Most Polluted U.S. National Parks Revealed

Yosemite and Joshua Tree are among the national parks with the most polluted air, according to new research by the National Parks Conservation Association (NPCA).

In a report released at the start of March, the non-partisan advocacy group found that 97 percent of America's national parks experienced unhealthy air, based on National Parks Service data collected in 2021.

It also found that around half of the 397 parks it analyzed were at high risk of suffering from at least one climate change-related threat, such as wildfire, drought or invasive species.

"Visitors to national parks expect clean air and clear skies whether they are visiting stunning wilderness landscapes or urban monuments," Ulla Reeves, interim director of NPCA's Clean Air Program, said upon the report's release. "The trouble with air pollution is that it knows no bounds and harms the places we love."

The NPCA researchers determined the healthiness of the different parks' air based on ground-level ozone concentrations.

While ozone in the upper atmosphere shields us from damaging rays from the sun, it can cause serious health problems when inhaled. The American Lung Association says the gas "aggressively attacks lung tissue by reacting chemically with it."

The report found that 42 national parks had levels of ozone that constituted a significant concern—down from 87 in 2019, suggesting some improvement in air quality. However, it noted that 85 percent of the parks were still deemed unsatisfactory.

"While these parks do not consistently reach unhealthy ozone levels, their conditions are far from optimal and can still adversely affect both park visitors and staff," it said. Only 12 parks had air pollution levels of little concern.

According to the report, the four national parks with the worst air pollution were all in California: Sequoia and Kings Canyon, Joshua Tree, Mojave and Yosemite. They were followed by Carlsbad Caverns in southern New Mexico, Death Valley in California, and Indiana Dunes, near Lake Michigan in Indiana.

Also making the top ten in terms of poor air quality were the Guadalupe Mountains National Park in Texas, Rocky Mountain in Colorado and White Sands in New Mexico.

Some 387 out of 399 national parks it reviewed were found to have unsatisfactory levels of haze in the skies, while a further four had significantly hazy skies. Haze is caused by combustion releasing smoke and particulate matter, and can significantly limit visibility.

The researchers also found that while ozone levels in many national parks were not enough to cause health problems in humans, they—along with sulfur and nitrogen pollution from burning fossil fuels and agricultural practices—were enough to cause harm to flora and fauna in 276 parks.

The NPCA urged lawmakers and the Environmental Protection Agency (EPA) to "hold states and polluters accountable" for air pollution, implement rules to curb greenhouse gas emissions from new power plants and finalize vehicle standards to cut emissions from the transportation sector.

Newsweek approached the EPA via email for comment on Wednesday.