Yosemite and most of the other national parks in California suffer from a malaise that would have sickened legendary conservationist John Muir, who reveled in Yosemite’s awe-inspiring views, among many other pleasures. “Another glorious day,” he wrote in 1911, “the air as delicious to the lungs as nectar to the tongue.”
More than a century later, California has nine national parks — the most of any state — spread across 6 million acres. From the foggy redwoods in the north to the desert Joshua trees in the south, these parks have long stood apart: Their air pollution levels are the worst in the national park system.