The chemical that leaked from a tanker truck in a city in Oklahoma is common but dangerous, and other leaks over the years have led to evacuations and deaths.

Hundreds of people were evacuated and dozens hospitalized because of the anhydrous ammonia leak from a tanker truck Wednesday night in Weatherford, about 70 miles (110 kilometers) west of Oklahoma City.

Here’s what to know about the leak and the chemical.

What happened in Oklahoma?

The tanker truck parked outside a Holiday Inn Express in Weatherford was spewing anhydrous ammonia. The driver had parked there to get a room for the night.

The cause seemed to be a mechanical failure on a valve or a faulty seal, the police chief said. Authorities have not said how much of the chemical was leaked.

Up to 600 people were staying in a s

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