HERMISTON -- Trace amounts of mustard chemical agent vapor were detected inside a Umatilla Chemical Depot storage igloo Tuesday.

As the weather warms, the mustard containers thaw and are more likely to leak. Mustard is a blister agent developed as a weapon.

The leak was detected during routine weekly monitoring and posed no danger to the public or environment, according to depot officials.

The igloos have a passive filtration system that prevents chemical agent vapor from escaping outside the structures. When a leak is discovered, a powered filtration system also is installed.