Research Triangle Energy Consortium

The Bootleg Fire (New York Times, July 23, A11) is a public health concern for much of the country. While the devastation from burning property is local, the emissions can travel thousands of miles. These emissions primarily comprise particles of solid carbon (soot) coated with organic molecules. Particles under 2.5 microns in diameter (a micron is one millionth of a meter and a human hair is about 70 microns in diameter) are designated PM_2.5 and are more toxic than the larger PM₁₀. This is because they can penetrate deep into the lungs. Regulations worldwide focus on PM_2.5.

Smaller particles travel further because they are lighter. The total concentration of particles distant from the fire will be lower than near the source. But these particles will largely be PM_2.5, also known as fine particles. Even smaller particles, known as ultrafine or nanoparticles, are a subset of these, and are referred to by scientists as PM_0.1, indicating particles smaller than 0.1 microns. Strong evidence points to these particles being even more toxic than the rest of the PM_2.5. There are two reasons for that. One is that the size allows them to penetrate individual cells in the organs, carrying their toxic organic payload. The other is that these tiny particles are much more likely to grab toxins from the atmosphere than the larger ones. That makes them more virulent. And finally, these ultrafine particles travel even further from the site of the fires than the fine particles.

For any given size of particle from a wildfire, toxicity of the particle is related to the type of wood and the conditions under which the burn occurs. A recent study concludes that combustion of eucalyptus, ubiquitous in Australia and common in northern California, results in particulate matter that is more toxic than from pine, peat, and red oak. The measures of toxicity in the study were markers for inflammation in the lungs of mice (1).

Fierce, hot burns produce particles with lower toxicity than smoldering burns. The natural reaction from a property protection standpoint is to douse the flames. However, if left to smolder, the detriment is to public health. The reason is that a smolder is relatively oxygen starved and produces more unburnt organic molecules (mostly from the outermost layers of the limb rich in aromatic compounds). These volatile organics then attach to the carbon particles emitted during combustion as soot, making them more toxic. The same lung toxicity study cited above noted a striking increase in toxicity in the smoldering condition (1).

The Bootleg Fire is the third largest fire in Oregon history. The burn area is close to 400,000 acres. To give that context, large fires are classified by statisticians as greater than 1000 acres of burn. Scientists use the metric of area burned rather than numbers of fires. Privation is certainly proportionate to the area of devastation.

EPA data show that over the three decades preceding 2018, the total numbers of fires have remained essentially constant. Not surprising, since most are caused by human behavior, which, absent intervention, tends to remain constant. Strikingly, however, over the same period, the area burned has nearly doubled (3). This statistic does not include the devastation of the 2020 fire season. Five of the ten largest fires ever in California were in that year. The largest was the August Complex fire, which burned just over a million acres.

The Bootleg Fire is believed to have been triggered by lightning strikes on dry underbrush. The mammoth August Complex fire in 2020 was believed to have been caused by 38 lightning strikes. Lightning as a cause of wildfires has not been in the top four causes, judged by the area burned (2). This is because lightning is ordinarily associated with a thunderstorm, and even 0.1 inches of rain is sufficient to materially reduce the ignition of underbrush. However, with high ambient temperatures increasingly being experienced, “dry lightning” can be produced. The associated rain evaporates prior to hitting the ground.

Until now the dogma has been that the causes of most wildfires are anthropogenic. In California, the four principal causes of ignition were arson, campfires, equipment (such as chain saws) and falling power lines, when the metric was area burned (2). All being related to human endeavor, amelioration was possible. But if the world is moving towards being hotter, the incidences of dry lightning could go up. Death Valley, CA recorded a high temperature of 130 F on August 16, 2020. This was the highest recorded temperature on the planet. August 16 was also the day of the first dry lightning strike associated with the August Complex fire. I am not suggesting causality, but the facts urgently suggest study*.

The literature does not offer promise for direct intervention in dry lightning caused large area devastation, other than controlled burns to clear out the under-canopy vegetation, which is already practiced to a degree. This stark reality applies not just to the fire prone western US, but also to the rest of the country.

Vikram Rao

* ” Lightning’s striking again” in Lightnin’ Strikes, by Lou Christie (1965), written by Lou Christie and Twyla Herbert

References

1. Kim YH, Warren SH, Krantz QT, et al. Mutagenicity and lung toxicity of smoldering vs. flaming emissions from various biomass fuels: implications for health effects from wildland fires. Environ Health Perspect. 2018;126(1):2018.

2. Syphard, A. D., & Keeley, J. E. (2015). Location, timing and extent of wildfire vary by cause of ignition. International Journal of Wildland Fire, 24(1), 37. https://doi.org/10.1071/WF14024

3. US EPA 2019 (Wildland Fire Research Framework 2019-2022)