(KRON) — Wildfire smoke has slowed down or reversed America’s progress toward achieving better air quality, a study by researchers with Stanford University found.

By spewing plumes of smoke laden with tiny toxic particles, called PM2.5, wildfires have a “rapidly growing influence” over air quality trends in at least 35 states, according to the study published this week. Victories toward improving air quality over the past two decades were erased by wildfire smoke, researchers said.

“We’re starting to see the fingerprint of wildfire smoke on overall air quality trends over the entire country, not just in the western states where these fires are burning most often,” said study co-author Marissa Childs. “This is because the larger fires that have been burning in recent years are lofting the pollution way up into the atmosphere, which can transport these tiny particles thousands of miles away.”

The Carr Fire sends a plume of smoke into the sky above Whiskeytown, Calif., on July 27, 2018. (AP Photo /Noah Berger)

Using ground and air sensors, researchers analyzed U.S. air pollution data. The average annual PM2.5 levels declined between 2000 and 2016. In recent years, however, wildfire smoke either slowed or fully reversed positive air quality trends, according to the study.

Stanford University researchers said drifting plumes “added enough particle pollution to erase about a quarter of the average air quality improvement.”

The San Francisco Bay Bridge and city skyline are obscured in orange smoke and haze as their seen from Treasure Island on September 9, 2020. (Photo by BRITTANY HOSEA-SMALL / AFP via Getty Images)

According to the study, at least one out of every four unhealthy air days were caused by wildfire smoke between 2011 and 2022 in the Northwest and Northern Rockies. 

San Francisco Bay Area residents are breathing smoky air this week due to several wildfires burning along the California-Oregon border. Winds blew smoke plumes south and lowered air quality to “unhealthy for sensitive groups” for four days straight.