Investigations are underway into a chemical incident at the Royal Mail depot at Moorside, Colchester.
Reports were received just after 3.30am that there was a strong smell of chemicals and that 12 people were unwell.
Firefighters at the scene said the chemical was thought to be diesel that had spilt into the River Colne, with witnesses describing a sheen on the water.
The casualties received medical treatment and were allowed to go home, a police spokesman said.
Divisional Officer Gary Jeffery said: “Crews have carried out investigation work and no trace of any chemical has been detected. One possible cause could be diesel in the River Colne and the Environment Agency is currently establishing any link to the incident.”
All roads are now re-opened.
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