Asian secondhand smog hits California.

• Position: Ozone

Approximately 10% of ozone pollution in California's San Joaquin Valley is estimated to be coming from outside of the state's borders, particularly from Asia, according to research by the University of California, Davis. 'To me, it's an exciting new chapter of how we think of air pollution," says atmospheric scientist Ian Faloona. "How do we deal with this not just as an air district of a couple of counties, but as a nation and a global citizen of the planet? Traditionally, air pollution has always been considered an issue to be handled locally, 'It's your backyard; it's your problem'--but we're going to have to treat air pollution to some extent how we treat greenhouse gases."

When someone smokes a cigarette next to you, you know that secondhand smoke is harmful to your lungs, even though you are not the smoker, but what about when your neighbor is thousands of miles away, and the pollution being emitted is from an industrial plant, millions of cars, or a raging wildfire?

Scientists long have known that a portion of ozone pollution was coming from overseas, but attempts to quantify just how much were hamstrung by coarse computer models that overlooked or broadly simplified California's complex terrain. Faloona describes California as if it were a human body: the Golden Gate is the mouth, breathing in air from across the Pacific Ocean, sucking it through the throat of the Bay Area and into the lungs of the San Joaquin Valley. Previously unknown is how much air comes over the coastal mountain range and mixes from above into the bathtub of the San Joaquin Valley.

The research comes as the U.S. Environmental Protection Agency has proposed tightening ozone limits from 75 parts per billion to between...