OAKLAND, Calif.— The Center for Biological Diversity and Center for Environmental Health filed a lawsuit today challenging the Trump Environmental Protection Agency’s failure to protect millions of California and Colorado residents from dangerous smog pollution.

“Trump’s EPA is forcing millions of people to breathe extremely harmful levels of smog, day after day,” said Ryan Maher, an attorney at the Center for Biological Diversity. “As this administration gives handout after handout to the fossil fuel industry and other polluters, we’re counting on the courts to step in and protect public health.”

The Clean Air Act requires the EPA to set national ambient air quality standards to protect people and the environment from pollutants like smog. When those standards are violated, the agency must ensure that states have valid plans in place to clean up the pollution.

The Denver metro area and the West Mojave Desert regions at issue in the lawsuit are home to more than 4 million people. Both regions suffer from air quality that does not meet federal standards for dangerous ozone pollution, commonly referred to as smog.

Ozone exposure has severe impacts on people, plants, and animals. Ozone pollution is particularly harmful to children and the elderly, causing asthma and reduced lung function. In the Denver metro area alone, more than 750,000 children and nearly 550,000 elderly people are put at risk by unsafe ozone levels, according to the American Lung Association.

Ozone also stunts the growth of trees and increases their susceptibility to disease, insect damage, and harsh weather. Unchecked levels of harmful air pollution threaten many much-loved spaces in the Western Mojave Desert, including Joshua Tree National Park, Mojave National Preserve, and the Mojave Trails National Monument.

“This pollution harms people in the worst ways possible,” said Tom Fox, senior legislative counsel at the Center for Environmental Health. “It’s imperative that Trump’s EPA uphold its duty to protect the public’s health by acting immediately, as the law requires.”

A variety of sources contribute to smog, including the fossil fuel industry and vehicle emissions. In the Denver metro area, for example, nearly half of the emissions causing smog arise from the production of dirty fossil fuels.

In the Western Mojave, the EPA’s failure to regulate local air quality is part of the Trump administration’s broader push to prioritize corporate profit over its lawful conservation duties. Earlier this year the president unlawfully approved mining in the Mojave National Preserve and, during the government shutdown, the Department of the Interior targeted Joshua Tree National Park for cuts to conservation and education resources.

Today’s suit was filed in the U.S. District Court for the Northern District of California.