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    Toxic pollutants, also called Hazardous Air Pollutants (HAP)s,  are a group of 188 pollutants identified by the federal Clean Air Act that have been associated with a wide variety of adverse health effects, including cancer. Toxic pollutants are emitted from various sources, including major stationary, area, as well as mobile sources. Although the focus on toxics in ambient air is relatively new, there are many local, state, and national programs aimed at reducing toxic pollutants in the air we breathe. Learn what you can do to help.

    Projects led by the Air Toxics Section tend to be special purpose monitoring to help characterize and determine the impact of unique emission sources in the Commonwealth. Projects may be designed to look at HAPs or other pollutants, including criteria pollutants, using both regulatory and non-regulatory monitoring methods. Projects may be targeted using a low number of discrete monitoring stations or distributed using a high number of small self-powered instruments. Projects may be in collaboration with other agencies to determine potential health impacts or develop follow up actions.

    Marcellus Monitoring

    Marcellus Shale Long-Term Air Monitoring Project Report

    The Department of Environmental Protection undertook a project to examine the impacts from air emissions from unconventional gas production within Washington County. Data was collected over a one-year period in 2012-2013 and results were subjected to a series of analyses conducted by DEP, DOH and the federal Agency for Toxic Substances and Disease Registry (ATSDR). Air quality measurements were collected from 5 Washington County sites, and an additional comparison was performed using data from an air monitoring station in Adams County. Air quality was monitored for ground-level ozone, particulate matter, carbon monoxide, nitrogen oxides, hydrogen sulfide, and methane. Additionally, the air was tested for over 60 toxic volatile organic compounds (VOCs). Continuous meteorological data was also collected.

    The air data was compared to National Ambient Air Quality Standards (NAAQS), ATSDR public health comparison values, Air Quality Index (AQI) indicators, and background values found in Pennsylvania. The data was also subjected to a screening-level risk quantification for hazardous air pollutants. The report from ATSDR can be found here (PDF).

    For more information, call DEP's Bureau of Air Quality at 717-787-9702.

    On Aug. 1, 2013, DEP released the Technical Support Document that outlines the methodology, study goals and technical information about the study.

    Short-Term and Investigative Monitoring Studies

    Established Monitoring Sites in Pennsylvania

    The Western Pennsylvania Oil and Gas Air Monitoring Project is setting out to measure volatile organic compounds (VOCs) and particulate matter (PM2.5 and PM10) at a developing unconventional natural gas well pad in West Finley Township, Washington County. The project began collecting data May 1, 2025 before any development activities took place and will continue into the production phase of the well pad, expected to last into 2029. Data, as it becomes available, will be linked below. As the project progresses, any reports developed for this project will be linked below.

    During the background phase, before any development occurs, DEP is collecting VOCs using evacuated canisters that are analyzed by a laboratory. As the data are reported to the DEP, an initial screening is performed by DEP staff for any suspect data. This is the preliminary data. Preliminary data is of high quality however, it is not fully reviewed for completeness and accuracy. The full review is expected to take several months and will be published when the process is complete.

    Preliminary Background VOC Data

    (Coming Soon) Final Quality Assured Background VOC Data

    (Coming Soon) Background Particulate Matter Data

    As the well pad develops, DEP will update monitoring strategies to include more robust monitoring equipment such as a Federal Equivalent Method grade PM2.5/PM10 monitor and a gas chromatograph to report hourly VOCs. As data from new phases become available, this page will be updated.

    The DEP, in partnership with the Delaware County Planning Department and Johns Hopkins University, is gathering two years’ worth of data to measure PM in and around waterfront communities near Chester and Marcus Hook, Delaware County using low-cost Quant-AQ Modulair and Modulair-PM sensors. Monitoring began in January 2025 and is planned to continue through July 2027. DEP is overseeing the project, while the Delaware County Planning Department is conducting community outreach and engagement, and Johns Hopkins University will perform a detailed data analysis.

     

    The full report and supplemental materials will be accessible below when available:

    (Coming Soon) Data Dashboard

    (Coming Soon) Final report

    (Coming Soon) Monitoring Data 

    DEP, in partnership with Lehigh University, conducted a year-long study to measure ethylene oxide and other volatile air toxics in and around East Side Allentown, Lehigh County. Monitoring took place from December 1, 2023, through November 30, 2024. DEP oversaw the project, while the Lehigh University research team conducted the on-the-ground work, including collecting air samples using canisters at seven locations, validating the data, and collaborating with a local community group to support outreach and engagement throughout the study.

     

    A visualization of the monitoring results is available in a dashboard.

    The full report and supplemental materials will be accessible below when available:

    (Coming Soon) Final report

    (Coming Soon) Monitoring Data 

    The DEP conducted a study to measure hydrogen sulfide (H2S) levels in Chester County from October 2021 to September 2023, with project oversight by DEP, and a health risk analysis prepared by the Pennsylvania Department of Health (DOH). The study was initiated following odor complaints from residents in London Grove and New Garden Townships, who contacted State Representative Christine Sappey, prompting the Southeast Regional Office (SERO) to collaborate with the Bureau of Air Quality Central Office to monitor H2S levels and assess changes since a 1999 study.

    Hydrogen sulfide is a colorless gas with a distinctive rotten egg odor, detectable at low concentrations and is commonly found in groundwater, crude oil, and as a by-product of natural and industrial processes. Pennsylvania’s ambient air quality standards for H2S, established in Title 25 of the Pennsylvania Code § 131.3 (1971), set two thresholds: a 24-hour average concentration limit of 0.005 parts per million (ppm) and a 1-hour concentration limit of 0.1 ppm. DOH wrote a Letter Health Consultation concluding that the ambient H2S concentrations detected during the study did not pose adverse health effects but acknowledged that the levels could negatively impact residents’ quality of life due to odor nuisance. DEP is working to restart monitoring.

    The supplemental materials can be accessed below:

    DOH Hydrogen Sulfide Factsheet

    DOH Letter Health Consultation Ambient Hydrogen Sulfide Near Mushroom Composting Operations

    (Coming Soon) Hydrogen Sulfide Monitoring in Ambient Air, Chester County (2025)