From their home on a hill along the western shore of the Monongahela River in Elizabeth, Nicole Salotti and her husband, Joseph Carringer, can see an inversion forming in the distance. 

A wispy layer of fog hugs the hillside and, just before sunset, they feel the cold air rush down into the valley, sinking into the many small ravines that line the banks of the Mon. It fills slowly — “like a bathtub,” said Carringer — as the cool, smoggy air settles against the basin nearly 400 feet beneath their home. Sometimes they can see an orangish-brown haze against the treeline across the river. Eventually it reaches their doorstep. It nearly always smells foul.

“It’s horrifying and impressive at the same time,” said Carringer. “It’s like being on the thin line of the yin-yang between industry and nature.”

For the relatively healthy couple, the inversions straddle a line between intermittent inconvenience and long-term health hazard. Their shared optometrist once blamed their unusually dry eyes on the particles in the valley’s air, and asked them, “Can you move?” But for people with preexisting conditions like asthma, the inversions can mean they can’t safely go outside, sometimes for days.

In late 2021, the Allegheny County Health Department enacted a rule requiring polluting industries in the Mon Valley to reduce particulate emissions during inversions. This year, the department is planning to evaluate the efficacy of the Mon Valley Air Pollution Episode Rule. 

Documents submitted to the Health Department show estimated reductions in particulate emissions at U.S. Steel’s facilities in the Mon Valley, though questions remain about whether the new rule protects public health. Some clean air advocates, meanwhile, are concerned that a lack of transparency could obscure the impending evaluation.

What is an inversion?

Temperature inversions, also known as weather inversions, occur when a layer of cool air is trapped close to the ground by warm air above it in the atmosphere. 

“We’ve always had temperature inversions,” said Jason Maranche, an air quality engineer for the Health Department. Generally, he said, inversions occur most commonly in the fall and winter when the sun’s angle is at its lowest and when large swings in temperature cause the ground to cool faster than the air just above it. 

A street-level view of U.S. Steel’s Edgar Thomson Works in Braddock on Jan. 30. (Photo by Quinn Glabicki/PublicSource)

Particulate pollution like PM2.5 — the kind emitted from the Clairton Coke Works and other industrial facilities in the Mon Valley — can be trapped against the earth by an inversion layer, concentrated at human breathing level and prevented from dissipating as it normally would. Exposure to PM2.5 has been linked to respiratory and cardiovascular illnesses like asthma, chronic obstructive pulmonary disease [COPD] and heart disease.

Dimpled topography and numerous sources of industrial pollutants combine to make inversions particularly dangerous in the Mon Valley. Seventy-five years ago, before the Clean Air Act was passed, the infamous Donora Smog killed 20 people in a town just a few miles south of Salotti and Carringer’s home.

“The inversion kind of puts a cap over the Mon Valley in that particular area, and it doesn’t allow for the normal disbursement of pollutants that we would have when we don’t have an inversion,” said Geoff Rabinowitz, deputy director of environmental health for Allegheny County. 

‘Peak shaving’ pollutants 

The Mon Valley Air Pollution Episode Rule, passed in September 2021, requires industrial facilities to reduce PM2.5 emissions when weather conditions “are likely to exceed acceptable levels.” It applies to 17 facilities in the Mon Valley, including all three of U.S. Steel’s Mon Valley Works facilities.

The goal, Rabinowitz said, is to “round the curve” and reduce the maximum potential for particulate pollution by curbing how much is emitted by industry during an inversion — a strategy the department calls “peak shaving.”

The rule works in two phases. First, if PM2.5 concentrations are forecast to exceed the National Ambient Air Quality Standard, a watch is issued via Allegheny Alerts, which signals to facilities that they should prepare to execute their mitigation plans. If that standard is exceeded during a rolling 24-hour averaging period, the department issues a warning and the mitigation plans must then go into effect. 

When the rule was first announced in late 2021, impacted Mon Valley facilities submitted mitigation plans to the department that detailed the steps that each would take to reduce PM2.5 emissions in the event of an inversion. 

U.S. Steel operates three major and longstanding facilities along the Mon and is the region’s largest industrial emitter of PM2.5. 

U.S. Steel’s Edgar Thomson Works in Braddock on Jan. 30. (Photo by Quinn Glabicki/PublicSource)

The corporation gave the Health Department estimates of cuts it could make to PM2.5 emissions during inversions at its three facilities. The reductions are based on four-year emission averages and range from less than 1% to nearly 20%:

  • At Edgar Thomson Works, estimated PM2.5 emissions reductions are 3.9%. 
  • At Irvin Works, 0.69%. 
  • At the Clairton Coke Works, the largest source of particulate pollution in Allegheny County, reductions range between 11.9% and 19.3%. 

