The leak occurred at the Catalyst Refiners plant, a silver recovery business in Institute, as workers were preparing to shut down at least part of the facility, Kanawha County Commission Emergency Management Director C.W. Sigman said.
A chemical gas reaction occurred at the plant involving nitric acid and another substance, Sigman said, speaking at a news briefing.
The reaction caused "a violent reaction of the chemicals and it instantaneously overreacted," Sigman added.
Among the injured were seven ambulance workers responding to the leak, officials said.
Other people were taken to the hospitals in private cars or even in one case a garbage truck, Sigman said.
The leak required a large scale decontamination operation where people had to remove all their clothes and be sprayed down, authorities said.
Ames Goldsmith Corp., the owner of Catalyst Refiners, said it is saddened by the deaths and its thoughts were with all those impacted and their families.
"This is an unfathomably difficult time," said the company statement released at the news briefing. "Our thoughts and prayers are with our colleagues and their families."
Ames Goldsmith promised to work with local, state and federal officials as they investigate what happened.
The plant is located near Institute, a community about 10 miles (16 kilometers) west of Charleston, the state capital.
This is a developing story. Please check back for updates.