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Asthma cases dropped when air pollution declined

The study found that cases of childhood asthma dropped as outdoor air became cleaner. bowdenimages / iStock / Getty Images Plus

Decreases in air pollution levels are known to reduce the number of asthma attacks experienced by children who have the disease. But whether pollution influences how many children are newly diagnosed with asthma has been less clear.

Researchers from the University of Southern California led by Dr. Erika Garcia used data from the Southern California Children’s Health Study to better understand this relationship. The study has collected health information from children in 16 diverse communities in Southern California for over 25 years.

The researchers followed large groups of children living in nine of these communities over three time periods from fourth grade through the end of high school: from 1993-2001, from 1996 to 2004, and from 2006 to 2014.

Air pollution declined steadily in the U.S. during the study period. Air pollutant monitoring stations tracked these changes over time in each of the participating communities. The stations have continuously measured air pollution since the start of the study.

The researchers used air pollutant measurements from the first year of each time period as a benchmark for the overall air quality each group experienced. They then compared new diagnoses of asthma between the three groups. The study was funded in part by NIH’s National Institute of Environmental Health Sciences (NIEHS) and National Heart, Lung, and Blood Institute (NHLBI). Results were published on May 21, 2019, in JAMA.

Air pollution declined overall in the communities during the study. Those communities that started out with more pollution experienced larger decreases.

Out of more than 4,000 children included in the study, about 500 developed asthma. Reductions in two of the pollutants measured—nitrogen dioxide and a type of fine particle called PM_2.5—were associated with reductions in new asthma diagnoses over the course of the study.

These reductions in nitrogen dioxide and PM_2.5 over two decades were associated with an approximately 20% reduced risk of asthma development. This was true regardless of children’s sex, ethnicity, race, exposure to tobacco smoke in the home, and other factors that can influence the development of asthma.

“This is encouraging news as it shows the number of new cases of asthma in children can be reduced through improvements in air quality,” says Dr. Kiros Berhane, one of the study’s authors.

This study doesn’t prove that these specific compounds are responsible for cases of asthma. They may be markers for the general mixture of traffic-related air pollution. But the findings add to growing evidence showing the health benefits of cleaner air.

—Sharon Reynolds