Wildfires blazed around the world last summer, burning land from California to Siberia - and the fires are causing huge amounts of pollution.
A new study focusing on fires in the Pacific Northwest suggests that the pollution could put the health of millions of people at risk.
A report published by the UN last month warned that wildfires are on track to increase 50% by 2050.
The research, led by scientists at the National Center for Atmospheric Research (NCAR), found that levels of carbon monoxide - a gas that indicates the presence of other air pollutants - increased sharply as wildfires spread in August.
Carbon monoxide levels are normally lower in the summer because of chemical reactions in the atmosphere related to changes in sunlight, and the finding that their levels have jumped indicates the extent of the smoke's impacts.
Lead author Rebecca Buchholz, a NCAR scientist, said: "Wildfire emissions have increased so substantially that they're changing the annual pattern of air quality across North America.
“It's quite clear that there is a new peak of air pollution in August that didn't used to exist."
The research team used satellite-based observations of atmospheric chemistry and global inventories of fires to track wildfire emissions during most of the past two decades.
Buchholz said the findings were particularly striking because carbon monoxide levels have been otherwise decreasing, due to improvements in pollution-control technologies.
The study was published this week in Nature Communications.
Buchholz said, "Multiple lines of evidence point to the worsening wildfires in the Pacific Northwest as the cause of degraded air quality.
"It's particularly unfortunate that these fires are undermining the gains that society has made in reducing pollution overall."
The findings have implications for human health because wildfire smoke has been linked to significant respiratory problems, and it may also affect the cardiovascular system and worsen pregnancy outcomes.
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