Children who are exposed to secondhand tobacco smoke early in life are at greater risk of being hospitalized for infections than those brought up in a smoke-free environment, researchers from Hong Kong report.

The risk of being hospitalized was greatest among babies 6 months old and younger, but the increased risk persisted up until the children were 8 years old, Dr. M. K. Kwok of the University of Hong Kong and colleagues found. Children who were premature or low birth weight were particularly vulnerable.

The findings suggest that secondhand smoke exposure may not only be harmful to children’s respiratory tracts, but to their immune systems as well, Kwok and colleagues say.

Hong Kong banned smoking in public places in 2007, but babies and children may still be exposed to secondhand smoke at home, the researchers note in their report in the journal Tobacco Control. While the danger smoke exposure poses to children’s developing respiratory systems is well understood, less is known about its effects on overall infection risks.

To investigate, the researchers looked at a group of 7,402 children born in 1997 who were followed up until age 8. At the study’s outset, nearly 42% were exposed to secondhand smoke at home.

Children who had been within 3 meters (or about 9.8 feet) of a person smoking cigarettes at any point during their first 18 months of life were 14% more likely to be hospitalized for any type of infection by 8 years of age, the researchers found.

The greatest difference was seen among infants, with 1 in 3 exposed babies being hospitalized for an infection by the time they were one year old. Exposure during the first 3 months of life had the strongest effect.

Preemies and low birth weight infants seemed to be more vulnerable to the effects of secondhand smoke, for a longer period of time; they were twice as likely to be hospitalized for an infection by age 8 than unexposed children.

“What this study adds is evidence of a window of greater vulnerability to secondhand smoke exposure in early infancy, which extends to all infectious illnesses not just respiratory and related infections, and which may have a larger and more long-lasting impact in developmentally more vulnerable subgroups, such as premature or low birth weight infants,” the researchers conclude.

SOURCE: Tobacco Control, online May 27, 2008.