaten the health of those who come in contact with them. It lingers on hair, clothing, environmental surfaces (including walls and ceilings), in cars where people have smoked and in dust in all those areas. You can smell it.

Children are especially vulnerable to this type of exposure both because of their small size and ongoing development and because they crawl and cruise through their environments, picking up residues of thirdhand smoke on their hands and then putting them in their mouths. Or they may suck on potentially contaminated objects such as furniture or toys. Children are exposed to environmental lead and pesticides in the same way.

Simply refraining from smoking when children are not present does not prevent their exposure to toxins.

Just how significant a source of tobacco contaminant exposure thirdhand smoke is for young child was made clear in a recent study. Researchers at Cincinnati Children's Hospital looked at 25 children, average age 5 years, who came to the emergency room with symptoms that were potentially attributable to secondhand smoke exposure — respiratory ailments, asthma, bronchiolitis — as part of a pilot project on thirdhand smoke.

Using a specialized wipe to collect the sample, they measured the levels of chemical components from thirdhand smoke on the children's hands. They also compared the hand nicotine residue to the amount of cotinine, a metabolic product of smoke, in the children's saliva. Salivary cotinine is a reliable marker for second hand smoke exposure. The higher the exposure, the higher the cotinine level.

All but one of the children had detectable hand nicotine levels and elevated salivary cotinine. The greater their hand nicotine levels, the higher the levels of cotinine in their saliva, showing that the hand nicotine levels were a reliable indicator of smoke exposure in children.

The message for parents is that children can pick up the chemicals in smoke on their hands, even in the absence of active smokers, and this can contribute to their overall exposure to the contaminants and carcinogens in tobacco. The researchers hope to raise parents' and physicans' awareness of thirdhand smoke as an important contributor to smoke-related health issues such as respiratory illnesses, asthma and ear infections that affect children. There may also be additional pollutants in the thirdhand smoke residue that have gone unrecognized and that could cause additional toxic health effects.

These other chemicals may remain in the environment for a longer time than those carried in smoke and pose a longer term health risk. Not only do younger children show higher exposure to tobacco smoke than older children and adults because of the ways that they interact with their environments, this exposure occurs at a particularly vulnerable age while children are undergoing periods of rapid development and growth.

Parents and adults who smoke need to be aware that simply refraining from smoking when children are not present does not prevent their exposure to toxins. Whenever children enter an environment where smoking has occurred, they are at high risk for secondhand and thirdhand smoke exposure, and its associated acute and chronic health consequences.

The study is published in Tobacco Control.