Air toxics monitoring around facilities and in communities


Fenceline and community monitoring programs provide information about the amount of air toxics at the fenceline of a facility or in a nearby community adjacent to a facility or facilities emitting air toxics. 

Fenceline air toxics monitoring

Several facilities in Colorado are required to continuously monitor for hydrogen cyanide (HCN), hydrogen sulfide (H2S), and benzene at the perimeter, or fenceline, of the facility. These air pollution sources must also make these data available to the public, as outlined in Colorado’s Air Toxics Act (HB21-1189). 

Aerial view of the Suncor Refinery

Fenceline monitoring is useful in identifying unintended, or fugitive, air pollution emissions from operations not captured by continuous emission monitoring systems (CEMS) on the stack (where the emissions are released). This information allows facilities to identify and repair leaks faster. 

Fenceline monitoring also provides notifications to surrounding communities when an exceedance occurs. Prior to monitoring, each covered facility must prepare and submit a fenceline monitoring plan to the Air Pollution Control Division for approval. The division also hosts public meetings and public comment periods prior to finalizing the plans.


Additional information on draft fenceline monitoring plans for each of Colorado’s covered facilities is available on the division’s Air Toxics Act website.

Final fenceline monitoring plans and data will be linked here as they become available: 

  • Suncor Refinery in Commerce City: Fenceline Monitoring Plan | Monitoring Data*
  • Phillips 66 Pipeline, LLC: Fenceline Monitoring Plan | Monitoring Data* - coming 7/1/2024
  • Holly Energy Partners (Sinclair) - Denver Terminal: Fenceline Monitoring Plan | Monitoring Data* - coming 7/1/2024
  • Goodrich Corporation (Collins Aerospace) in Pueblo: Fenceline Monitoring Plan | Monitoring Data* - coming 7/1/2024

*Disclaimer: Please note, these sites are managed and updated by third-parties. The Air Pollution Control Division cannot confirm the accuracy of the data published at any given time.

Community air toxics monitoring

Colorado’s Air Toxics Act also requires monitoring of hydrogen cyanide (HCN), hydrogen sulfide (H2S), and benzene in the communities within a 3-mile radius of covered facilities. This monitoring is conducted with a combination of mobile and stationary measurement tools. 

Mobile community monitoring 

The Community Air Toxics (CAT) and Emissions Monitoring Utility (EMU) mobile monitoring vans employ state-of-the-art technology to monitor a wide range of volatile organic compounds (VOCs), including benzene, hydrogen sulfide and hydrogen cyanide in near real-time while driving.

Monitoring routes focus on disproportionately impacted communities and cover a wide area within the 3-mile radius of the covered facilities. Repetition of these routes provides information on community impacts over time.

The CAT is focused on meeting the community monitoring requirements of HB21-1189. The EMU can serve as a backup to the CAT for HB21-1189 monitoring, but is also free to investigate other compounds and facilities across Colorado. 

Community Air Toxics, or CAT, mobile monitoring van parked in a lot
EMU mobile monitoring van parked in a lot

Ground-based community monitoring

Stationary monitoring site located on a roof top, includes solar panels, a protective case to hold the instrument, and a meteorological station for measuring wind speed and direction

From January to December 2023, there were three ground-based community monitors operating within a 3-mile radius of the Air Toxics Act covered facilities (Suncor Refinery in Commerce City, Phillips 66 Terminal in Commerce City, Holly Energy Partners (Sinclair) Denver Terminal in Henderson, and Goodrich Carbon Products (Collins Aerospace) in Pueblo).

Each ground-based site used passive samplers to collect emissions of benzene, hydrogen cyanide, and hydrogen sulfide over a period of 14 days. The ground-based measurements supplemented mobile community air toxics monitoring from the CAT mobile monitoring van. 

Data and reports


Targeted emissions monitoring


The Mobile Air Remote Monitoring Trailer (MARMOT), which is a new solar-powered trailer platform, is under construction and scheduled for completion in early 2024. This platform can be deployed to areas of concern and operate without ground power. The trailer platform can be customized with different analyzers depending on the pollutants of concern in a community. Some potential targeted compounds that could be measured using various analyzers include: 

  • Volatile organic compounds, including BTEX (benzene, toluene, ethylbenzene, and xylene) and ethylene oxide (EtO)
  • Hydrogen sulfide
  • Sulfur dioxide
  • Methane
  • Ethane

Data and reports

Solar-powered modular trailer, outfitted with solar panels on the side and top