HERMISTON, Ore. -- A monitoring crew found trace amounts of GB, or sarin, nerve agent vapors Tuesday during a check of a storage structure at the Umatilla Chemical Depot.

The trace discovery was not a threat to the public, depot spokesman Jim Hackett said, and the nerve agent did not escape the structure, or "igloo," in which it was stored.

Hackett said munitions in that igloo had previously leaked and are double-packed in larger containers for additional protection.

He said the igloo has a "passive" filter system that prevents vapor from leaving and an added filter would be installed.

He said it was the first gas leak detected since November and that leaks have diminished since workers finished incinerating the last of the M55 GB sarin rockets, which were the worst leakers.

Currently, he said, 155 mm sarin artillery shells are being incinerated.

When the treaty-mandated disposal program started in 2004 the depot had about 12 percent of the national stockpile of chemical weapons.

Hackett said plans are to complete destruction of the sarin munitions this summer, then move on to VX, another nerve agent, and finally to mustard gas.

Sarin is a deadly, colorless, odorless, tasteless liquid that vaporizes quickly. It was used in at least two terrorist actions in Japan in the 1990s and probably was used in the Iran-Iraq war of the 1980s. It was developed as a pesticide in Germany in 1938.

About 100,000 of the original 220,000 chemical munitions remain at the depot, he said. Contractors hope to have destroyed the last of them by 2010-2012.