The study, one of the few to focus exclusively on the effects of passive smoking on children age four and older, was conducted by researchers at the Centers for Disease Control and Prevention (CDC) and published in the January issue of Archives of Pediatrics and Adolescent Medicine.

Lead author David M. Mannino, MD, a medical epidemiologist with the National Center for Environmental Health at the CDC, says his team found children of all ages could have health problems from exposure to environmental tobacco smoke.

The researchers looked at biomarkers of exposure to secondhand smoke. That means they actually looked for evidence of exposure in blood samples from kids ? in contrast to previous studies that depended on parents to report if people smoked in the home. Mannino says the blood-test method, which detects exposure from the past three to four days, allowed the researchers to better determine exposure levels in all children.

Mannino and his team evaluated levels of a blood chemical called cotinine, which increases with exposure to tobacco smoke, in 5,400 children ages four to 16. Of the study participants, 85% had measurable levels of cotinine in their blood and were more likely to develop asthma and wheezing and miss days from school.

"Cotinine is a very sensitive marker for nicotine exposure," the researcher says. "In general, people who are exposed to passive smoke have cotinine levels between 10 and 20 times higher than levels in people who aren''t exposed to passive smoke. Active smokers have levels 100 times higher than those exposed to passive smoke."

Younger children ? those aged four to six ? are still the most vulnerable to developing health problems from exposure to secondhand smoke. Mannino believes part of the reason for this is that younger children are at home and typically around adults more than older children.

In their paper, the CDC researchers also provide more information on the long-debated question of whether postnatal exposure to smoking might have the same effects on young children as prenatal smoking by the mother. "Our study also found that postnatal exposures seem to be just as important as prenatal exposure," notes Mannino.

His team found that, among children aged four to six with high cotinine levels, the incidence and severity of asthma and wheezing was as prevalent in children whose mothers did not smoke during their pregnancies but smoked afterwards, as it was in those whose mothers smoked during their pregnancies.

An expert says the study provides more strong evidence for doctors who want to educate parents and child-care providers about the effects of passive smoking on children.

"This [study] draws attention to the fact that environmental tobacco smoke exposure is something that clinicians and others involved in child health programs should discuss with parents of young children," says Alan Henderson, DrPH, past president of the California Division of the American Cancer Society and a member of the tobacco control team at California State University in Long Beach, Calif. "This can only help in reducing exposure and encouraging those who smoke in the home not to do it or to quit."