An environmental law firm has asked federal regulators to reject air permits the state Department of Natural Resources has granted for sand mines in western Wisconsin.
Reports say Midwest Environmental Advocates has asked the U.S. Environmental Protection Agency to reject air permits for Superior Silica Sands plants in Barron and Polk counties.
MEA alleges the DNR allowed the company to expand those operations without considering the potential emissions of ultrafine particles that can cause health problems.
The MEA cited as an example that the EPA has objected to a review process the DNR used in approving an air permit this summer for the Barron County plant.
The DNR has maintained that mechanical processes such as crushing and grinding don't produce ultrafine particles.