Residents were asked to close their doors and windows after a "major" acid leak caused a vapour cloud over an east Hull dock.
Humberside Fire and Rescue said crews were called to a leak in a tank containing 580 tonnes of hydrochloric acid at the King George Dock late on Monday.
The acid caused a vapour cloud as 50 firefighters were called to deal with the incident, near to the River Hull.
They said the leak did not lead to any evacuations and was at the east end of the dock with waste land to the side of it.
The fire service said crews had contained the leak by 4.20am and were scaling down the operation.