Officials detected trace amounts of mustard vapor inside a storage igloo that houses artillery shells at Blue Grass Army Depot in Richmond.
The leak poses no threat, officials said in a news release.
The mustard vapor was detected Tuesday morning during routine weekly monitoring, the release says.
Workers have started the isolation process. Once the leaking munition has been located, it will be packed into a leakproof container and moved to a separate igloo, the release said.
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The Madison County Emergency Management Agency and Kentucky Emergency Management Agency were notified of the leak.