Editor’s note: This article was supported by funding from the Pittsburgh Media Partnership. It is the fourth in a series on pollution and misinformation in greater Pittsburgh from a consortium of outlets including The Allegheny Front, Ambridge Connection, The Incline, Mon Valley Independent, Pittsburgh City Paper, Pittsburgh Independent, and Pittsburgh Union Progress. Read the previous installments in the Pittsburgh Independent, Ambridge Connection, and Pittsburgh Union Progress.On a clear September morning in 2021, hundreds of Allegheny County residents woke up reporting an invasive burning odor — so bad that many placed 911 calls.
The health department later revealed thousands of pounds of toxic chemicals had been released during a leak at a chemical plant on Neville Island. But the agency’s initial communications focused not on the source of the emissions but instead the weather conditions that exacerbated them.
"The Allegheny County Health Department is aware of a strong odor being reported by residents primarily in McKees Rocks and other nearby areas,” the department wrote in a press statement that day. “The odor appears to be the result of a weather inversion. The Health Department continues to monitor air quality in the area and will provide updates when available."
Neville Chemical was ultimately fined $62,075 for the episode — but some environmental advocates were left wondering whether the invisible cloud of carcinogens and toxins would have even been noticed without the unique weather conditions at play.
While usually less severe than the one recorded that early fall morning, inversions occur on 47% of all days in a given year in Allegheny County, according to Michael Brown, a meteorologist at the National Weather Service Pittsburgh.
“Think about oil and vinegar — they don’t dissolve into each other,” Brown says. “Oil rises to the top but vinegar sinks because it’s more dense.”
The impact of this is that harmful emissions are kept close to the ground instead of dispersing into the wider airshed. Typically inversions “burn off” as the midday sun starts to heat the ground, but they can, in some cases linger for days. When a strong inversion sets in around the site of toxic emissions, the consequences can be deadly.
In 1948, 21 people died during a 6-day inversion that trapped a thick cloud of factory smog around the small Mon Valley community of Donora. Hundreds more were hospitalized as the caustic fumes infiltrated their lungs. The now infamous Donora Death Fog eventually caught the attention of the federal government and set in motion some of the country’s first environmental regulations.
Brown says inversions can and do occur over any kind of landmass — but Western Pennsylvania’s distinctive tangle of rolling hills and steep valleys encourages them far more than say the plains of Western Kansas.
Pittsburgh’s rugged topography complicates air movement in other ways, too.
Air quality modeling developed by the Environmental Health Project suggests emissions from Shell’s sprawling Beaver County ethane cracker plant don’t disperse outward in even circles but instead cling to the Ohio River valley on which it sits, traveling west toward Ohio and east toward Pittsburgh.
Other mapping developed by Carnegie Mellon University’s Breathe Project shows heavy concentrations of black carbon settling around Pittsburgh’s three main waterways. This, experts say, reflects both how roads, factories, and other pollution sources tend to be nestled within river valleys, and also how the steep terrain can keep emissions trapped within them.
“There are plenty of communities that are on the rivers that are getting the one-two punch,” says Nathan Deron, a data scientist at EHP.
From the earliest days of Pittsburgh’s industrialization, manufacturers set up mills and factories in the flat plains surrounding the Monongahela River south and east of Pittsburgh. Access to river water was crucial for operations, as was access to waterways and railroad lines for transportation.
Even after decades of deindustrialization, the Mon Valley remains one of the most heavily polluted areas in the country. The region is home to nearly 20 large industrial polluters, including U.S Steel’s Clairton Coke Works and Edgar Thomson Steel Works.
“And so that's why often the worst air pollution is in the Mon River Valley, because the valley itself is really steep,” Albert Presto, a research professor at CMU’s Center for Atmospheric Particle Studies, tells Pittsburgh City Paper. “And then you have a source there as well … and that’s how you end up with high concentrations.”
To help mitigate the worst cases of pollution in the Mon Valley, the Allegheny County Health Department adopted the Mon Valley Episode Rule in 2021. The rule requires industrial polluters to take additional steps on days when strong temperature inversions heighten air quality concerns.
Since its passage, major facilities have submitted individualized mitigation plans to the health department that spell out the steps to be taken when an episode is declared.
These plans are triggered whenever the Air Quality Index exceeds 100 throughout a rolling 24-hour period. The index, created by the U.S. Environmental Protection Agency, rates air quality from zero to 300, with the 101 to 150 range color-coded orange and deemed “unhealthy for sensitive groups.”
For conditions to move into that range, the region has to be experiencing a fairly potent inversion where the temperature differential between the cooler ground air and warmer overhanging layer is five degrees or more, says ACHD planning and data analysis program manager Jason Maranche.
The air index in the Mon Valley has dipped into the orange zone about 12 times since the rule was passed in 2021, including one day last week.
The goal during those orange days, says ACHD environmental health director Geoff Rabinowitz, is to “shave the peak ... to both lessen the maximum magnitude and duration of an episode.”
Nearly two years after its launch, some environmental advocates have voiced skepticism about the rule’s effectiveness. But health department officials say it’s too soon to know quite how effective the rule is proving. Maranche says the department is still collecting data and will release new modeling and reports over the coming years to evaluate the rule’s impact.
Outside the Mon Valley, Southwestern Pennsylvanians are still subject to the whims of earth and wind, although experts say it’s hard to generalize about the ways air pollution is dispersed and concentrated.
Deron says prevailing winds carry air northwest and southwest, but the endless contours complicate this pattern closer to the ground, where air at the bottom of a valley may be traveling in the opposite direction.
“It doesn’t always turn out how you would expect based solely on prevailing winds,” Deron says. “And I think that has a lot to do with topography.”
Presto says his research hasn’t always confirmed the existence of pollution hotspots in some of the places he would expect to find them based on meteorological and topographic factors. But, he says, the data confirms a clearer picture with respect to the people and communities that bear the brunt of industrial pollution.
“Unsurprisingly, people of color systematically live in places with higher air pollution. To some extent, people with lower incomes live in higher places with higher air pollution,” Presto says.
“Air pollution is a huge environmental injustice, right, systematically like at the national level and a local level."