Schools and colleges were forced to shut today after a chemical leak caused hydrochloric acid to form over a town.
Firefighters told residents in Thurrock, Essex, and the surrounding areas to stay indoors and keep their windows closed after investigating the acidic smog.
Fumes started to emerge from a building at an industrial unit in the town on Monday night and lingered into the air today.
"We understand that the cloud includes an amount of hydrochloric acid," said Neil Fenwick, area manager at Essex County Fire and Rescue Service.
"A 100-metre exclusion zone was created at the site as a precautionary measure.
"We now know that these fumes were caused by a tank that has failed, the leak is contained to within the site.
"Firefighters are now safely managing the leak and this process is expected to continue for some time."
South Essex College closed today and schools were also told to shut.
"We have deployed Air Quality Monitoring equipment overnight to support the Health Protection Agency.
"This is a regulated site, which means it is subject to regular monitoring and inspections."
Employees who work at other factories on the industrial site told of the scenes last night.
Chris Bark, a 52-year-old technician, said: "I was coming on to the site and I saw fire engines, I thought that it must be from a crash.
"We started getting reports from other workers that they couldn't get in because the roads were shut by police. A little bit later management told us that we needed to lock the factory down.
"No one was allowed to leave the site and no one was allowed on to the site.
"There is a big cloud coming from the incident on to our site. We were ordered to evacuate."
