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Two workers were injured at the Tesoro Corp. refinery near Martinez, Calif., on Monday, March 10, according to reports. The two contractors were carrying out maintenance work at a processing unit at the refinery when sulfuric acid spilled on their necks, the San Francisco Chronicle reported.

Maria Duazo, a Contra Costa County hazardous materials specialist, explained that both contractors were wearing protective clothing and although they were taken to a nearby hospital for a treatment, their injuries were not serious.

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The exact circumstances of the sulfuric acid spill were not known, but employers in the refinery told reporters that the spill occurred while the two contractors were working on a pipeline. Although burns were initially prevented by their garments, some of the acid drained onto them after they took decontamination showers later, reports said. In an email to the San Francisco Chronicle, Tina Barbee, spokeswoman for Tesoro, confirmed that the workers had been involved in planned maintenance but did not provide any further details.

This incident raises a series of questions because another incident with sulfuric acid occurred at the same processing unit in the refinery just a month ago. State and federal investigators are still looking into the previous incident, but the U.S. Chemical Safety Board has criticized Tesoro for not cooperating with federal regulators on the investigation and alleged that the incident might be more serious than the refiner claimed it was.