Six Anglian Water workers were taken to hospital after mixed chemicals produced noxious fumes at a Southend depot.
An Essex Police spokesman said the incident happened at the company's works in Stock Road, Southend-on-Sea, on Saturday.
At around 8.30am police issued a precautionary warning to householders in the Shoeburyness area to stay indoors and keep windows shut to give any fumes time to disperse harmlessly.
No-one other than the six workers is believed to have been harmed after the scare.
Police said the incident - which they labelled as comparatively low-risk - was caused when two chemicals were accidentally mixed. The fire brigade attended and damped down to dilute the chemicals.
Strong winds helped to disperse any fumes and the all-clear was issued at about 12.15pm.
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