Hydrogen Sulfide
Hydrogen Sulfide (H2S)
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.