Air pollution levels have been declining for years, and researchers can now show that cleaner air is linked to fewer kids developing asthma.

The new study looked at nine California communities. The researchers found that reductions in certain pollutants were tied to about a 20% reduction in the odds of children developing asthma, a chronic airway disease.

"The findings are really encouraging. A benefit of lower air pollution levels is that the number of new cases of asthma could be reduced," said study author Erika Garcia. She's a postdoctoral scholar at the Keck School of Medicine at the University of Southern California, in Los Angeles.

But Garcia said policy makers need to continue to push for better air quality.

"We need to reduce air pollution in Los Angeles and other high pollution areas. We've made great progress, and it's important that we don't stop," she added.

In asthma, the airways are inflamed, making it difficult to breathe. The study findings were published May 21 in the Journal of the American Medical Association.

The research was published on the heels of an announcement that the Trump administration's Environmental Protection Agency (EPA) has proposed a change to the way it estimates the serious effects of air pollution. The change assumes there is no additional benefit from increasing air pollution standards above current regulations. This new method will likely result in deaths due to air pollution being missed, published reports have said.

George Thurston, a professor of environmental medicine and population health at New York University School of Medicine, said he's very concerned by this proposed change.

Thurston is co-author of an editorial accompanying the new study. He said when it comes to air pollution, "we haven't found a threshold where there's no effect."

If the government sets a regulatory threshold, it doesn't mean there is no danger below that level, he noted. Thurston said regulations governing particulate matter -- fine particles of pollution that get inhaled -- are akin to speed limits for cars. The speed limit might be 65 miles per hour (mph) on the highway, but that doesn't mean if you crash going 40 mph that there's no risk of serious injury because you're under the legal limit.

"The [Trump] administration thinks if you stay within the pollution 'speed limits,' you're not at risk. But from everything we've seen to date, the risk from air pollution is proportional to your exposure," Thurston said.

Asthma isn't the only health concern linked to air pollution. A recent study published in Chest by an international team of scientists, including Thurston, found that outdoor fine particulate matter of 2.5 micrometers or less accounted for more than 4 million deaths worldwide. (A hair from your head is approximately 30 times wider than particulate matter that's less than 2.5 micrometers.) While typically associated with breathing and heart troubles, air pollution can potentially damage every organ in the body, the researchers said.

Thurston said the new research by Garcia and colleagues is "a landmark study that shows if you clean the air, you will get health benefits. It really does validate the concept of setting regulations and receiving benefits from those efforts."

Dr. Len Horovitz is a pulmonary specialist at Lenox Hill Hospital in New York City.

"Just as increased air pollution results in increases in asthma, this study demonstrated that a decrease in air pollution decreased the number of pediatric asthma cases," Horovitz said.

The study looked at more than 4,100 kids from nine California communities. The children were recruited at different points in time over 20 years -- 1993 through 2014.

The researchers collected all kinds of information on the children and their families, including factors that might affect asthma risk, such as income and smoking. The investigators also checked to see whether a physician had diagnosed a child with asthma. And they gathered information about air pollution levels in each community.

The findings showed that decreasing levels of two pollutants, in particular, were linked to fewer asthma cases -- nitrogen dioxide and particulate matter less than 2.5 micrometers. Both are linked to the burning of fossil fuels by cars, trucks, buses, off-road equipment and power plants, according to the EPA.

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