Every time air pollution spikes in the Mon Valley — whether it be from wildfires or industrial emissions — the Allegheny County Health Department issues an alert. This year alone, at least 20 watches or warnings have been issued for the region so far.
The Mon Valley Air Pollution Episode rule — created by the health department in 2021 — specifically targets air pollution in the Mon Valley. When a warning is in effect, industrial facilities there must limit their activities, and residents, especially vulnerable ones, are discouraged from spending time outdoors.
But who, exactly, is at risk when an air quality alert for the Mon Valley is issued? After all, the Monongahela River — for which the valley is named — stretches 130 miles, from northern West Virginia to the Point in Downtown Pittsburgh.
That query was at the heart of a message from Good Question! asker Frances Hartnett. The window of her South Side home looks out onto what appears to be a river valley, with the Mon just a stone’s throw away.
Hartnett said she pays attention whenever a Mon Valley air quality alert is issued.
“But I have a friend who has lived in or near Pittsburgh her whole life, and insists I'm not in the Mon Valley, that the Mon Valley is farther upriver,” Hartnett wrote. “This doesn't make sense to me, as I'm only five blocks from the river, with higher elevation visible on both sides.”
To figure out whether Hartnett should heed those air quality warnings, it’s helpful to address a different question first: Where exactly is the Mon Valley?
Johnna Pro with the Pennsylvania Department of Community and Economic Development says asking the locals could yield you 10 different definitions. For the purposes of this story, we’ll go with hers.
“I define the Mon Valley as those communities that are located between the City of Pittsburgh and Elizabeth Borough,” Pro said. “Those are communities like Homestead, West Homestead, Duquesne, Clairton, McKeesport. All of those communities that are on either bank of the Monongahela River, but they are all located within Allegheny County.”
Municipalities in the Mon Valley, Pro said, were known for the steel mills that once dotted the banks of the Mon River south and east of Pittsburgh. In the cases of Clairton and Braddock, some are still home to active steel or steel-related facilities.
The Allegheny County Health Department applies similar boundaries to its map of municipalities subject to the Mon Valley Air Pollution Episode rule. The measure required 17 facilities, from Braddock to Elizabeth, to submit mitigation plans to ACHD for approval.
Anytime an air pollution episode warning is in effect, the plants are expected to put those plans into effect, modifying their work practices in an effort to reduce industrial emissions of fine particle pollution.
Regulators say that, in turn, should lower the risk to public health. Breathing unhealthy levels of fine particle pollution has been linked to respiratory illnesses like asthma, as well as low infant birth weight and cardiovascular disease.
Why apply this rule to only a specific section of Allegheny County?
The Mon Valley is not alone in experiencing hours or days of poor air quality. Residents throughout Allegheny County — and across Pennsylvania — have been subject to Code Orange air quality warnings this week, which are issued by the state’s Department of Environmental Protection to denote high concentrations of fine particulate matter.
And yet, according to the health department’s Jason Maranche, a few factors differentiate air quality issues in the Mon Valley. For one, the area’s river valleys create topographic barriers to pollutants trying to escape.
Maranche said that’s especially true on days with temperature inversions, when the air closer to the ground is cooler than the air above it.
“We experience temperature inversions everywhere in the county. Actually, everywhere in America, you can have a temperature inversion,” he said. “It's really exaggerated, though, at specific locations in the river valley. The bottom of the valley cools a lot quicker than the air just above the valleys.”
On top of that, communities in the Mon Valley are in close proximity to many industrial sources of fine particulate matter, also known as PM 2.5, that is easy to breathe in.
“The largest presence of those currently in Allegheny County is at the facilities in that Mon Valley region,” Maranche added.
That includes U.S. Steel’s Clairton Coke Works, the single-largest coke plant in the country, and by far the largest source of particle pollution in Allegheny County.
Maranche said communities in Liberty, Glassport and Lincoln tend to get the highest downwind impacts from the plant in the Mon Valley, and while emissions can escape to other areas over several miles, they won’t be dispersed “at the same concentration that they would be in the Mon Valley region.”
That is to say, Maranche continued, that South Siders and other Pittsburgh residents see far less air pollution on days when the Mon Valley Episode Rule is in place. Maranche said that’s based on monitor levels the county uses, as well as modeling adopted from the U.S. Environmental Protection Agency.
A different model, though, can tell a different story.
Inside his office on the Carnegie Mellon University campus, Randy Sargent pointed to a timelapse depicted through PlumePGH, a program that combines weather models from federal meteorologists with air quality monitoring data from regulatory and citizen-owned networks.
“As we go through the 24 hours, you're seeing emissions from some of the biggest polluters, such as Clairton Coke Works, go all across the South Side [and] across Pittsburgh proper,” Sargent said.
Sargent and the team at the CMU CREATE Lab wrote the model to demonstrate how emissions can travel miles from a source at significant concentrations.
While valleys and temperature inversions can contain air pollution within one localized area, Sargent said it’s more common to see inversions that sit a little higher off the ground, giving the air space to blow for miles.
“What you'll see with these inversions is that the pollution can actually get out of the valley and go to the next valley over,” he added.
Because of that, even if a resident doesn’t live in the Mon Valley, Sargent said they’re likely still impacted by some of that particulate matter that triggers a Mon Valley Air Pollution Episode.
But the inverse would also be true: if particulate matter spreads farther than just the Mon Valley, then more Allegheny County residents will be positively impacted by regulations seeking to curb industrial emissions, provided the nearly two-year-old rule is working the way it should.
Whether that’s the case isn’t clear. Maranche with the health department said the county needs to gather more data before it can determine whether or not that’s the case.