The majority of reductions are implemented at the Clairton Coke Works “where emissions and impacts are greatest” and mitigation efforts “are most feasible,” per U.S. Steel’s submission. Actual steps taken to reduce emissions at Clairton will be determined on a “per episode basis” due to safety considerations and “complex and dynamic” operations at the facility. 

U.S. Steel spokesperson Amanda Malkowski said the company uses its own “predictive air quality monitoring capabilities” and “may proactively adjust our operations in response to atmospheric changes.” She added: “We are often able to detect an inversion and initiate our mitigation plans prior to issuance of an air quality watch or warning.”

Mitigation plans differ between facilities, said Rabinowitz, but each is required to conduct a post-episode analysis and document the steps taken to reduce PM2.5 emissions during an episode. 

In response to a records request from PublicSource, the department supplied records of actions taken at all three of the company’s Mon Valley facilities during a single inversion event in late April. The actions range from extending coking times and enacting battery outages at Clairton to minimizing “unnecessary truck travel” and applying dust suppressants at Edgar Thomson.

The regulation does not require facilities to submit reports following a warning, but companies must make the records available to the Health Department upon request. The department said it does not possess and has not requested U.S. Steel’s reports for any of the other four warning episodes in 2022. 

“Mon Valley Episode Rule compliance is determined based on the review of semi-annual (twice a year) reports and certifications of compliance submitted to the ACHD’s Air Quality Program from the facilities,” said Health Department spokesperson Neil Ruhland in an email to PublicSource. 

Some environmental advocates view this as a lapse in oversight and transparency.

“There’s no evaluation of effectiveness happening in real time,” said Matt Mehalik, executive director of the Breathe Project. “That’s problematic. It’s all being done after the fact, but that doesn’t help Mon Valley residents.” 

“I would like to see at least some kind of report that would describe, during a warning, what the facility actually did to comply with their mitigation plan,” added Patrick Campbell, executive director of the Group Against Smog and Pollution [GASP]. “I’d like to see that submitted to the Health Department. I’d like to see that be available publicly.”

“It’s horrifying and impressive at the same time,” said Joseph Carringer of the inversions in the Mon Valley. “It’s like being on the thin line of the yin-yang between industry and nature.”(Photo by Quinn Glabicki/PublicSource)

Air quality impacts from inversions are a “regional issue,” U.S. Steel’s Malkowski said. “We urge the Allegheny County Health Department to broaden the requirements for episode mitigation plans to all industrial sources in Allegheny County, not just those in the Mon Valley.”

Will the public have input?

A “major goal” of the Health Department’s Air Quality Program this year will be assessing the Episode Rule to determine if it is effective in its goal of reducing PM2.5 pollution during inversions in the Mon Valley. The department will do a “very complicated analysis,” said Rabinowitz, who acknowledged that the small sample size of just seven warnings to date makes a robust assessment difficult. 

Air quality advocates are concerned about transparency.

PM2.5 levels shown at the department’s air quality monitors eventually decrease after a warning, said Campbell. “Is it a decrease due to the rule being implemented and facilities actually implementing their mitigation plans?” he asked. “Or is it simply because weather conditions have changed and that an inversion has broken?”

To assess that, the department plans to look at historical meteorological data — things like wind speed and direction, atmospheric pressure, humidity and the strength of each inversion — to model what it may have looked like without the mitigation plans in effect. The department will also likely need to review the implementation reports from Mon Valley facilities to assess what measures were put into place. “We just don’t have that information at this time,” said Maranche.

“We would love to see the Health Department actually solicit public opinion and public input of their criteria” for how the rule will be assessed, said Campbell, who advocated for resident testimony to be a part of that process. “It’s really difficult to have a complete opinion of the effectiveness of the rule without some kind of a robust analysis.”

Rabinowitz said there will “more than likely” be opportunity for public involvement “at some point.”

“We don’t really know how ACHD checks whether companies actually implement their plans,” said Mehalik. “We don’t know how ACHD evaluates whether the plans actually do anything.”

Emissions rise from U.S. Steel’s Clairton Coke Works on Jan 18. (Photo by Quinn Glabicki/PublicSource)

Regardless, inversions and pollution continue to be a persistent issue for people living in the Mon Valley.

“Basically it’s taking away folks’ rights to breathe clean air,” said Germaine Patterson, a community health worker with Women for a Healthy Environment in Clairton. She tells people in her community to stay indoors until an inversion passes and for children to make sure they have their inhalers.

“Stay inside if you can.” she said. “Which is sad. What else can you do?”

Quinn Glabicki is the environment and climate reporter at PublicSource and a Report for America corps member. He can be reached at and on Twitter and Instagram @quinnglabicki.

This story was fact-checked by Betul Tuncer.

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Quinn Glabicki is a writer and photographer covering climate and environment for PublicSource. He is also a Report for America corps member. Quinn uses visual and written mediums to tell stories about...