(Reuters Health) - Childhood asthma rates appear to be dropping in many communities across southern California, and a new study suggests this may be due at least in part to improved air quality.

Air pollution has long been linked to a variety of respiratory problems and an increased risk of severe asthma attacks for people who already have the breathing disorder. But research to date hasn’t offered as clear a picture of how much reducing levels of toxic chemicals in the air might help stop asthma from developing at all.

For the current study, researchers followed 4,140 kids without asthma for eight years, starting in 1993 when children were in fourth grade and typically around 9 to 10 years old. The study spanned two decades and included three cohorts of children living in one of nine southern California communities with historically poor air quality.

Regional air pollution levels generally declined during the study period, with decreases in nitrogen dioxide, ozone gasses and fine particulate matter, researchers report in JAMA.

Asthma rates also dropped. While 2.7 new asthma cases were diagnosed annually per 100 kids in the first cohort (1993-2001), this dropped to 1.8 cases a year per 100 kids by the last cohort (2006- 2014).

“Exposure to air pollution is a known risk factor for the exacerbation of pre-existing asthma,” said lead study author Erika Garcia, an environmental health researcher at the Keck School of Medicine at the University of Southern California in Los Angeles.

“What distinguishes our study is that we are not looking at exacerbation of pre-existing asthma, but at the contribution of air pollution exposure to the development of new cases of asthma in children,” Garcia said by email. “We found that reductions in air pollution, specifically nitrogen dioxide, were related to reduction in rates of new-onset asthma in children.”

The pollutants in the study have long been associated with traffic fumes and smog blanketing Los Angeles and many surrounding communities in southern California.

Researchers examined concentrations of ozone, an unstable form of oxygen produced when various types of traffic and industrial pollution react with sunlight; nitrogen dioxide, a byproduct of fossil fuel combustion that can contribute to smog; so-called PM 2.5, a mixture of solid particles and liquid droplets smaller than 2.5 micrometers in diameter that can include dust, dirt, soot and smoke; and PM 10, a blend of particles and droplets up to 10 micrometers in diameter.

Reductions in levels of the pollutants varied across communities over two decades. But at least half experienced reductions of 4.3 parts per billion (ppb) in annual average concentrations of nitrogen dioxide, as well as reductions of 8.9 ppb ozone, 8.1 micrograms of PM2.5 per cubic meter of air (ug/m3), and 4 ug/m3 of PM10.

Neighborhoods where pollution levels were highest in 1993 experienced the greatest reductions, the study team notes.

They study wasn’t designed to prove whether or how declines in air pollution exposure might directly reduce childhood asthma rates, and it also didn’t look at changes in the frequency or severity of symptoms for kids who did develop asthma.

“I don’t think we have enough data to say that early life exposure increases the risk of severe asthma per se,” said Dr. Steve Georas, an environmental medicine researcher at the University of Rochester Medical Center in New York.

“But it is clear that acute exposure to poor air quality increases the risk of asthma exacerbations, which is a marker of asthma severity,” Georas, who wasn’t involved in the study, said by email. “It seems logical to speculate that improving the air quality that children with asthma breathe would improve their disease control and possibly lead to a sustained remission of the disease, but we can’t conclude that from this present study.”

SOURCE: bit.ly/2EmViY5 and bit.ly/2JwhNhC JAMA, online May 21, 2019.