As fires burned tens of thousands of acres across Los Angeles County, officials were warning residents that the air was a toxic soup of pollutionfueled by the fact that not only vegetation but cars, buildings, homes, and all the plastics and electronics inside them were going up in flames.
But to some residents surprise, the Air Quality Index (AQI) on their phones didnt relay that same message. That’s because AQI doesnt capture the full scope of air pollutionwhich, during the fires, was made up of toxins including lead, chlorine, and bromine. To give residents a fuller picture of what exactly was in the air around L.A., scientists with an air monitoring project made their advanced air pollution measurements available to the public.
Lead data, Los Angeles 1/1/2025-1/28/2025 [Screenshot: ASCENT Data]
What doesnt AQI capture in air pollution?
The Environmental Protection Agency developed the AQI to measure five major pollutants: ground-level ozone, particle pollution (also called fine particulate matter or PM2.5, meaning particles that are 2.5 microns or less in size), carbon monoxide, sulfur dioxide, and nitrogen dioxide. PM2.5 and ozone tend to be the primary pollutants. The index gives all this air pollution a value based on the total mass concentration.
That means the general AQI reading can lack specificity, says Roya Bahreini, a professor of atmospheric science at University of California Riverside and a co-principal investigator of the Atmospheric Science and Chemistry mEasurement NeTwork, or ASCENT project. Its also missing certain toxins that may be released during events like urban wildfires. Just looking at the value of PM2.5 cannot tell you how toxic the air is.
The way many cities measure the aerosols in their air doesn’t show a full, real-time picture, she adds. While some may measure air pollutants continuously, they may only analyze the air filters every three or five days. Theyre also aggregating all that data into a 24-hour period. You dont get these very dynamic profiles, Bahreini says, We dont know if the high concentration that one filter sees is because of something that happened in the morning rush hour or the afternoon rush hour or at night.
These gaps are what the ASCENT project, which was funded by the National Science Foundation in 2021, hopes to fill. We wanted to have a continuous look at the chemical composition of the PM2.5, and this fast, sort of state-of-the-art network has not existed in the U.S., Bahreini says. ASCENT set up its network at 12 sites across the country, including two locations in Los Angeles. With this network, we see continuously how dynamic the aerosol composition can be, and also how things change.
Chlorine data, Los Angeles 1/1/2025-1/28/2025 [Screenshot: ASCENT Data]
Lead and chlorine in L.A. wildfire air
ASCENTs goal is to have all its measurements public by September 2025, but it rushed to make a website with its L.A. readings available during the fires as researchers realized the importance of what they were capturing.
Early on, ASCENTs monitoring site in Pico Rivera, which is part of southeastern Los Angeles County, saw massive spikes in airborne leadjumping, at its worst, to 472 nanograms of lead per cubic meter of air. The EPAs safe levels for lead in the air is 150 nanograms per cubic meter averaged over three months. Before the fires began, L.A.s levels were less than 3 nanograms.
Around the same time. ASCENTs tools also clocked spikes in chlorine to more than 13,000 nanograms per cubic meter of air, when the background level in the region is usually less than 50 nanograms. Bromine levels also saw significant spikes.
Exposure to all of these can come with health impacts: Breathing in lead has been associated with brain and nervous system damage; chlorine can damage the respiratory tract and lungs, and bromine exposure can cause lung inflammation. (Though for the latter two, its difficult to know the risk given how they were distributed through the air during the fires.) Breathing in lots of particulate pollution in general can cause respiratory issues and even premature mortality.
These three chemicals are very unique to something like an urban wildfire, Bahreini says. Lead could have gotten into the air from soil deposits that were burning, as well as lead pipes or paint from older homes. Chlorine is often added to plastics, including PVC piping, computer casings, or the insulation around wires. You can imagine how much of that got burned, Bahreini says. Bromine could be in all sorts of household materials, from mattresses to sofas to carpets, because its often used a component of flame retardants.
Checking the air qualityand masking up
The highest levels of those three toxins did decrease after the most active periods of the fires. Still, even once they decreased, their levels were still above normalpossibly because of wind, cleaning efforts, or other disturbances that can redistribute the particles in the air.
ASCENT is planning to keep its publicly-available L.A.-area data online as long as there is interest, and fire risk, so residents can keep an eye on these pollutants with real-time data. (L.A. did rcently get some rain that eased the wildfires, though officials warned that fire conditions could still persist afterwards, especially if the Santa Ana winds return.) Bahreini encourages residents to wear a mask, like an N95 or Kn95, that protects against PM2.5.
