Workers were turned away from the site on Friday morning
Three workers were hurt when a toxic gas cloud escaped at a plant on the banks of the River Humber.
Described as "serious", the incident took place at 0030 GMT at the Cristal Global plant, North East Lincolnshire.
The men, who suffered burns and toxic inhalation, were taken to hospital after titanium tetrachloride escaped at the plant near Grimsby.
Humber Coastguard confirmed all shipping in the area was halted for several hours.
One of the injured men was later transferred to a specialist burns unit at Pinderfields Hospital in Wakefield, West Yorkshire, where he is said to be in a critical but stable condition.
The other men were taken to Diana Princess of Wales Hospital in Grimsby.
The emergency services released the site back to the company at 1330 GMT.
Police said air samples had been taken on the north bank of the River Humber and were reading normal.
Phil Leake, from Humberside Fire and Rescue Service, said the escape occurred after an explosion in a chemical vessel at the factory.
"We've currently got an officer mobile on the north bank and he's assessing the air samples in the area to see if the gas cloud has dissipated.
"There's no immediate danger to the public but we're still advising people in the immediate area to keep their windows and doors closed as a precaution."
He added: "The advice that the ambulance service has given us is that if anyone does feel unwell in the immediate vicinity of the incident... they should contact NHS Direct."
At least 10 fire crews attended the scene, with firefighters using a blanket of foam to contain the spillage.
The emergency services said up to 100 people could have been working at the plant overnight.
The fire service said the number of crews had been scaled down but firefighters would remain on site.
Humber Coastguard said the 'no-sail order' in the estuary was lifted at around 0500 GMT.
Cristal Global, situated in Stallingborough, specialises in making titanium-based products.
A spokesman for the site said the company was still investigating what happened.
The Health and Safety Executive is expected to start an investigation.