Two environmental groups have put PPG Industries on notice that they intend to sue over mercury releases from a West Virginia plant.
The West Virginia Rivers Coalition and the Washington, D.C.-based group Oceana, filed their notice on Monday against PPG's Natrium plant.
Both claim the plant continues to exceed the legal limits governing mercury releases into the Ohio River.
The Marshall County plant makes chlorine by pumping saltwater through vats of pure mercury, which is a neurotoxin harmful to human health.
In 2007, the state Department of Environmental Protection issued a permit that increased the plant's average monthly mercury discharge into the river.
A PPG spokesman did not immediately return a call seeking comment.