Research Triangle Energy Consortium

A March 14 story in the New York Times describes research efforts to quantify health outcomes from the Altadena fire. Once you get past the “toxic soup” sensationalism in the title, the content is good in identifying the long-term killer: ultrafine particles which can enter cells, cross the blood brain boundary, thus causing ailments ranging from cancer to dementia. The near-term privation wrought by fires, especially ones such as this one, close to urban areas, is overt. The long-term effects tend to get discounted, much the same as long term profits in business. But they are real, and worse for fires proximal to populated areas. Here’s why.

Forest fires usually originate and burn in remote areas.  Direct impact is usually on small communities. They are evacuated and the burn is contained as well as possible. However, when they do occur close to major metropolitan areas, a somewhat different set of rules appear to apply. The priority is the protection of people and property. While this may appear to be a truism, it can lead to unintended consequences. Fierce burns are subdued to smolders and attention is shifted to the next fierce burn. This certainly achieves the priority purpose and may be the only practical way to achieve it, given limited resources.

However, in terms of effects on human health, all fires are not created equal. A fierce fire is one where oxygen is available for a complete burn of the flammable material. This principle applies to the burner on your stove as well. A complete burn is desirable on your stove to give maximal heat and to minimize unburnt hydrocarbons (you should shoot for a blue flame, not yellow). Same for forest fires. Parts of the tree, especially the bark, has aromatic (organic) molecules. An incomplete burn leads to the organic molecules to be released into the air. This matters because these molecules are directly injurious to human health. There is another, somewhat more insidious, negative. These molecules coat the carbon particles (soot) emitted, making them much more toxic.

Forest Fire, painting, courtesy Falguni Gokhale,

The importance of being small (with apologies to Oscar Wilde). Finally, soot particles are more toxic when they are small. Particles below about 100 nanometers (nm) are classified as nanoparticles. In public health circles these are known as ultrafine particles. Whereas larger particles impair respiratory pathways, ultrafine particles go a step further. They can enter cells and are transported over the body. They are known to cross the blood brain barrier and are strongly implicated in exacerbating dementia.

As if this were not bad enough, ultrafine particles are particularly prone to deposition of organic molecules on their surface. This is because they have a large surface area for their mass, and that ratio is known to make them more reactive, a property that is used beneficially in chemical engineering. Here, it is all bad. Toxic organic molecules are now attached to particles which readily enter cells. Game, set and heading to match.

Now to another negative attached to a smoldering burn as opposed to a fierce one. A smolder leads to a relatively high proportion of ultrafine particles when compared to a complete burn. A study of the Getty Fire in 2019 demonstrated this effect. This too was a fire near dense housing, a couple of miles from UCLA and very close to the Getty Museum.  The study found that the median particle diameter of the particles was 130 nm in the flaming state and dropped to 40 nm in the smoldering state. Potentially confounding variables were eliminated in arriving at this conclusion.

The inescapable inference is that smoldering fires are a greater health risk than flaming ones because they create ultrafine particles and emit volatile organic compounds which preferentially attached to the ultrafine particles. Admittedly, the priority on dousing the flames is well placed, but more attention ought to be paid to putting out the smolders.

The Getty Fire, and several other large ones in California, were started by tree limbs falling on power lines, sparking them and igniting the underbrush. This brings into focus the fact that high voltage power lines are bare, devoid of insulation.  No practical solutions exist to insulate the lines. Underground lines are found in communities but are not feasible across forests. Chalk one up for distributed power.

Toxicity of fire emissions is also a function of the type of wood. Lung toxicity studies demonstrated that eucalyptus was by far the worst, followed by red oak and pine. This is bad news for Australians, where the species is native. But California has it in abundance because it was imported as a construction wood for its characteristics of fast growth and drought tolerance. The use in construction did not pan out (brought the wrong variety) but eucalyptus loved the west coast setting and is now considered an invasive species and highly prevalent.

One final point regarding ultrafine particulate emissions from wildfires. These can, and do, travel across the continent. The smaller the particles, the further they will travel. Ergo, the most toxic particles impact much of the nation, not just the areas close to the fires.

And the pain in the form of disease will be felt long after the fires are a distant memory *.

Vikram Rao

March 16, 2025

*All my troubles seemed so far away, now it looks as though they’re here to stay, from Yesterday, performed by The Beatles (1965), written by John Lennon and Paul McCartney.