(CBS/AP) SEQUOIA NATIONAL PARK, Calif. - The California forest that is home to some of the world's biggest and oldest living things, the giant Sequoia redwoods, also suffers a dubious distinction. It has the worst air pollution of any national park in the U.S.
"Ozone levels here are comparable to urban settings such as LA," said Emily Schrepf of the nonprofit advocacy group the National Park Conservation Association. "It's just not right."
Signs in visitor centers warn guests when it's not safe to hike. The government employment website warns job applicants that the workplace is unhealthy. And park workers are briefed every year on the lung and heart damage the pollution can cause.
Although weakened trees are more susceptible to drought and pests, the long-term impact on the pines and on the giant redwoods that have been around for 3,000 years and more is unclear.
"If this is happening in a national park that isn't even close to an urban area, what do you think is happening in your backyard?" said Annie Esperanza, a park scientist who has studied air quality there for 30 years.
It's a problem in a handful of the nation's 52 parks that are monitored constantly for ozone, including Joshua Tree National Park in California's Mojave Desert and North Carolina's Great Smoky Mountains National Park. But none is as severe as Sequoia and its neighbor, Kings Canyon.
While forest fires create some pollution, most comes from the San Joaquin Valley, the expanse of farmland that is home to California's two busiest north-south trucking highways, diesel freight train corridors, food processing plants and tens of thousands of diesel tractors.
Smog is created when the sun's rays hit pollutants such as oxides of nitrogen and volatile organic compounds that are in motor vehicle exhaust, solvents, pesticides, gasoline vapors and decaying dairy manure.
"There is no simple answer to ozone pollution," said Thomas Cahill, a researcher at the University of California, Davis who studies air problem in Sequoia and across California.
Breathing ozone at high levels for even a short time can blister the lungs like UV rays blisters skin, scientists agree. The problem in quantifying exposure levels, however, is that some people suffer pulmonary damage at lower doses than others.
The only way to improve air in the park is to improve the San Joaquin air basin, something that so far has proved elusive given the myriad sources of pollution. Even with hundreds of millions of dollars spent to retrofit diesel engines and replace gasoline lawnmowers with electric ones, residents pay a federal fine for the region's failure to meet even minimal EPA ozone limits.
"We don't create a disproportionate amount of pollution; it's just that we have these natural challenges so that the pollution we do create can take literally weeks or months to clean out. It just builds up over time," said Jaime Holt, spokeswoman for the valley air district.
Already this year, the level of ozone in Sequoia park has exceeded federal health standards, even though it's early in the summer ozone season. During the June-to-September summer season last year, the park violated the National Ambient Air Quality standard at least 87 times, compared with 56 at Joshua Tree and 12 at Great Smoky Mountains.
"It's tragic that the National Park Service is known for clean air, and then you see a sign saying it's unhealthy to breathe," Esperanza said. "It's so contrary to the national parks idea."