An analysis of air quality and childhood asthma in Los Angeles found that kids’ health improved when smog declined, reports NPR.

The research focused on Southern California, long notorious for its smog. Over the past two decades, air quality has been markedly getting better, as stricter rules limit pollution from vehicles.

The change led Erika Garcia, a postdoctoral fellow at the University of Southern California, to ask a simple question. “We know there have been reductions in air pollution,” she says. “Let’s see if concurrently there are improvements in health outcomes during that same time period.”

Garcia wondered whether new cases of asthma would be lower in communities where the air improved the most. The answer was yes, especially for nitrogen dioxide, which is an indicator of tailpipe emissions, and fine particles, which are a major type of pollution from diesel engines.