The Northern Ireland Department of the Environment said workers at Albion Chemicals Ltd. poured nitric acid into a disposal container that was contaminated, causing a reaction that produced plumes of nitrous oxide.
The gas, inhaled as an analgesic, has been known since the late 18th century as "laughing gas." It has a range of medical and industrial uses and is not toxic.
Greig Laing, Albion Chemicals' director of human resources at its headquarters in Leeds, England, declined to describe the exact chemicals and reactions involved in the accident. He said the fumes created were "fully controlled," and caused "no injuries and no environmental damage."
Belfast police and firefighters launched an emergency plan following first reports of the accident, which suggested an explosion had created a haze of unknown, potentially toxic fumes.
Una Devlin, spokeswoman for the Northern Ireland Fire and Rescue Service, said firefighters sprayed the gas plume with water mist and shut off the plant's sewage connections to nearby Belfast Harbor. The operation lasted about three hours.
She said fire crews donned full-body protective suits and gas masks as they doused the container, which continued to emit fumes of the gas for more than an hour.
