By Jeff Wheelwright, National Geographic

SEQUOIA NATIONAL PARK — This great park in the Sierra Nevada Mountains of California attracts a million visitors a year, who come mostly in summer to see the largest trees on Earth. After Labor Day the heat and the tourists dissipate. So does a less welcome visitor: air pollution.

By a misfortune of topography, Sequoia National Park sits above an industrial-agricultural basin, the San Joaquin Valley, whose dirty, sun-baked air has nowhere to go but up. Along with its neighbor, Kings Canyon National Park, Sequoia has the worst air quality of any park in the country. In 2013 it exceeded the national health standard for ozone on 59 days, according to measurements taken at the Ash Mountain entrance station, at 1,400 feet. Up at 6,500 feet, where the giant sequoias grow, excessive levels of ozone occurred on 41 days.

On such days visitors arriving from Fresno or Los Angeles, among the most polluted cities in the nation, are greeted at the Sequoia visitor center by the same “code orange” air quality alert they left back home. Hiking or biking in parks that are almost entirely designated as wilderness, “untrammeled by man,” they may find themselves wheezing or suffering chest pain. (Read “Fifty Years of Wilderness” in National Geographic magazine.)

And yet the air is much better than it used to be.

The health standard for ozone is 75 parts per billion (ppb), averaged over eight hours. Recently the staff of the U.S. Environmental Protection Agency recommended that the standard be lowered to between 60 and 70 ppb — sparking the usual heated debate in Washington between business representatives and environmentalists. The argument may seem a million miles from Sequoia-Kings Canyon, but Annie Esperanza, the air resources specialist at the parks, is cheering the regulators on.

“We tell them, if they do the work, we’ll see it here first,” Esperanza says. “We’re the canary in the coal mine.”

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