Industrial facilities that reduce their ability to emit toxic air pollutants will be exempted from some requirements to control their emissions in a move the Trump administration said will encourage companies to reduce pollution voluntarily.

The Environmental Protection Agency Jan. 25 issued a memorandum withdrawing the “once-in-always-in” policy under the Clean Air Act, which prevents major sources of toxic air pollutants from removing pollution control devices after emissions fall below a certain threshold. Instead, those facilities can now be...