CHELSEA (CBS) -- An MWRA spokesperson said an alarm detecting hydrogen cyanide went off early Tuesday morning at a sewage treatment facility, prompting a large hazmat response.
There were several firetrucks outside the facility on Marginal Street.
Chelsea Fire Chief Leonard Albanese said the source of the chemicals was not known, but that there was no threat to the public.
"The firefighters on the entry team were able to isolate the area where the hydrogen cyanide was located," Chief Albanese said. "With that, MWRA is flushing that area and continuing to ventilate."
An MWRA spokesperson said a gas meter went off inside the facility around 4 a.m. An MWRA operator didn't find anything dangerous, but said the meter was still flashing.
The Chelsea Fire Department said a detail firefighter detected hydrogen cyanide and hydrogen sulfide around 6:30 a.m., prompting the hazmat response.
Chief Albanese said 28 people were inside the facility when the alarm went off, and they were all safely evacuated.
A Chelsea firefighter was taken from the scene for a stress-related illness, but Chief Albanese said it wasn't related to exposure to the detected materials.
The MWRA said the building is currently under construction and is minimally staffed.
The facility was ventilated, and Chief Albanese said tests for the chemicals found results near zero percent.
The MWRA spokesperson said it could have been a false reading.
Chelsea Police Chief Brian Kyes said traffic is being diverted away from the area, and asked the public to find alternate routes.
The Boston Fire Department assisted and cleared the scene around 9:45 a.m. Chelsea Fire was still on the scene.
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