CHELSEA, Mass. Officials were called to the MWRA Headworks building just before noon Monday after high levels of hydrogen sulfide were detected.
One floor of the building, located in Chelsea, had to be evacuated until the air levels returned to normal. No one was hurt.
Last week, traces of hydrogen cyanide were detected in a sewage treatment area beneath the facility.
The chemical concentration was not strong enough to pose danger to any workers.
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