The Marathon Petroleum Company refinery in Catlettsburg, Ky.
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For the second time this year, a cloud of highly toxic acid used at about a third of the nation’s oil refineries has escaped at a Marathon Petroleum Company unit, sending workers to the hospital.

Earlier this year, the site of the accident was Canton, Ohio, where refinery workers had to knock down the acid with water cannons and one worker was hospitalized. Tuesday night, it was Catlettsburg, Ky., where a small vapor cloud formed and three workers were hospitalized.

Across the country, at least 16 million Americans live in the potential path of the toxic substance – called hydrofluoric acid, or HF – if it were released in an accident, an investigation by the Center for Public Integrity’s iWatch News found.

Some 50 refineries use the acid, despite the availability of a safer alternative.

In Catlettsburg, a worst-case release could threaten 300,000 people living near the refinery. Marathon spokesman Shane Pochard said the release Tuesday was small, and, “There was no off-site impact.”

The three exposed contract workers were treated and released, and Pochard said he couldn’t comment on the nature of their injuries.

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