Cotinine is one of the chemicals produced when the body breaks down nicotine ingested with inhaled smoke in order to eliminate it.

Researchers at the University of Leicester Medical School and Warwick University found that having a mother who smoked quadrupled the level of cotinine, while having a father who smoked doubled it.

Sleeping with parents -- a known risk factor for sudden infant death syndrome (SIDS) --also was linked to higher levels of cotinine in infants. The authors speculated that inhaling smoke particles from clothing or other nearby smoke-contaminated objects during sleep might play a role.

Sleeping in lower temperature rooms was found to be another risk factor.

"Higher cotinine levels in colder times of year may be a reflection of the other key factors which influence exposure to passive smoking," the authors suggested, "such as poorer ventilation or a greater tendency for parents to smoke indoors in winter." Cotinine was measured in 104 urine samples taken from infants for the study. Seventy one of the babies had at least one parent that smoked; the parents of the other 33 were nonsmokers..

An estimated 40 percent of children under five years old exposed to tobacco smoke at home, according to the authors, who suggested that smoke may be responsible for up to 6,000 deaths of young children per year in the US alone.

"Babies affected by smoke tend to come from poorer homes," they observed.

The problem is a particularly thorny one for healthcare officials to address, since smoking in private residences cannot be regulated as it is in public places.

The solution depends on educating parents and other caregivers to the harmful effects of secondhand smoke.