This past weekend and Monday, local residents took to social media and the Smell Pittsburgh app to voice their concerns and complaints about abysmal air quality, noting noxious odors and physical symptoms like runny nose, itchy eyes, breathing difficulties, and an inability to sleep.

The air quality data and atmospheric conditions were grim, with the Allegheny County Health Department (ACHD) issuing a Mon Valley Air Pollution Warning Sunday through Monday afternoon as levels of fine particulate matter (also known as PM2.5) exceeded national health-based standards at the Liberty Borough air quality monitor.

Quick aside about PM2.5 and why it’s such a concern: Fine particulate pollution is harmful to human health because it’s smaller and more able to infiltrate the body through the nose and mouth. This means these particles can travel deep into the lungs and even into the bloodstream. PM2.5 is linked to heart attacks, strokes, arrhythmias, and lung cancers. Exposure to PM2.5 is also linked to everything from baldness to dementia to mental illness.

But back to this recent bout of bad air…

As is so often the case, PM2.5 wasn’t the only air pollutant of concern: Levels of hydrogen sulfide (also known as H2S, which has a distinct rotten-egg odor) handily exceeded Pennsylvania’s 24-hour average air quality standard of 0.005 parts per million (ppm) on Saturday, Sunday and Monday.

For most of Sunday and Monday H2S levels were between two and FOUR TIMES higher than the allowable level at ACHD’s Liberty Borough monitor.

Pennsylvania also has a one-hour H2S standard of 0.100 ppm. Between 2 a.m. and 3 a.m. Sunday the one-hour H2S concentration at the Liberty Borough monitor reached 0.089 ppm, the closest we’ve come to exceeding that threshold since 2015.

While these H2S exceedances are often framed by officials as “just a quality of life issue,” the Environmental Protection Agency (EPA) notes that exposure to hydrogen sulfide can cause irritation to the eyes, nose, or throat, as well as breathing problems, headaches, and fatigue.

“We are particularly concerned about the outrageously high one-hour H2S values that helped drive those exceedances,” GASP Executive Director Patrick Campbell said. “They were high enough for GASP to contact the Allegheny County Health Department this weekend to inquire whether there was an issue at U.S. Steel’s Clairton Coke Works.”

GASP staff also reviewed Breathe Cam footage and noted heavy smoke and flame emanating from U.S. Steel’s Clairton Coke Works. We filed a complaint with Allegheny County Health Department.

ACHD on Thursday morning said it is investigating.