New study reveals how to make prescribed forest fires burn safer and cleaner

Prescribed burns literally fight fire with more fire. Often referred to as a “beneficial fire,” they target areas at risk for wildfires and burn away material that could otherwise fuel a future blaze.

However, all fires, whether accidental or planned, produce smoke that can cause health and respiratory issues, especially in nearby communities. Burning fires release harmful chemicals, like polycyclic aromatic hydrocarbons (PAHs), that are carcinogenic – PAHs can cause cancer, lung damage, and lead to weakened immunity in those who inhale smoke. 

Recently, in a study published in Atmospheric Pollution Research, scientists at Stanford University suggested ways to perform prescribed burns with drastically reduced health implications. They’ve determined that simply tweaking some of the burn conditions can slash PAH emissions by up to 77%. The researchers estimate that this could cut cancer risks from smoke exposure by over 50%.

“There is clearly potential for improving prescribed burn procedures, such that the health impact is reduced,” said Karl Töpperwien, lead author of the paper and postdoctoral fellow in the Department of Mechanical Engineering in the School of Engineering. “We can essentially kill two birds with one stone – protect ecosystems while simultaneously protecting communities that would be otherwise at risk.”

Many hands make light work

To bring this new method to fruition, Töpperwien’s team took a multidisciplinary approach, collaborating with medical researchers at the Harvard T.H. Chan School of Public Health, physicists at the SLAC National Accelerator Laboratory, and chemical researchers at Aerodyne Research Inc. 

The medical researchers identified the most toxic pollutants from wildfires, in terms of potential harm to humans. Many PAHs are listed as carcinogenic pollutants by the U.S. Environmental Protection Agency (EPA) and can cause long-term health impacts. “We were focusing on these as our priority pollutants because exposure to these pollutants can cause cancer, inflammation, and other types of diseases,” said Töpperwien.

Once identified, the next obstacle was how to measure these pollutants precisely. This is where the chemists brought in their expertise. “They are excellent experimentalists who really advanced the frontier of measuring chemical species at high selectivity and precision,” said Matthias Ihme, senior author of the paper and professor of mechanical engineering in the School of Engineering and of photon science at SLAC National Accelerator Laboratory.

Töpperwien and Ihme’s team tied their collaborative efforts together by building the experimental apparatus that enabled these measurements and providing expertise on the fundamentals of fire behavior. They built a laboratory-sized combustion chamber where they burned samples of Eastern White Pine, one of the most common trees found on the eastern U.S with the highest emissions of PAHs among commonly studied hardwoods and softwoods.

Fine-tuning fires

Adjusting the moisture, heat, and oxygen level of the fires, the researchers conducted controlled burn experiments in their lab. They used high-resolution atmospheric and aerosol mass spectrometers to measure gaseous PAH emissions and particle-phase aerosols in real time.

Their experiments revealed that fine-tuning three conditions can reduce PAH emissions by up to 77%.

The first condition concerns the moisture content of the fuel. They found that the wood should have a moisture content of 20-30% – too dry and it burns too fast, producing more smoke, but too wet and it smolders, emitting high levels of PAH. Second, the heat intensity of the fire needs to be between 60-70 kW/m². Different levels of heat load promote different chemical reactions, which ultimately form PAHs: dialing in the heat means finding the “sweet spot” where the least PAH is formed. Finally, the fires need to burn with oxygen levels of 5-15%. Too little oxygen can cause inefficient burns, leading to too much smoke, while too much oxygen can make a fire burn uncontrollably.

These three burn parameters can effectively make prescribed burns safer and cleaner for the environment and for communities living near fire-prone regions.

Fire in the field

Ultimately, the question is whether these laboratory fires can be replicated in real-life prescribed burns. The researchers believe it to be highly possible and note that many of the techniques that could control these conditions are already used in field tests. “There will be some limitations to upscaling this, but I clearly see a path towards making this technique more viable for a broader range of environmental conditions,” said Ihme, who is also a principal investigator with the Stanford PULSE Institute at SLAC. 

In fact, Töpperwien remarks that forest managers and burn crews often prepare the wood by pre-burn treatments such as chopping, drying, and measuring moisture content to increase burn efficiency. Moisture content is perhaps the most straightforward parameter to control. Oxygen level and fire intensity are affected by the size of the wood burned and the arrangement of the burn pile, but it will take further research to understand how to precisely influence those parameters in a real-world burn. 

For the team, the next steps include replicating the laboratory burns in a field experiment and observing how their findings translate to real-life prescribed burns. The researchers will also expand this work by experimenting on different woods and by finding the best balance between cleaner burns, fuel consumption, and the cost of employing these methods. 

“Fire is more complex than we think,” said Ihme. “It’s not only finding where the flame is, but also how the smoke is transported, how it affects long-term health, and how it is admitted into the environment as it settles from the air onto the soil.”

For more information

Additional Stanford co-authors of this work include postdoctoral scholar Guillaume Vignat, former postdoctoral scholar Alexandra Feinberg, and Matthias Kling, professor of photon science at Stanford and SLAC. Additional co-authors are from Aerodyne Research Inc. and Harvard T.H. Chan School of Public Health.

This research was funded by the Stanford Sustainability Accelerator, the Google Academic Research Awards program, the Moore Foundation, and the U.S. Department of Energy.