New cases of asthma have fallen when Los Angeles smog has declined

Children living in areas where air pollution is bad are more likely to develop asthma, which is the most common chronic disease among young people. But when you purify the air, does it really protect the health of children?

A study published Tuesday in JAMA, the journal of the American Medical Association, sought to answer this question.

The research focused on southern California, long known for smog. Over the past two decades, air quality has improved significantly, with stricter rules limiting pollution from vehicles.

This change led Erika Garcia, a postdoctoral fellow at the University of Southern California, to ask a simple question. "We know that there have been reductions in air pollution," she said. "Let's see if there are simultaneously improvements in health outcomes over the same period."

Scientists have regularly learned about student health in 12 different communities in the Los Angeles area. The study on children's health in southern California looked at three different time periods: 1993-2001, 1996-2204 and 2006-2014.

Garcia and his colleagues have reconstructed the trends in air quality in these regions. "Some communities have declined a bit," she says. "Some communities have declined a lot, some have not decreased at all."

Garcia asked if new asthma cases would be less common in communities where the air is most improved. The answer was yes, especially for nitrogen dioxide, which is an indicator of exhaust emissions, and fine particles, which are a major type of pollution caused by diesel engines.

Overall, USC scientists indicate that these pollutants have decreased by about 20% over the 20 year period.

"This corresponds to a decrease of about 20% in the number of new cases of asthma in children," Garcia said. "It's pretty substantial."

There were also other positive trends in these communities. The study found that pregnant women significantly reduced their smoking during this period and that children were significantly less exposed to second-hand tobacco smoke.

Garcia says his results show improvements beyond what these changes would have brought. The study can not prove the cause and effect, but the results provide strong circumstantial evidence that reducing air pollution reduces disease.

John Balmes, professor of environmental health at the University of California at San Francisco and a physician member of the California Air Resources Board, says it's not surprising to find that clean air means fewer diseases. But the ongoing debate about how much money to spend on improving air quality is important.

"Throughout this study, concerted efforts have been made in California to reduce motor vehicle emissions in general and diesel emissions in particular," said Balmes. "And I think this study shows that this is paying off."

State policy, focused on both health and climate change, calls for the phasing out of gasoline and diesel vehicles for cars, trucks and electric buses.

The industry, formerly opposed to these rules, is now split in two, electric car and renewable energy companies being in favor of a regulation of the quality of the air.

"It allowed us to move forward, I think, perhaps at a faster pace than we would have otherwise," said Balmes.

It's a different story at the federal level, though. The Trump Administration's Environmental Protection Agency is skeptical about the link between clean air and health.

For example, George Thurston, a professor of environmental medicine at New York University, says new discoveries in southern California are coming at a key moment, "because they confirm the current EPA's concerns that there are real health benefits to purify the air.

"We think it should be obvious," Thurston said, "but that's a question they're asking themselves." He said the new asthma study is "a benchmark" for demonstrating the benefits of air cleaning.

You can contact NPR Scientific Correspondent Richard Harris at [email protected]