Denver (Aug. 15, 2024): The state of Colorado has effectively kept outdoor carbon monoxide pollution low for the past 20 years. It is now more than 80% below the U.S. EPA health-based standard. Carbon monoxide is a dangerous outdoor and indoor air pollutant. Outdoor carbon monoxide pollution originates primarily from the tailpipes of gas-powered vehicles. It can cause headaches, dizziness, nausea, chest pain, and exacerbate pre-existing conditions such as heart diseases.

From 1999-2003, the EPA designated various Colorado areas as back in compliance with the federal carbon monoxide standard. The federal Clean Air Act requires a state to stay in compliance with an air quality standard for 20 years before it can request that EPA ends its oversight. The 20 years are considered a maintenance period, during which the state proves it can keep air pollution low. Colorado went above and beyond achieving the federal standard—the state has kept carbon monoxide levels more than 80% below EPA’s standard. The federal standard is an average of 9 parts per million over 8-hour time periods. Colorado has achieved and maintained carbon monoxide levels below 4.7 parts per million over 8-hour time periods for 20 years and reached levels below 0.8 parts per million.

On Aug. 15, 2024, the Colorado Air Quality Control Commission approved revisions to the state’s maintenance plans for outdoor carbon monoxide pollution after 20 years of meeting federal standards. The state health department’s Air Pollution Control Division developed the revisions to demonstrate the state’s progress in managing and improving air quality.

“This success story shows that Colorado can meet and even exceed federal air quality standards,” said Air Pollution Control Division Director Michael Ogletree. “We achieved attainment for carbon monoxide, and we’re making headway to do it again for other air pollutants. This is a case study in how state and federal policies, combined with technological improvements, can work in concert to reduce air pollution and protect public health across Colorado.”

“The air commission celebrates this success and was thrilled to approve these revisions to the state’s air quality plan,” said Commissioner Patrick Cummins, Chair of the Colorado Air Quality Control Commission. “Colorado experienced high levels of carbon monoxide pollution in the 1970s and 1980s, and this milestone shows how far we’ve come in protecting and improving air quality for all Coloradans.”

The majority of outdoor carbon monoxide emissions reductions stem largely from improvements in technology, such as catalytic converters, and cleaner fuels. In 1975, the EPA required all vehicle manufacturers to include catalytic converters in new cars through the Clean Air Act. Modern gas-powered vehicles emit a small fraction of the amount of carbon monoxide that they used to.  

Other federal and state policies also helped Colorado achieve and maintain federal air pollution standards for carbon monoxide. For example, since 1988 Colorado Air Quality Control Commission Regulation 4 has required wood burning stoves to meet emissions requirements for carbon monoxide. Wood burning stoves are a smaller source of carbon monoxide emissions when compared to gas-powered vehicles.

Although Colorado has attained federal standards, the state will continue to reduce carbon monoxide emissions through clean transportation initiatives and other policies to help reduce tailpipe emissions of pollutants like carbon monoxide.

The division provided updates on the maintenance plan revisions during public meetings in summer 2024, and stakeholders expressed their support.  

EPA must next review the maintenance plan revisions. Colorado will submit the revisions to EPA following the state’s 2025 legislative session. The EPA’s review process will also include a public comment period.

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