Sequoia National Park --

On a clear day, the view from Beetle Rock in Sequoia National Park extends west for 105 miles across the patchwork of crops in California's agricultural heartland to the Coast Mountains and the Pacific Ocean beyond.

The problem is there are few clear days, even at 6,200 feet.

The Sierra Nevada forest that is home to the biggest living things on earth - the giant Sequoia redwoods - also suffers a dubious distinction. It has the worst air pollution of any national park in the country.

Mountaintops that should offer awe-inspiring views of California's geologic grandeur often are muddled by a disorienting gray soup of smog.

"Ozone levels here are comparable to urban settings such as L.A.," said Emily Schrepf of the nonprofit advocacy group the National Park Conservation Association as she beheld the diminished view. "It's just not right."

Smog is so bad that signs in visitor centers caution guests when it's not safe to hike. The government employment website warns job applicants that the workplace is unhealthy. And park workers are schooled every year on the lung and heart damage the pollution can cause.

Ozone also is to blame for weakening many stands of the park's Jeffrey and ponderosa pines, leaving telltale yellowing of their long needles. Instead of absorbing carbon dioxide, they soak up ozone through the stoma in their needles, which inhibits photosynthesis. Ozone also stresses young redwood seedlings, which already face challenges to survival.

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, which is ringed by power plants and several major highways including Interstate 40, a major tractor-trailer shipping route. But none is in the ballpark with Sequoia and its neighbor, Kings Canyon.

While forest fires create some pollution, the lion's share 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, 1.7 million dairy cows, food-processing plants and tens of thousands of diesel tractors plowing dusty fields. Its trough shape traps pollutants, and high-pressure systems act like a lid on a pot.

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 UC Davis who studies air problems in Sequoia and across California.

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.