From studying stardust to investigating Los Angeles wildfires, François Tissot made the transition naturally. Inside a clean room at Laboratory 216 of the California Institute of Technology (Caltech), the French professor set aside his meteorite samples for a while to make space, in this ultra-filtered air environment, for wipes covered in the black dust left behind by the smoke from the January 7, 2025 fires.
Since the fires that killed 31 people and destroyed 16,000 homes in the heart of Los Angeles, this cosmochemist – whose doctoral work has advanced the understanding of the origins of the solar nebula – has been studying urban dust for traces of arsenic, cadmium and lead. These are all heavy metals hazardous to health, which the scientist suspects settled in the wake of smoke from the megafires, especially in homes spared by the flames – on average, one out of every two in the affected areas. At present, there are no state or federal regulations governing this new terrain of urban megafires, nor any definition of what constitutes a “habitable” house in terms of post-fire pollution.
François Tissot in his laboratory at the California Institute of Technology, Pasadena, California, January 4, 2026. STELLA KALININA FOR LE MONDE
A few days after the disaster, Tissot at first acted out of curiosity, taking early samples from window sills and inside 100 offices in Caltech’s geosciences building. The test results showed that, even 10 kilometers from the fire, concentrations of heavy metals far exceeded authorized standards. Sitting in his university office on Sunday, January 4, the 39-year-old researcher recalled the online seminar organized nearly a year ago with other specialists in water and air pollution: “2,000 people logged in and, in the chat, many asked us to come test their homes.”
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