Smoke from wildfires across the continental U.S. is stalling — in some places, reversing — years of progress on air quality.

A new study published in Nature found that since 2016, wildfire smoke has undone 25 percent of air quality improvements achieved since 2000.

“We’re not back to 2000 levels. But in some parts of the country we’re headed in that direction,” said Marshall Burke, the study’s lead author and professor of environmental policy at Stanford University.

That’s concerning because previous studies found that wildfire smoke is bad for human health. It’s not just that it exacerbates respiratory illnesses like asthma; breathing in wildfire smoke is also associated with an increased risk of cancer, heart attacks, and preterm birth.

“Basically we find that there is no safe level of exposure,” Burke said.

His latest study found that air pollution varied by state — in Oregon, wildfire smoke has become so severe as to erase much of the air quality progress of the past two decades.

The problem isn’t limited to the West. Burke noted that the study found smoke influenced pollution levels even in the South, Midwest, and Northeast — regions where wildfires are far less common.

“The influence of wildfire smoke is broad, and it is affecting populations that did not used to be affected,” Burke said. “We are seeing influence in states that basically have none of their own wildfires. They are getting affected by wildfires from thousands of miles away.”

The new study analyzed data up to 2022, meaning it didn’t include any data from the Canadian wildfire smoke that shrouded New York City in an orange haze earlier this year, nor the air pollution produced by the West Maui wildfire last month, the deadliest in modern U.S. history.

Researchers used ground and satellite-based data that was only available comprehensively for the 48 contiguous states, Burke said. The study also notes that air pollution from wildfires is not currently included under any federal air quality regulations.

The number of wildfires globally is expected to grow by 50 percent by 2100, according to a report published by the United Nations Environmental Programme and the Norway-based environmental nonprofit group GRID-Arendal last year.

Read the full article about U.S. air quality by Anita Hofschneider at Grist.