As flames incinerated whole blocks in Southern California, fierce winds pushed dark, speckled ash through Rossana Valverde’s door frames, windows and vents. Her home stood a short drive from the worst destruction caused by January’s Eaton Fire, but she had gotten lucky: Apart from a singed tree, her property appeared unscathed.

Yet the acrid stench in the bungalow she shared with her husband suggested otherwise. The remains of other people’s homes now permeated hers.

As the Los Angeles wildfires died out in January, firefighters trekked through the burn zones to take stock of the destruction. For every home they found leveled, they counted another still standing. The structures looked fine from the outside, but ash and oily soot often coated the floors and furniture, while invisible chemicals burrowed into clothes, blankets and even walls.