SUSAN JAMES, a 50-year-old probation officer in Bakersfield, Calif., has been a competitive runner for almost three decades. “I’ve spent a lot of hours running through this city,” she said.

Which is beginning to worry her.

“Twenty years ago, I didn’t have asthma or allergies,” she said. Today, she has both, probably due to the same improbable cause. “My doctor told me I’m allergic to Bakersfield air,” she said. “I’m actually allergic to it.”

In May, the American Lung Association called Bakersfield the third-sootiest city in the country, behind Los Angeles and Pittsburgh.

The news didn’t surprise Ms. James. “Sometimes my chest aches” midrun, she said. To combat the pollution, she may soon join a gym for the first time. “I’ve got a lot of years to run still, and I’m not sure if I can do it outside,” she said.