The state Department of Labor and Industrial Relations is investigating the leak of a potentially deadly gas at the Sand Island Wastewater Treatment Plant last month.

No workers at the city facility were injured during the leak of hydrogen sulfide and methane during the week of July 6-11. Two workers have filed worker's compensation claims in connection with the leak.

The leak was detected after employees in a neighboring building complained of a foul odor.

Workers discovered the gas leaking from a manhole cover between buildings and remedied the situation in an hour. The city also brought in an outside consultant to evaluate repairs and air quality.

The leak occurred after water in a plumbing p-trap — which keeps the gases from backtracking — dissipated to a point that allowed the gas to escape, according to the city.

Investigators with the state Department of Health's Clean Air Branch found "no evidence of hazardous conditions or that the situation was mishandled," the city said.

There is no way to tell how long the leak was occurring, the city said.

Markus Owens, spokesman for the city Department of Environmental Services, said there were no reports of employees throwing up, passing out, requiring hospitalization or experiencing any other adverse reactions because of the leak.

"If someone were to inhale that (gas), they would have passed out; if they inhaled it at the highest level, they would have been dead," Owens said.

A city subcontractor, Synagro Wastewater Treatment Inc., is responsible for the system that maintains the gas.

Synagro plant manager Kenny Huy declined comment yesterday, and said he could respond only to written questions.

Over the last four years, the company has had similar incidents at treatment plants on the Mainland, according to the U.S. Occupational Safety and Health Administration.

Brief exposures to high concentrations of hydrogen sulfide (greater than 500 parts per million) can cause a loss of consciousness and possibly death. according to the U.S. Agency of Toxic Substances and Disease Registry.

Exposure to low concentrations of hydrogen sulfide may cause irritation to the eyes, nose, or throat and may cause difficulty in breathing for some asthmatics, according to the agency. Hydrogen sulfide is produced by human and animal wastes and remains in the atmosphere for about 18 hours.

Methane gas is a greenhouse gas and a principal component of natural gas. It is formed and released to the atmosphere by biological processes occurring in anaerobic environments, according to the U.S. Environmental Protection Agency.