Two people were believed to have died and five were being treated for injuries on Thursday after a chemical release at Pemex's 312,500-barrels-per-day Deer Park oil refinery in Texas, the Mexican national oil company said.

A contractor working at the refinery is believed to have opened a line containing hydrogen sulfide and was killed by exposure to the deadly gas, according to two people familiar with the plant's operations who spoke on condition of anonymity.

The county sheriff's office confirmed one person was dead, another had been taken by Lifeflight and several others were injured.

The leak was reported around 4:40 p.m. CDT (2140 GMT), causing emergency protocols to be activated and the coking and hydrotreatment units were proactively stopped, Pemex said in a statement.

Hydrogen sulfide is used by or produced as a byproduct in industrial processes including petroleum production and refining, according the U.S. Occupational Safety and Health Administration (OSHA).

Hydrogen sulfide gas can lead to various health issues based on the amount inhaled and the duration of exposure. According to OSHA, inhaling very high concentrations can quickly lead to death.

"The Harris County Fire Marshal's Office and Harris County Pollution Services are on the scene to assist and monitor the air," Harris County executive Lina Hidalgo said in a statement posted on X.

The city of Deer Park issued a shelter-in-place for all its residents. A state highway on the south side of the refinery was shut by the shelter-in-place order.

Earlier in the day, Pemex had reported it was performing activities that might cause flaring.