Four parks in California have the most polluted air of all national parks, a study released Tuesday said.

Yosemite, Sequoia, Kings Canyon and Joshua Tree were the only national parks of the 48 analyzed that earned an “F” grade for polluted air, according to the National Parks Conservation Association.

“This air pollution is unhealthy for most park visitors and rangers and also heavily hampers visibility in the parks. Scenic views and clean air are treasured by park visitors,” according to the report.

“Air quality doesn’t know boundaries,” said Luke Sabala, chief science officer for Joshua Tree National Park.

The approximately 30 days annually of unhealthy air in Joshua Tree National Park is due to ozone, he said. He added that the air situation in Joshua Tree is stable, or even improving, due to the proliferation of more fuel-efficient vehicles.

The NPCS study estimates it will take until 2106 to reach natural visibility at Joshua Tree at projected pollution cleanup rates.

“With four national parks in California amongst the dirtiest in the country and projections showing it may take more than a century to clean the air in these parks, it’s clear we need a different strategy,” said Ron Sundergill, senior director of the NPCA’s Pacific regional office.

The association says that the situation is “one of those rare problems with a simple solution.”

With a stroke of pen, the president can close loopholes to make ensure that states and the Environmental Protection Agency are poised to restore the air in national parks, the NPCA says.

The Obama Administration is expected to revise the Regional Haze Rule before end of its term.

NPCA analysis recommends that changes must include setting park-centered targets for reducing pollution impacting parks for the next decade, as well as closing loopholes that allow some polluters to escape the rule’s requirements and greater accountability for states to reduce pollution within their borders.