A CHEMICAL reaction sent a cloud of chlorine gas over the Huddersfield and Mirfield areas.
People were warned to keep doors and windows closed after the gas escaped from the Techfill chemical packaging company on Calder Road, Ravensthorpe.
The chemical reaction - involving 400 kilogrammes (nearly 900lb) of chlorine granules - happened in a machine at 9.30am yesterday.
The machine began to overheat and sent a dusty cloud of the gas from the building.
Traffic was diverted away from the site as firefighters wearing chemical protection suits dealt with the problem.
They flooded the machine with thousands of gallons of water to stop the reaction - but were called back in the afternoon when there was a chemical reaction in some packaged boxes.
It is thought the process involved was repackaging chlorine so it could be used in swimming pools.
A fire service spokesman said: "Hazardous material specialists and scientists are investigating to find out what happened and why."
A police spokes-woman said: "No-one was hurt, but chlorine can cause a sore throat, reddening of the eyes and skin irritation."