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Air quality regulations in Southern California have led to large reductions in air pollution over the past two decades. A new study reports that the cleaner air has been accompanied by a significant decrease in childhood lung problems.

Researchers followed three groups of children in Southern California: fourth graders from 1993 until their high school graduation in 2001; fourth graders from 1996 to 2004; and a group of kindergartners and first graders from 2003 through 2012. A total of 4,602 children were involved.

The study, in JAMA, used data on ozone, nitrogen dioxide and particulate matter for each year. Parents also provided regular updates about symptoms like coughing and phlegm production in their children.

Among children with asthma, air-pollution reduction was consistently associated with reductions in respiratory symptoms. For example, in children with asthma, reductions in fine particulate matter were associated with a 32-percent reduction in symptoms, while lower levels of ozone were linked to a 21-percent reduction.

The associations were weaker, but still significant, in children without asthma.

“Clearly, the reduction in air pollution levels has translated into improvements in respiratory health,” said the lead author, Kiros Berhane, a professor of preventive medicine at the University of Southern California. “Especially for parents of children with asthma, this is very good news, but we see significant improvement in children without asthma as well.”