Air pollution can contribute to asthma and heart disease. And it puts children at greater risk of developmental and behavioral problems.

But not everyone is equally likely to be exposed to air pollution.

While regulations and cleaner energy have meant the air’s getting a little cleaner for everyone, a new study by University of Washington researchers shows that, at every income level, people of color are still exposed to more air pollution than white people.

The researchers looked at how those differences changed over time—and they didn’t. Communities of color were exposed to about forty percent more air pollution than white communities in both 2000 and 2010.

During that decade, things improved a little in Oregon, but not in Washington.

“The exposure disparity between people of color and white people in Washington state was higher than in neighboring states,” says Lara Clark, one of the co-authors of the study published Thursday in Environmental Health Perspectives.

The inequity is due to “the siting of heavily trafficked roadways, of polluting plants, in these communities,” says Frederica Perera, a researcher at Columbia University’s Mailman School of Public Health who was not involved in the study.

Perera says the solution is stricter regulation and more clean energy.

“We know how to control pollutants. We can do that at the end of stacks or tail pipes,” she explains. “But even better is to transition to a cleaner, carbon-free economy.”

Watch: an explainer video produced by the University of Washington