For many years, scientists have wondered what makes the coal in one rural Chinese county so deadly. Lung cancer mortality rates among non-smoking women living in Xuan Wei are the worst in the world and up to 20 times the Chinese average.

The culprit, according to a new study: silica in a seam of coal formed during Earth’s worst extinction event, 250 million years ago.

Geologist Bob Finkelman of the University of Texas at Dallas believes a major volcanic episode in Siberia at the Permian-Triassic boundary radically altered the geochemistry of the atmosphere, causing highly acidic rain.

As the rain fell, it killed life on the surface of the Earth and even dissolved rocks, causing silica to be carried by groundwater into the peat that would eventually become coal.

That particular seam of coal has been used indoors for heating and cooking in Xuan Wei, often in basic cook stoves without proper ventilation.

The high fatality rates in Xuan Wei are highly localized because there’s so much variation in the world’s coal, even from one region to the next.

“What we’re saying is that the geologic and climatic events that nearly extinguished life 250 million years ago is still having an impact because its imprint is in the coals that the people are using,” says Finkelman.

In effect, Finkelman explains, that mass prehistoric die-off is causing human fatalities today.

“They are inhaling this material with nanoquartz that was precipitated 250 million years ago, and in a sense it’s extinguishing life in the community.”