HERMISTON (AP) - About 500 workers at the Umatilla Chemical Depot were ordered to wear their gas masks when vapor from a chemical weapon was detected outside one of the igloos where the munitions are stored.

No one was injured or exposed during Monday's incident, depot officials said.

Emergency operations centers in Umatilla and Morrow counties and Benton County in Washington were notified of the incident, as was the Confederated Tribes of the Umatilla Indian Reservation and the state.

The low reading of GB Sarin chemical agent was detected outside an igloo where 500-pound bombs are stored. Subsequent tests next to and farther away from the igloo detected nothing, said depot spokeswoman Mary Binder.

Many of the weapons are 60 years old and the munitions are deteriorating. But all the weapons are housed in protective bunkers that are monitored regularly.

"To the best of our knowledge, this is the first time we've detected vapors outside the structure," Binder said Tuesday morning.

The depot stores about 12 percent of the nation's chemical weapons supply. The United States has agreed through an international treaty to destroy its chemical weapons. An incineration complex has been completed at the depot and is undergoing testing.

Incineration is expected to begin next year.