Strong inversion in the Mon Valley at Clairton Coke Works, 23 Oct 2026, 7:09am (screenshot from BreatheCam Project)

23 October 2026

If you live in Pittsburgh’s Monongahela Valley and have smelled a rotten egg smell this week or you’re finding it hard to breathe here’s why.

This week’s beautiful sunny days and cold nights cause inversions when warm air above traps cold air in our valleys. All of our valleys are affected but the Monongahela Valley has three heavy industries that produce air pollution: Clairton Coke Works, Irvin Works and Edgar Thomson Works. All of the pollution is trapped and so is the rotten egg smell, which is hydrogen sulfide (H2S). Where does it come from?

According to a recent 31-page study conducted by the Allegheny County Health Department, several years’ worth of H2S exceedances in the Mon Valley “can be attributed entirely to emissions from U.S. Steel’s Clairton coking facility.” 

Despite U.S. Steel scapegoating landfill facilities, we want to point out that the study also stated: “ACHD did not identify evidence of any other source contributing to the exceedances.”

This morning we have another strong inversion. It displays like low-lying fog on the Breathe Cam screenshot above and as orange and red dots on the Purple Air map below. The red spot on the map is the neighborhood of Clairton Coke Works.

Purple Air monitor map focused on Allegheny County, PA, 23 Oct 2026, 6:37am (screenshot from Purple Air)

As the day heats up the air will start moving and dissipate the pollution. Meanwhile stay indoors if you can. Hospital admissions increase during bad air weeks, especially for those with respiratory issues. It’s happened this week to a friend of mine.