Emergency vehicles on the Springwood Industrial Estate were responding to a false alarm.
This photo shows several fire vehicles and an ambulance at the scene just off Rayne Road, in Braintree.
One witness said they first arrived at around 4pm this afternoon (June 11).
Traffic is heavier than usual along Pod's Brook Road and Rayne Road approaching the industrial estate as a result.
The Essex County Fire and Rescue Service said it was a false alarm.
A spokesman for East of England Ambulance Service said: "We have sent a number of our resources, including two Hazardous Area Response Team vehicles to reports of a chemical incident at Rayne Road, Braintree.
"At this time (16.45pm) crews are still on the scene."
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