Hornchurch firefighters went to Colchester on Friday of last week to help Essex crews deal with a suspected chemical incident.
The Fire Rescue Unit left Hornchurch to provide support for colleagues at the incident in The Crescent.
A noxious smell was noticed in the area, an Essex County Council building was evacuated, and a 400m cordon was set up around it.
Ambulance crews assessed 29 people at the scene, and three were taken to hospital for further examination.
Firefighters were unable to find any hazardous material in the area.
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