Indoor air cleaners may help asthmatic children who live with a smoker to breathe a little easier, a new study finds.

The devices are no cure-all and parents of children with asthma should always strive for a smoke-free home, researchers stress. But in cases where that's not yet a reality, they say, air cleaners may lessen the ill effects of secondhand smoke.

The study looked at the effects of HEPA air cleaners among 115 inner-city Baltimore children who had asthma and lived with a smoker. The researchers gave one-third of the families an air cleaner for their living room and one for the child's bedroom. Another third received the devices plus education sessions with a nurse on the dangers of secondhand smoke. The final third served as the "control" group, with families receiving air cleaners after the study was over.

Over six months, the study found, children in the air-cleaner groups showed improvements in wheezing, coughing and other symptoms, based on surveys of their parents. The benefit would translate into an extra 33 symptom-free days per year, the researchers estimate.

"They'd actually have an extra month free of symptoms. That's significant," said lead researcher Arlene M. Butz, an asthma specialist at Johns Hopkins Children's Center in Baltimore. However, she told Reuters Health, an air purifier is no substitute for a smoke-free home. Her study found that while HEPA cleaners did lower levels of airborne particles in families' homes, it did not give them the kind of air quality found in a smoke-free home.

"You want to eliminate the source of the smoke," Butz said. "When a child has asthma, there shouldn't be a smoker in the home."

Levels of nicotine in the household's air and nicotine breakdown products in the children's urine did not change with the air filters. Ideally, parents should quit smoking. But until that happens, they should limit all smoking to outside the home, according to Butz.

The issue of indoor air quality is particularly important for inner-city children, Butz said. Not only do they have higher rates of asthma than other U.S. kids, but they also have heavy exposure to secondhand smoke.

Studies have found that between 40 percent and two-thirds of inner-city children with asthma live with a smoker, Butz noted. Another issue in inner-cities is that many families live in apartment buildings, and even if parents ban smoking from their home, their next-door neighbors may be lighting up.

"We know that in multi-unit buildings, smoke does leak into other rooms," Butz said. So she recommended having a HEPA cleaner even in non-smoking apartments. If families can afford it,

Butz said two devices -- one for the living area and one for the child's bedroom -- would be best.

HEPA devices vary in price; the ones used in this study cost between $100 and $150, according to Butz. Then there's the cost of replacement filters and the energy used to run the machine -- which can consume about as much power as a standard light bulb.

The study was funded by federal grants and Johns Hopkins University. None of the researchers reports any financial conflicts of interest.