The smoke from the 2025 Los Angeles fires exposed millions of residents to toxic lead levels 80 times higher than normal background levels within a 24-hour period, according to a new study.
Researchers from the California Institute of Technology compared the chemical composition of smoke from the Eaton Fire, an urban blaze, with that of a wildfire. They discovered that the urban plume contained more lead, arsenic, and halogens—elements including chlorine and bromine—which are associated with burning vehicles, building materials, and electronics.
"The main, key finding is really that wildfire smoke is very different fundamentally from the urban smoke," said Nga Lee “Sally” Ng, a study author. Using new technology to measure particles in the smoke, researchers were able to "actually see the very detailed component of these particles that we are breathing."
The study, published Wednesday in the journal Science Advances, highlights a gap in research concerning the distinct chemical makeup of smoke from urban versus wildland fires. As wildfires grow larger, more intense, and more frequent due to climate change, and as urban areas expand, more fires are occurring in cities. This trend necessitates an adaptation in monitoring and emergency response strategies, according to the researchers.
Ed Avol, professor emeritus at the Keck School of Medicine at the University of Southern California, who has extensively researched the health effects of air pollution, stated that the findings of significant heavy metal exposure are consistent with expectations but were surprising to document. He noted that a key advantage of this study was its ability to take hour-by-hour measurements, unlike typical air quality monitoring that occurs only a few times a week.
The two wind-driven fires that ignited in Altadena and the Pacific Palisades in January 2025 resulted in at least 31 deaths and destroyed over 16,000 structures, primarily homes. Millions of people were exposed to the smoke. Smoke from wildfires across the United States has been linked to tens of thousands of deaths annually. A previous study on the 2023 Maui wildfires found that metal mixtures in the residual smoke, ash, and debris were harmful to the lung health of nearby residents long after the fires subsided.
During the Los Angeles fires, researchers utilized instruments from the Atmospheric Science and Chemistry mEasurement NeTwork (ASCENT), a relatively new network of air quality monitors across the U.S. that measures the size and chemical composition of fine particulate matter (PM2.5) in real time. Ng stated that ASCENT is the first U.S. measurement network capable of real-time chemical composition analysis of airborne particles.
This real-time data allowed scientists to identify the chemicals present in the smoke, track changes in its composition throughout the fire's duration, and differentiate emissions from burning structures. Based on this, they estimated the number of people exposed to heavy metals, particularly lead, during the initial days of the fire.
Comparing data from the Eaton Fire plume with the chemical makeup of smoke from a 2024 wildfire in Riverside County that burned mostly vegetation, the Los Angeles fire plume showed significantly higher concentrations: 50 times more lead, three times more arsenic, and seven times more halogens. At the peak of the burning, an estimated 7 million people were exposed for 24 hours to lead levels more than 80 times the typical background concentration.
Lead is a known neurotoxin that can impair child development, reduce IQ scores, and increase blood pressure in adults. Scientists are still investigating the health effects of short-term, high-level exposure to lead. Arsenic exposure is linked to various types of cancer.
Yaguang Wei, an assistant professor at the Icahn School of Medicine at Mount Sinai who researches air pollution and wildfire smoke, commented that the study's findings were not surprising. He noted that when fires burn residential properties, they combust materials like lead paint, plastics, and electronics, all of which release toxic substances.
Wei suggested the study has "important policy implications," recommending that governments increase investment in wildfire monitoring, public alerts, and prescribed burns to better protect public health during such events.
Avol described the study as a reminder of an "increasingly dangerous new world" where urban and wildland environments intersect and increasingly face fires. He urged caution and a focus on minimizing exposure, acknowledging that completely eliminating materials containing metals and halogens from homes is not feasible.