Hydrofluoric acid (HF) leaked from an alkylation unit at BP's 406,570-barrels-per-day refinery in Texas City, Texas on Tuesday morning, triggering alarms in the plant and warnings to area residents, company and city officials said.

No injuries were reported at the refinery, the fifth-largest in the United States, or in the surrounding community, the officials said.

"We have a small leak of hydrofluoric acid at the refinery," BP spokesman Tom Mueller said shortly after 10 a.m. local time (1500 GMT). "Water is being sprayed on it. We expect to secure the leak shortly."

Water cannons surrounding alkylation unit 3 were triggered when monitors detected the leak, Mueller said. Workers in the refinery not battling the leak were ordered to shelter in place.

The cause of the leak has not been determined, and Mueller declined to say whether gasoline production had been affected.

Hydrofluoric acid can damage skin, eyes, lungs, bones and the heart in humans. Exposure can be fatal, and HF acid can form into vapor clouds that are able to spread over large distances.

Water cannons are used to prevent a vapor cloud from forming. The HF clings to water droplets and falls to the ground. Workers trying to secure a leak wear protective clothing.

Environmental groups and the United Steelworkers union, which represents most U.S. refinery workers, have campaigned for HF alkylation units to be replaced with units using sulfuric acid in order to prevent HF vapor cloud exposure.

Texas City officials notified residents of the leak shortly before 9 a.m. local time. No orders for residents to remain indoors had been issued as of 10:30 a.m.

BP's Texas City plant was the site of the worst refinery disaster in the past decade when 15 workers were killed and 180 others injured by an explosion on March 23, 2005.

BP's term of probation for a violation of federal environmental law in the 2005 explosion ended this month.

An alkylation unit uses refining byproducts to make octane-boosting components that are added to gasoline.

The U.S. Chemical Safety Board and U.S. Occupational Safety and Health Administration are investigating an HF acid leak earlier this month at Citgo Petroleum's 163,000-bpd Corpus Christi, Texas refinery.

The Chemical Safety Board called a 2009 explosion on the HF alkylation unit at Citgo's Corpus Christi plant a near-miss of widespread exposure of HF acid in a community surrounding a refinery.