The U.S. is producing less air pollution, but smog levels are still rising in the western U.S. because of pollutants released in Asian countries that then drift over the Pacific Ocean. Researchers say their findings show the importance of a global approach to preserving air quality.

"Scientists found Asian air pollution contributed as much as 65 percent of an increase in Western ozone in recent years," NPR's Rob Schmitz reports from Shanghai. "China and India, where many consumer products are manufactured, are the worst offenders."

The problem, scientists say, is that Asian countries' emissions of nitrogen oxides — which sunlight then breaks down in reactions that produce ozone — have tripled since 1990. When those harmful gases circulate to North America, they offset gains in U.S. air quality that have come from cutting nitrogen oxide emissions by 50 percent.

The researchers say that as China continues to cut its nitrogen oxide emissions, "rising global methane and NOx emissions in the tropical countries (e.g., India) in Asia, where O3 [ozone] production is more efficient, may become more important in the coming decades."

"A global perspective is necessary when designing a strategy to meet US O3 air quality objectives," the scientists wrote.

The research was published in the journal Atmospheric Chemistry and Physics.

The study looked at levels of ground-level ozone (the key component in smog) from 1980 to 2014. To determine U.S. trends, pollution levels in cities, rural areas and national parks were collated. Scientist Meiyun Lin of the National Oceanic and Atmospheric Administration led the work, along with others from her agency and the Environmental Protection Agency.

They concluded that the spike in man-made emissions in Asia "is the major driver" of the rise in ozone levels in the western U.S. for both spring and summer in recent decades. The researchers cited data that ranges from Joshua Tree National Park in Southern California to observations in Denver, Colo., and the eastern U.S.

According to the EPA, high levels of ground-level ozone "can be harmful to people, animals, crops, and other materials." The agency adds, "Ozone can aggravate asthma, and can inflame and damage cells that line your lungs."

The study's authors said their work was funded by NASA grants and also used ozone data that's freely available online.

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