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Smell That? First H2S Exceedances of 2026 in the Books; GASP Calls on ACHD to Address Stench Issue

Yinz smell that?! That tell-tale stench of rotten eggs? You’re not smelling things: Concentrations of hydrogen sulfide (or H2S for short) are sky-high in and around the Mon Valley today.


And you’re not alone in your suffering - social media and Carnegie Mellon University’s Smell Pittsburgh app lit up with complaints starting overnight Monday and continuing into the afternoon Tuesday:





For the uninitiated: H2S is a toxic gas that occurs both naturally (from sources such as swamps, manure pits, and oil, gas, and water wells) and as a result of industrial activity (including, most notably for our region, coke making).


According to the Agency for Toxic Substances and Disease Registry, H2S is a colorless gas recognizable by its rotten-egg odor. Exposure to the levels of hydrogen sulfide we see in the Mon Valley can cause:


  • irritation to the eyes, nose, or throat

  • headaches

  • poor memory

  • tiredness

  • breathing problems for some people with asthma

  • balance problems


Just how elevated were levels of the foul-smelling pollutant? The short answer: They were high enough to exceed the Pennsylvania Department of Environmental Protection’s 24-hour average standard, marking the first exceedances of the new year.


But those exceedances don’t tell the whole story on today’s H2S event. There were also crazy-high H2S spikes at Allegheny County Health Department’s air quality monitors in both Liberty and North Braddock.


For reference: Pennsylvania’s 24-hour average standard for H2S is 5 parts per billion. As of 11 a.m. Tuesday, the 24-hour average for H2S at the Liberty monitor was more than twice that.


Here’s a look at those spikes:



You might be asking yourself at this point, “But where does the stuff come from?” Fortunately, several studies have placed the blame squarely on U.S. Steel’s Clairton Coke Works facility. You can read more about all that here.


So, what can you do about it? Two words: Report it. Make a complaint to the Allegheny County Health Department, which is tasked with enforcing DEP’s H2S standard. We also implore you to report poor air quality on the Smell Pittsburgh app.


GASP is following this issue closely and has - and will continue - to demand ACHD do more to stem stench in the Mon Valley through more robust enforcement.


“This issue has been ongoing for decades. Knowing the source is awesome, but it’s not enough. More must be done to reel in these episodes of hours-long stench that residents say impact everything from their sleep to their breathing to their quality of life,” GASP Executive Director Patrick Campbell said. “There were just as many H2S exceedances in 2025 as there were in 2024. So, we’re NOT making progress on this issue, just holding the line at best.”




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