Four Aberdeen Proving Ground employees were taken to a local hospital Thursday after a report of a chemical leak, the third incident in about a week during which workers were at risk of exposure to lethal substances at the Army base in Harford County.

Though none of the workers was injured, the incident was the second at the Edgewood Chemical Biological Center, a 1.5 million-square-foot research and engineering facility within APG for chemical and biological defense, prompting the facility's director to pledge a thorough review of safety procedures.

Thursday's incident involved phosgene, a toxic industrial chemical employed as a choking agent in World War I and used to make plastics and pesticides.

Army officials say the three incidents are unrelated, and their timing coincidental. Still, the accidents have roused members of the surrounding communities, which had largely been calm since the base cleared a 1,600-ton stockpile of mustard agent last year.

"That nobody has gotten hurt in the past week -- that's luck, in my opinion," said Arlen Crabb, an environmental activist and member of the Restoration Advisory Board at APG.

As part of a full-disclosure policy, base officials send alerts detailing accidents that require an emergency response. There are more than 65 tenants on the base, but of the handful of incidents disclosed in the past year, three have occurred at ECBC.

In March last year, two employees were taken to a hospital for possible exposure to phosgene after a morning experiment mistakenly coincided with maintenance work on the building's air filter system.

Then last week, a power outage at an ECBC laboratory temporarily shut down the powerful fans that protect researchers from dangerous vapors. Employees working with mustard agent, GB, hydrogen cyanide and cyanogen chloride, were treated on the base and released.

A similar incident the day before at U.S. Army Medical Research Institute of Chemical Defense sent two workers to a local hospital for observation.

APG is not the only major military facility in the region facing recent concerns about dangerous materials. It was reported this week that the Army's biological weapons defense laboratory at Fort Detrick in Frederick probably had multiple episodes of anthrax contamination as workers strove to process a flood of samples sent there for testing in 2001 and 2002, according to an internal report.

The 361-page report, a copy of which was obtained by the Frederick News-Post, contains previously undisclosed details about the sometimes-sloppy practices that allowed anthrax spores to es cape from biosafety containment labs at the U.S. Army Medical Research Institute of Infectious Diseases. No one was hurt by the released spores.

Thursday's mishap at APG was attributed to corrosion within the Thermos bottle-sized container used to store the phosgene, said Jim Zarzycki, director of the ECBC. A subsequent investigation will help determine whether additional precautions must be implemented, he said.

"We are very concerned about our workforce," Zarzycki said. "We work with very dangerous materials, and we can only do our job if we know how to work safely."

But Crabb noted that corroded storage containers have led to explosions in the past. He called for research facilities to consider policies that would require substances to be rotated among containers to avoid corrosion.

Residents have long had a tenuous relationship with the military base. Lauded as Harford County's economic engine, APG's distinction as the region's largest employer will be bolstered in the next several years as a base realignment plan shifts thousands of jobs there.

But over the years, residents and military officials have often sparred over concerns about to the base's dangerous work, from reports of a leaking stockpile of mustard agent to unexploded ordnance buried within a half mile of homes and schools.

"It's happening too soon and too much," said Judy Blomquist, a Havre de Grace resident and president of a grassroots activist group called Friends of Harford. "When accidents continue to happen, we would hope that there would be some accountability from the command forces and from the people who are in charge when things went wrong."

At about 8 a.m. Thursday, laboratory workers were conducting experiments with phosgene when one of them smelled the gas, Army officials said. Operations were shut down and emergency personnel later found the valve on the phosgene cylinder was not completely closed as the bottle moved from one part of the lab to another.

No phosgene was detected in the room, and any chemicals that escaped the container would have been trapped by laboratory filters and the building's environmental containment system, said APG spokeswoman Pat McClung. Four workers were sent to Upper Chesapeake Medical Center in Bel Air for evaluation. None displayed symptoms.

A spokesman for the Maryland Department of the Environment said the agency believed the recent incidents had been handled appropriately, and agreed with APG's assessment that there have been no public health threats