Hydrogen Sulfide

Data Removal
As part of the quality assessment and quality control process, it has been determined that the monitoring equipment at the Liberty Monitoring Station was not fully functional between March 20, 2024 and May 14, 2024 and on May 17, 2024. This may have resulted in inaccurate data. The data from this time period is being invalidated by the ACHD. Please see the FAQs for further details on this occurrence.

H2S Overview

Hydrogen sulfide (H2S) is a colorless gas with a rotten egg-like smell. The most common sources of H2S are from industrial processes like oil and natural gas refineries, craft paper production, coke ovens and tanneries. These activities form H2S by combining high heat, sulfur and organic matter. It is also released at sewage treatment facilities, landfills, as well as during bacterial breakdown of human and animal waste. The smell of H2S is noticeable to most people around 0.3 parts per million (ppm), though as with any other smell, some people with higher sensitivity can recognize the odor at lower levels.

Regulation of H2S in Pennsylvania

The U.S. Environmental Protection Agency (EPA) does not have federal air quality standards for H2S. Pennsylvania is one of a few of states with a hydrogen sulfide regulation. In 1971, the Allegheny County Health Department incorporated the Pennsylvania Department of Environmental Protection (PA DEP) H2S regulations into its Air Quality Rules and Regulations. The Air Quality Program enforces the Pennsylvania H2S odor control standard and has issued enforcement orders based on it.

All Air Quality Program enforcement actions, including H2S, are posted on the Health Department’s website.

The Pennsylvania 24-hour standard for H2S is 0.005 ppm and the one-hour standard is 0.1 ppm. The commonwealth’s regulation is based only on the odor’s nuisance and not on a health standard, meaning the enforcement limits for H2S are based on annoyance of the smell, not on potential health impacts.

Hydrogen Sulfide Standard Average Concentration
Federal Ambient Air Quality Standards None -
Pennsylvania Ambient Air Quality Standards 24-Hour 1-Hour 0.005 ppm 0.1 ppm

Most H2S emissions are fugitive, meaning they are released at non-specific points during an industrial process and cannot be easily traced. Though there is a state odor limit, if the odor cannot be traced to one facility or process, then no enforcement action is possible.

What the ACHD Is Doing to Monitor and Enforce H2S Standards

The ACHD understands that the smell of H2S is unpleasant, and a concern for residents. The Air Quality Program has two permanent H2S monitors in the Mon Valley, one in North Braddock and the other in Liberty Borough. Staff review daily H2S data and investigate when monitors are nearing or exceeding state standards.

There are known sources in the county that emit H2S. Through monitoring and data analysis the Air Quality Program did a study on H2S: Analysis and Attribution of Hydrogen Sulfide (H2S) Exceedances at the Liberty Monitoring Site from January 1, 2020 through March 1, 2022(PDF, 3MB). This study looked at the permanent H2S monitors and portable H2S sensors, weather conditions, emissions inventory statements from facilities, and how these factors were used to determine the source of H2S exceedances.

Where to Find Data on H2S Levels in Allegheny County

A week’s worth of hourly H2S data from the North Braddock and Liberty monitors are continuously updated on the Hourly Air Quality Data Report(PDF, 718KB) . This report also includes other air pollutants that are monitored in Allegheny County: fine particulate matter (PM2.5), coarse particulate matter (PM10), nitric oxide (NO), nitrogen oxide (NO2), nitrogen oxides (NOx), total reactive nitrogen (NOY), ozone (O3), carbon monoxide (CO), black carbon and sulfur dioxide (SO2). There are various pollutants measured at the ACHD's monitoring sites. The types of pollutants measured vary by station, based on the location and purpose of the site. Not all pollutants measured by the ACHD are measured on an hourly basis and therefore not reported on the Hourly Air Quality Data Report.

Hourly readings and 24-hour averages from the Hydrogen Sulfide Dashboard can also be found on the Western Pennsylvania Regional Data Center. 

What You Can Do

If you think you smell a rotten egg-like odor linked with H2S, report it to the Health Department through the Self-Service Portal. This is the best way to alert staff and have your voice heard. These complaints are reviewed daily.

To learn more about air quality and the Air Quality Program, residents can sign up for Allegheny Alerts and attend Air Advisory Committee meetings. The department’s air quality dashboard is updated hourly. Every weekday a daily dispersion report is released with a forecast for the day's atmospheric dispersion, meteorology, air quality index, and surface temperature inversion information on the air quality home page.

Data Removal FAQ

What caused the monitor issues?

  • The monitoring issue was caused by a probe line that became dislodged from its exterior housing on the sulfur dioxide monitor. This caused the ambient sulfur dioxide readings to become lower than normal despite the instrument passing all regular quality assurance checks. Hydrogen sulfide concentrations in this setup require two independent sulfur dioxide monitors to operate in together. The hydrogen sulfide data was also affected. 
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Does this impact any enforcement actions?

  • No current enforcement actions are impacted by this. However, the specific data affected during this time period will not be used in any future enforcement action(s). 

Is there a backup with which to understand any data for these sources during this period?

  • Only sulfur dioxide and hydrogen sulfide data were affected during this time period. The second sulfur dioxide monitor which measures hydrogen sulfide and sulfur dioxide was unaffected and those data can still be evaluated. However, without the other monitored data it cannot be known what percentage of the total concentration in the air at that time period was hydrogen sulfide or sulfur dioxide. All other monitored data at the Liberty monitoring station operated normally and can be used to evaluate ambient air quality conditions during this time period.

How can the public be sure that any data that the ACHD is collecting is legitimate and that monitors were not impacted?

  • In order to inform the public of real-time air quality conditions, the ACHD posts hourly data generated by automated instruments that is not considered official and valid until fully reviewed by qualified Air Quality program staff members. Monitored data at the  ACHD goes through a thorough quality assurance and quality control (QA/QC) system where instruments are regularly checked against United State Environmental Protection Agency (EPA) and National Institute of Standards and Technology (NIST) standards. In this instance, this same system identified the issue and invalidated the data while monitoring technicians repaired the damaged infrastructure.
  • In addition to multiple levels of internal and independent QA/QC, the monitors at the ACHD air monitoring network are audited by the EPA at regular intervals through the National Performance Audit Program (NPAP) and National PM2.5 Performance Evaluation Plan (PEP). The dislodged probe line was a unique situation that has never been encountered before at any of the ACHD’s air monitoring stations.

What is ACHDs plan for fixing the impacted monitors?

  • The monitoring systems were diagnosed and fixed within days of identifying the issue. After that, the monitored data resembled historical data trends at the site.

How will the ACHD ensure they are maintained in working order moving forward?

  • The ACHD has increased the frequency of station infrastructure inspection by the monitoring technicians who regularly visit the stations. Recent upgrades to the ACHD’s air monitoring database will allow for an expansion of automated routines to identify inconsistent data more quickly.  
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