The leak occurred on Wednesday at the Catalyst Refiners plant in the community of Institute as workers were preparing to shut down at least part of the facility.
A chemical gas reaction occurred at the plant involving nitric acid and another substance, Kanawha County Commission Emergency Management Director CW Sigman said.
He added that there was "a violent reaction of the chemicals and it instantaneously overreacted".
"Starting or ending a chemical reaction are the most dangerous times," Sigman said.
Among the injured were seven ambulance workers responding to the leak, officials said. One person was in critical condition.
Vandalia Health Charleston Area Medical Center, one of several hospitals in the area, was treating multiple patients, some brought by ambulance, while members of the community were arriving asking to be checked, a hospital spokesman said.
A shelter-in-place order was issued for the surrounding area and lifted more than five hours later. Officials said all the deaths and injuries occurred on the plant site.
The leak required a large-scale decontamination operation in which people had to remove their clothes and be sprayed down, authorities said.
Catalyst Refiners works to remove silver from what remains of chemical processes and can find thousands of dollars of the precious metal just by vacuuming the floors in a plant's offices.
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," company President Frank Barber said in a statement.
"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 15km west of Charleston, the state capital.
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