The shelter-in-place extended from the Mound in South Charleston west and includes Spring Hill and Dunbar, said Kanawha County Deputy Emergency Manager C.W. Sigman.
Manager Jennifer Sayre said one employee was treated for overexertion after carrying equipment up and down stairwells, but was back at work in the afternoon.
Sigman also said two firefighters were treated on site for heat exhaustion.
Sigman and Taylor said air monitoring tests conducted at the Clearon plant and surrounding area showed low concentrations of chlorine following the release.
The Clearon facility employs about 118 people.
"The situation was brought under control with the help of our emergency team and the local fire department," the company said in a statement. "All employees are accounted for and are safe. Clearon is investigating the cause of the incident and will take measures to prevent a recurrence."
The U.S. Occupational Safety and Health Administration had an inspector at the facility Wednesday afternoon to investigate, Johnson said.
Staff writer Rusty Marks contributed to this report.
Reach Ken Ward Jr. at kw...@wvgazette.com or 304-348-1702.