The U.S. Chemical Safety Board has dispatched a four-member team to investigate the cause of the release of a toxic chemical at an oil refinery last week near Joliet.

Two employees — one of them in critical condition — were transported to the hospital in the accident. Their conditions today are not available.

ExxonMobil said a propane leak resulted in the hydrogen fluoride release at its plant in Channahon. The unit’s water deluge system, which is designed to contain airborne hydrogen fluoride, was activated and the alkylation unit was shut down.

Will County Emergency Management Agency Director Harold Damron said there was no cause for alarm outside the building, “based on everything we know.” The chemicals “were released through the flare stack, which essentially burns up everything,” he said.

According to the Centers for Disease Control and Prevention, hydrofluoric acid, which is created when hydrogen fluoride combine with water, is used to make high-octane gasoline. It goes easily and quickly through the skin and into the body’s cell tissues.

Breathing hydrogen fluoride can burn lung tissue and cause swelling and fluid accumulation in the lungs. Skin contact may cause severe burns that develop after several hours and form skin ulcers.

John Bresland, chairman of the U.S. Chemical Safety Board, said the incident is the third in five months at a U.S. plant.

“We are concerned about the three apparent releases of hydrogen fluoride from refinery alkylation units in Pennsylvania, Texas, and now Illinois that have been reported since March 2009,” Bresland said in a release. “Because of its high toxicity, any loss of primary containment for hydrogen fluoride is a serious matter.”

The other releases were at the CITGO refinery in Corpus Christi, Texas, on July 19 and at the Sunoco refinery in Philadelphia on March 11. Chemical Safety Board investigators are currently at the CITGO Corpus Christi refinery examining that incident.

The CSB is an independent federal agency charged with investigating industrial chemical accidents.