A worker at Southern Water Treatment was mixing three chemicals to make a product used to treat water at sewage plants when there was an explosion or flash fire, said Thom Berry, spokesman for the state Department of Health and Environmental Control.

"The employee was able to shut off the valve and stop the process," Berry said.

"The employee recognized what happened and stopped, turned off the nozzles and then was overcome by the fumes."

The worker was mixing sodium hydroxide, ferris chloride solution and sodium hydrosulfide.

Workers were evacuated from the building.

The Greenville hospital system treated 19 people for symptoms ranging from headaches and nausea to vomiting, respiratory distress and tingling, burning skin, Berry said.

There was no word on how serious the injuries were.

The explosion or fire released a vapor cloud which drifted down the road, so some homes were evacuated and occupants of nearby buildings, including an elementary school, were instructed to stay inside, Berry said.

"The vapor cloud dissipated a few minutes after the incident," he said.

Members of a Greenville County EMS unit were treated at a nearby restaurant after driving through the cloud and becoming ill during lunch.