In Dunn County, North Dakota, the roads are paved with a unique danger. Over 300 miles of them are covered with gravel taken from the local North Killdeer Mountains. This rock is rich in a mineral called erionite that behaves not unlike asbestos. Both cause cancer, but according to animal studies, erionite is anywhere from 200 to 800 times more effective at it than its more famous counterpart.

Dunn County’s erionite gravel releases small brittle fibres into the air when lightly disturbed. They’re released by wheels driving overhead, the footfalls of pedestrians, or even the gentle scrapes of brooms and rakes. Once airborne, the fibres can find their way into the lungs of passers-by, accumulating in the surrounding cavity