Asthma rates among U.S. children have quieted down after a decades-long increase, a government study found, and researchers are trying to pinpoint reasons that would explain the trend.

A possible plateau in childhood obesity rates and declines in air pollution are among factors that may have helped lower cases in kids, the 2001-13 study suggests. Overall, average asthma rates among those ages 17 and younger increased slightly, then leveled off and declined by the study's end, when 8.3 percent of children were affected.

Still, rates increased in those ages 10 to 17, children from poor families, and those living in the South. Rates increased but then plateaued among blacks.

Asthma's causes are uncertain but authorities believe several factors play a role or trigger attacks, including air pollution, obesity, tobacco smoke, premature birth, and respiratory infections in infancy.

The study was published online Monday in the journal Pediatrics.

- AP