Prescribed fire is unlikely to reduce net PM2.5 emissions in most locations

Researching prescribed fire, an essential tool for conservation

The benefits of prescribed fire are supported by decades of research and long-standing cultural traditions. Prescribed fire helps reduce the risk of catastrophic wildfire, protects communities, improves wildlife habitat and supports healthier fire-adapted forests and grasslands.

A new study in the journal PNAS examined whether prescribed fire also reduces net smoke emissions. The authors developed a method for evaluating total fine particulate emission reductions and used it to review 73 sites from published studies across the globe. They found that prescribed fire is unlikely to reduce total particulate emissions over time, largely because the reductions in emissions only occur if a wildfire encounters a treated area. They highlighted that more research is needed to understand the potential health effects of prescribed fire and wildfire management.

The study notes that few areas treated with prescribed fire are later affected by wildfire. Across the U.S., for example, only about 7% of prescribed fire treatments encounter a wildfire within 10 years, on average. At the same time, prescribed fires can help limit the size of wildfires by enabling firefighters to more effectively and safely fight wildfires.

The study finds that while prescribed fire may add more smoke than it avoids, it may still help reduce the risks to human health from smoke. Prescribed fire practitioners take steps to reduce risks to nearby communities by giving residents advance notice, conducting burns during favorable weather conditions and avoiding areas at risk of prescribed fire escape, all of which helps control prescribed fires and provide for favorable smoke dispersion.

Prescribed fire remains one of the most effective ways to support healthy, resilient landscapes and protect people and property from wildfire. (The Nature Conservancy)

Abstract

Wildfire smoke poses a growing global health risk, largely from fine particulate matter (PM2.5) emissions. Prescribed fires, which are critical for maintaining resilient forests in many locations, can also reduce wildfire emissions in treated areas that later burn. However, prescribed fires also produce smoke, creating a tradeoff in their net impact on PM2.5 emissions. We develop a mathematical framework showing that, under most current conditions globally, prescribed fire emissions are rarely offset by reduced wildfire emissions and therefore increase PM2.5 emissions overall. An analysis of 73 prior study sites reveals that reported net emission reductions are often based on unrealistic assumptions, for example that all treatments are subsequently encountered by wildfire during their effective lifespan. Using empirically based estimates of expected encounter rates, prescribed fire increases net emissions at 99% of sites, with a median 10-year increase of 210% (IQR: 70 to 475%), and only 0.06 tons of wildfire emissions avoided per ton of prescribed fire emissions (IQR: 0.03 to 0.15). However, prescribed fires are intentionally conducted under favorable meteorological conditions, allowing smoke to disperse more safely than during wildfires. Thus, whether prescribed burning can reduce health risks while increasing emissions—and whether its emissions impact can be lowered beyond current practice—is a critical area for future study. Regardless, prescribed fires have multiple objectives and benefits; they remain essential for forest management and hazard reduction.