An Indiana County power plant already facing a series of pollution lawsuits was the scene of a steam pipe blast that injured six workers on Thursday.

A 6-inch pipe containing steam under high pressure burst at 7:45 a.m. on the sixth floor of the plant's Unit One, said Charley Parnell, a spokesman for Edison Mission Group, the parent company of the plant's operator, the EME Homer City Generating LP.

The break in the pipe tripped the unit's automatic safety systems, shutting the unit down. The other two units were operating normally, he said.

The rupture prompted an explosion of steam but did not cause a fire. The extent of damage has yet to be determined. EME Homer City is investigating, Mr. Parnell said.

Firefighters from the Coral/Graceton Volunteer Fire Department and the Homer City Fire Department were called to clear a landing site near the plant for three medical helicopters.

Helicopters transported three workers to West Penn Hospital, where a hospital spokeswoman said the men were in fair condition in the burn unit. The three other workers were driven to the Indiana Regional Medical Center, where they were treated and released, a hospital spokeswoman said.

Firefighters said at least one of the six workers had burns on his face and hands.

Katy Gresh, spokeswoman for the state Department of Environmental Resources, said the department's emergency response unit didn't get involved in the incident because no pollution-control equipment was damaged.

The massive plant, with the nation's largest smokestack, has three generating units that burn coal to heat water and make steam, which turns a turbine to produce electricity.

All of the employees at the 1,884-megawatt plant were evacuated and accounted for, including the six who were injured. Mr. Parnell said he didn't know exactly how many people were working when the pipe burst, but the plant employs 255 over three shifts.

Soon after the accident, EMG Homer City alerted the federal Occupational Safety & Health Administration.

"They opened the investigation this morning and have up to six months to complete the investigation," said Joanna P. Hawkins, spokeswoman for the U.S. Department of Labor's office of public affairs in Philadelphia.

Mr. Parnell said the company takes safety seriously, pointing out that the plant had gone 856 days without an injury that caused an employee to miss a day of work.

"Our safety record is exceptional," he said. "Nothing we do matters unless our workers go home safely each day."

Ms. Hawkins said the company's safety record would be determined during the investigation, "so at this time we do not have that information."

On Jan. 4, the U.S. Environmental Protection Agency filed a civil lawsuit claiming past and present corporate owners and operators, including two past owners -- the New York State Electric and Gas Corp. and Pennsylvania Electric Co -- operated the plant without proper permits. They also are accused of modifying plant operations without using the best available technology, as the law requires. The violations, the EPA suit said, have contributed to premature mortality and asthma attacks and have generated acid rain among other "adverse effects in downwind communities and natural areas."

The DEP and the New York attorney general previously joined the lawsuit, and New Jersey filed suit Thursday.

On Jan. 7, two residents, Scott and Maria Jackson of Homer City, filed a separate lawsuit in U.S. District Court in Pittsburgh against the plant claiming its emissions are adversely affecting their health and property values.

Peter Villari, a Philadelphia attorney representing the Jacksons, said the lawsuit would seek class-action status, which would open it to other plaintiffs who feel they and their properties have been damaged by plant pollution. If a judge certifies the citizen lawsuit as a class action, the suit says, it eventually could include "thousands of Pennsylvania residents, homeowners and individuals."

The Post-Gazette's "Mapping Mortality" series in December on air pollution in the region noted that the plant in 2009 emitted 610 tons of fine particles, 10,511 tons of nitric oxides, and 101,337 tons of sulfur dioxides, among other pollutants. The EPA said the sulfur dioxide emissions alone made the plant "one of the largest air pollution sources in the nation."