Marathon Petroleum Corp. is investigating what caused a toxic chemical release that injured two workers on May 4 at a Texas City oil refinery, a spokesman said, as the U.S. industrial safety watchdog also began looking into the incident.

A release of hydrofluoric acid at the 585,000-bpd refinery on Texas’ Gulf Coast prompted workers and nearby residents to take shelter indoors with doors and windows closed and air conditioning turned off before the leak was contained.

The U.S. Chemical Safety Board (CSB) described the release as an undetermined amount of hydrogen fluoride, a toxic chemical that turns into a ground-hugging vapor cloud at room temperatures and can lead to severe health problems, even death. It was released from piping near an alkylation unit, said Hillary Cohen, a CSB spokeswoman.

Marathon has confirmed the release of hydrogen fluoride (HF) and said it had been contained and diluted by 1.1 million gallons of water. Two workers were injured. One is expected to be released today and the other is being treated for eye irritation, said Marathon spokesman Jamal Kheiry.

“The automated response system worked as intended, deploying water cannons that prevented the estimated 90 lbs (40kg) of HF from moving beyond the unit,” said Kheiry. Federal and state regulators have been notified, he said.

Hydrofluoric acid is used as a catalyst in alkylation units to convert refining by-products into octane-boosting components added to gasoline.

“Subsequent testing of the water in the vicinity indicated no detectable levels of acid, and all wastewater from the site drained to the facility’s wastewater treatment,” Kheiry said.

The leak from the 31,500 barrel-per-day (bpd) alkylation unit 3 began at about 3:30 p.m. local time (2030 GMT) on May 4, people familiar with plant operations told Reuters, adding that the refinery was later operating normally.

The company has said its air monitoring did not detect any impact of the release outside its facility.

Reporting by Erwin Seba; editing by John Stonestreet, Marguerita Choy and Lincoln Feast.

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