Infants whose parents smoke are much more likely to get bronchitis and other breathing problems than those living in smoke-free homes, a new study concludes.

This so-called "passive smoking" -- breathing in cigarette smoke from the air -- "is dangerous to the health of infants," Dr. Frank A. Pedriera and several other Washington-area physicians write in the journal Pediatrics.

The five-year study followed 1,114 infants during their first year of life. Overall, about 8 percent came down with bronchitis, or inflamed bronchial tubes, and 3 percent with tracheitis, inflammation of the trachea.

Among the key findings was that the amount of smoke, rather than the mere fact that one parent smoked, seemed to play the greatest role in making the infants sick. Those whose mothers smoked were 44 percent more likely than the others to contract bronchitis, while when the father was the smoker there was only a 10 percent greater risk of bronchitis. This disparity occurred, apparently, because the mothers in the study spent more time at home.

For tracheitis, the risk went up 92 percent when the mother smoked, and only 7 percent when the father did.

The research confirms other findings that passive smoke is a hazard, including some that suggest a tie to pneumonia.

"Damage to the airways caused by bronchitis and pneumonia in early childhood," the study says, "may make children more susceptible to subsequent wheezing and/or chronic cough."