A chemical release at Pemex's 312,500 barrels per day Deer Park refinery in Texas has killed two people, the county sheriff said.
Up to 35 people at the refinery were treated at the plant or taken to area hospitals for exposure to potentially deadly hydrogen sulfide gas, county emergency officials said.
A state highway adjacent to the refinery was closed for several hours, and Deer Park residents were told to turn off their air conditioners and shelter in place.
Highways have since reopened and shelter-in-place orders have been lifted.
Contractors working at the refinery apparently opened a line containing hydrogen sulfide and died from exposure to the gas, two people familiar with the plant's operations said on condition of anonymity.
Harris County Sheriff Ed Gonzalez said the flange may have opened, allowing gas to be released.
Gonzalez also said it could take seven to eight hours for authorities to enter the area where the gas was released.
Pemex said the leak was reported around 4:40 p.m.
The company said in a statement that it had shut down the refinery's 92,000 barrel-per-day coker and hydrotreater after releases began.
Officials said contractors were working on a sulfur recovery unit (SRU) at the refinery when the release occurred.
The SRU converts hydrogen sulfide from cokers and hydrotreaters to elemental sulfur.
According to the Occupational Safety and Health Administration (OSHA), hydrogen sulfide is used in refineries or produced as a byproduct.
Hydrogen sulfide gas can be rapidly fatal if inhaled in very high concentrations.
Pemex reported on the same day that it was carrying out maintenance work that could cause a flare. The company announced Thursday night that it would use safety flares at its refinery following the incident.
