Activists with Pittsburgh’s Breathe Project met outside the U.S. Steel Tower Tuesday afternoon and declared “enough is enough,” calling for an independent audit of U.S. Steel’s troubled Clairton Coke Works plant.

Rachel Filippini, executive director of the Group Against Smog and Pollution, said an independent audit would help to ensure the plant’s “structural and operational integrity.”

“Such an independent audit would help to ensure deficiencies at the plant are identified, that appropriate actions are taken, and future incidents and fires that have the potential to adversely affect air quality in the Mon Valley are averted,” Filippini said.

The plant has experienced two fires in the last six months — a large incident on Christmas Eve, and a smaller one Monday — both of which temporarily shut down the plant’s desulfurization, or pollution control, systems. While the Monday incident was short-lived, the December fire led to widespread damage and resulted in a combined 28 exceedances of environmental standards for three different regulated materials — sulfur dioxide, hydrogen sulfide and particulate matter 2.5.

Allegheny County received an “F” rating from the American Lung Association in this year’s “State of the Air” report in every category due to particulate matter and ozone concentrations that exceeded national air quality standards. The health department has levied increasing fines on Clairton Coke Works for emitting pollutants in violation of environmental regulations.

A spokesperson for Allegheny County Executive Rich Fitzgerald did not immediately respond to a request for comment on whether or not he supported an independent audit of the plant.