Prescribed burning, a wildfire management technique that involves controlled fires set intentionally in forests or grasslands, has long been recognized as a valuable ecological tool. These so-called “beneficial fires” actively reduce accumulations of dry vegetation and other combustible materials that would otherwise fuel uncontrollable and catastrophic wildfires. By carefully applying fire under monitored conditions, land managers can maintain ecosystem health, prevent large-scale fires, and protect biodiversity. However, these controlled burns also produce smoke laden with harmful pollutants, including a class of toxic compounds known as polycyclic aromatic hydrocarbons (PAHs). As it turns out, navigating the delicate balance between ecological benefits and public health risks of prescribed burns demands a more refined understanding of combustion chemistry and burn conditions.
PAHs, formed during the incomplete combustion of organic matter, are a major health concern due to their carcinogenic and immunotoxic properties. These compounds, released in wildfire and prescribed fire smoke, pose significant respiratory risks to communities downwind of burn sites. Exposure to PAHs has been linked to lung cancer, chronic respiratory diseases, and systemic inflammation. The quandary facing wildfire managers is how to leverage prescribed burning to mitigate wildfire severity, while minimizing smoke emissions that can adversely affect vulnerable populations. This challenge reached new prominence as wildfire events worldwide have increased in frequency and intensity, heightening interest in optimizing burn protocols to reduce toxic emissions without compromising ecological outcomes.
In a groundbreaking study recently published in Atmospheric Pollution Research, a multidisciplinary team led by researchers at Stanford University made significant advances toward this goal. Their investigation focused on identifying the precise burn conditions under which PAH emissions can be dramatically curtailed. The research demonstrated that by fine-tuning parameters such as fuel moisture content, fire heat intensity, and environmental oxygen levels, it is possible to reduce emissions of toxic PAHs by as much as 77%. This reduction potentially translates into more than a 50% decrease in cancer risk from smoke exposure, offering a promising pathway to safer prescribed fire practices.
The project’s success hinged on an integrative approach that combined expertise from mechanical engineering, atmospheric chemistry, environmental health, and physics. Stanford’s lead author, Karl Töpperwien, along with senior author Matthias Ihme, convened collaborators across diverse institutions including Harvard’s T.H. Chan School of Public Health, the SLAC National Accelerator Laboratory, and Aerodyne Research Inc. Medical researchers pinpointed the specific PAH species most hazardous to human health, while chemists deployed cutting-edge mass spectrometry techniques to precisely quantify these pollutants in real time during combustion. Meanwhile, fire behavior specialists engineered a laboratory-scale combustion chamber that replicated forest fire conditions with exceptional control.
The experimental setup employed Eastern White Pine wood samples, selected for their prevalence in eastern U.S. forests and their known propensity to emit higher levels of PAHs compared to other hardwoods and softwoods. By systematically varying moisture content, heat flux, and oxygen concentration, the researchers were able to monitor how these factors influenced chemical reactions during combustion. Their high-resolution mass spectrometric data revealed intricate relationships between fire dynamics and toxic pollutant formation, confirming that combustion efficiency and smoke toxicity can be optimized simultaneously.
One of the study’s key findings is the pivotal role of fuel moisture. Wood moisture levels between 20% and 30% were identified as optimal. If the wood is too dry, combustion proceeds rapidly but ineffectively, generating excess smoke and elevated PAH production due to incomplete burning. Conversely, overly wet fuel tends to smolder, also producing high quantities of PAHs. Achieving this “moisture sweet spot” mitigates these extremes, promoting more complete combustion and cleaner emissions. This insight points to existing forest management practices like pre-burn drying and moisture monitoring as beneficial measures in smoke reduction.
Heat intensity, measured as the fire’s heat flux in kilowatts per square meter, emerged as another critical parameter. Fires maintained between 60 and 70 kW/m² heat flux facilitate chemical pathways that limit PAH synthesis. Lower or higher heat intensities shift combustion regimes, favoring reactions that increase smoke toxicity. This nuanced understanding underscores the importance of controlled flame intensity—not too vigorous to cause runaway combustion, and not weak enough to provoke smoldering. In the same vein, oxygen concentration must be carefully regulated. The study found that burning with oxygen levels maintained at 5–15% enables efficient oxidation of volatile compounds while preventing the excessive smoke produced by oxygen-starved or oxygen-rich flames.
Translating these laboratory results to operational prescribed burns holds great promise but is not without challenges. Processes influencing oxygen availability and heat distribution in the field are more complex and variable than controlled laboratory settings. Factors such as wind conditions, terrain, and fuel arrangement all impact burn behavior. Nonetheless, the researchers emphasize that many existing forest management techniques—like pile arrangement, wood sizing, and moisture control—already touch upon these influential factors. Future work will need to focus on fine-tuning these parameters in situ, perhaps informed by real-time sensor technologies and burn monitoring tools.
The Stanford team’s next steps involve validating their laboratory findings through pilot field studies and expanding analyses to include different wood species and fuel types. Investigating trade-offs between emissions reduction, fuel consumption efficiency, and economic costs will be essential to making these optimized burn protocols practical for widespread adoption. Their research also highlights the multifaceted nature of fire—from flame propagation and pollutant formation to smoke transport and deposition—emphasizing the need for integrated scientific approaches to fire management.
This study advances the frontier in understanding wildfire smoke toxicity, offering concrete strategies to improve air quality outcomes connected to prescribed burns. By identifying specific burn conditions that minimize harmful PAHs, the researchers pave the way for enhanced wildfire mitigation techniques that safeguard both ecosystems and human health. Their work underscores a critical paradigm shift: that prescribed fires, when meticulously managed, can serve dual roles as guardians of nature and protectors of public well-being. With climate change intensifying wildfire risks globally, innovations like these become ever more vital in promoting resilient landscapes and healthier communities.
Subject of Research: Prescribed fire combustion parameters and polycyclic aromatic hydrocarbon (PAH) emissions reduction.
Article Title: Burn parameters affect PAH emissions at conditions relevant for prescribed fires
Web References:
- https://www.sciencedirect.com/science/article/abs/pii/S1309104225000406
- https://profiles.stanford.edu/karl-toepperwien
- https://engineering.stanford.edu/
- https://hsph.harvard.edu/
- https://www6.slac.stanford.edu/
- https://aerodyne.com/
- https://profiles.stanford.edu/werner-ihme
- http://dx.doi.org/10.1016/j.apr.2025.102438
Keywords: Forest fires, Environmental methods, Pollution control, Smoke, Environmental health
