By any measure, the fires that tore through Northern California were a major disaster. Forty-two people are dead, and 100,000 are displaced. More than 8,400 homes and other buildings were destroyed, more than 160,000 acres burned—and the fires aren’t all out yet. That devastation leaves behind another potential disaster: ash. No one knows how much. It’ll be full of heavy metals and toxins—no one knows exactly how much, and it depends on what burned and at what temperature.

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