Octel extracts bromine from seawater at the Amlwch site. The company is owned by Great Lakes Chemical.
The incident occurred at 4pm on 19 July. Clouds of corrosive orange-brown bromine fumes wafted across the nearby village and police warned residents to stay indoors and close their windows.
The Octel works is a top-tier "major hazards" site because large stores of both chlorine and bromine are maintained there. Its emergency plan was put into operation and six fire engines attended. Firefighters wearing protective suits and breathing apparatus took 90 minutes to bring the incident under control. Four Octel staff suffered burns or breathing difficulties, and one fireman was treated in hospital the following day.
An Octel spokesman said that the company is investigating the cause of the incident. The release occurred when bromine entered a heated acid tank, vaporised and escaped through a vent, but why this happened is not yet known. The firm's emission monitors showed that "at no time were levels off-site sufficient to cause significant health effects," he said.
The Environment Agency's site inspector commented that the incident was the fourth bromine release from the site since it was authorised under integrated pollution control in 1994. One of the earlier incidents resulted in an enforcement notice, but "all were very much smaller than this," he said. "The company have not been able to give a realistic estimate of the release," he added, but he suggested a maximum figure of two tonnes. The incident is being investigated by the Health and Safety Executive.