After some recent intense wildfires in Northern California, scientists tested samples of singed soil and were disturbed by their findings: It was laden with a cancer-causing metal called hexavalent chromium.

Scientists think the heat of severe wildfires can transform a benign version of the metal, which is found commonly in California soil, into a notorious carcinogen, according to new research published in the journal Nature Communications on Tuesday.

As climate change intensifies wildfires, scientists are trying to figure out just how dangerous their smoke can be for human health. Researchers have found dangerous metals — from burned-out cars, houses and farms — in previous fires. The new finding adds a surprising twist to the growing body of research and suggests that wildfires burning in natural areas could be pumping smoke containing a toxic metal into the atmosphere, too.