As wildfires move across forests, grasslands, and peatlands, they release large amounts of gases and particles into the air. Scientists now say the pollution from these fires may have been underestimated. A study published in ACS' Environmental Science & Technology reports that wildfires and prescribed burns (i.e., wildland fires) around the world likely emit much higher levels of gases that contribute to air pollution than earlier estimates suggested. The research also highlights several regions where emissions from fires overlap with pollution from human activities, creating especially difficult air quality challenges.

"Our new estimates increase the organic compound emissions from wildland fires by about 21%," says Lyuyin Huang, the first author of the study. "The inventory provides a foundation for more detailed air-quality modeling, health-risk assessment, and climate-related policy analysis."

Each year, wildfires burn through vast areas of vegetation, sending a complex mixture of water vapor, ash, and carbon-based chemicals into the atmosphere. Some of these chemicals are volatile organic compounds (VOCs), which readily exist as gases. Others only evaporate and become gases at warmer temperatures and are classified as intermediate- and semi-volatile organic compounds (IVOCs and SVOCs, respectively). Once in the air, these partially volatile compounds more easily form fine particles that can be harmful if breathed in, compared with VOCs.

Overlooked chemicals in wildfire smoke

Despite their importance, IVOCs and SVOCs are often missing from wildfire emission studies. Their large numbers and chemical complexity make them difficult to measure, leading many past assessments to focus mainly on VOCs. Researchers led by Shuxiao Wang aimed to include IVOCs and SVOCs alongside VOCs to better capture how wildland fires affect air quality, human health, and climate.

To do this, the team first examined a global database tracking burned land from forest, grass, and peatland wildland fires between 1997 and 2023. They then gathered information on the VOCs, IVOCs, SVOCs, and other extremely low volatility organic compounds released as different types of vegetation burn. When direct field measurements were not available, the researchers relied on laboratory experiments to estimate the chemicals produced. These data were combined to calculate yearly wildfire emissions worldwide.

Global totals and pollution hotspots

Using this approach, the researchers estimated that wildland fires released an average of 143 million tons of airborne organic compounds each year during the study period. This figure is about 21% higher than previous estimates, indicating that wildfire emissions, particularly IVOCs and SVOCs, contribute more to air pollution than scientists had recognized.

When wildfire emissions were compared with earlier estimates of pollution from human activities, the researchers found that human sources produced more airborne compounds overall. However, both sources released similar amounts of IVOCs and SVOCs. The comparison also revealed shared emission hotspots, including Equatorial Asia, Northern Hemisphere Africa, and Southeast Asia. According to the researchers, air pollution in these regions is especially complex and will require different strategies to reduce emissions from both wildfires and human activities.