Federal ozone pollution standards and Colorado nonattainment areas

Update: During its November 2025 meeting, the Air Quality Control Commission approved revisions to a State Implementation Plan for ozone pollution and air quality regulations 3, 7, 11, 25, 26, and air quality standards. Read the press release.

Colorado's ozone pollution challenge

Ozone and air quality fact sheet

Background on Colorado’s nonattainment areas

Through the federal Clean Air Act, the U.S. Environmental Protection Agency sets health-based standards for six primary pollutants in outdoor air. One of these primary air pollutants is ground-level ozone pollution. Since the 1990 Clean Air Act Amendments, the EPA has slowly lowered the ozone pollution standard from 85 parts per billion to 70 parts per billion in accordance with the latest scientific information.

Colorado’s air quality has improved over the past several decades, but not quickly enough to meet the EPA's updated standards. In 2008, the EPA lowered the 8-hour standard for ozone pollution to 75 parts per billion. In 2015, the EPA again lowered the 8-hour standard to 70 parts per billion. Parts of Colorado do not meet either the 2008 or the 2015 standards.

Map of Colorado’s ozone pollution nonattainment areas.

The EPA evaluates compliance with the standards using three years worth of ozone pollution data. The EPA determines classifications for nonattainment with ozone pollution standards based on how far from compliance an area may be. The EPA’s potential classifications start at “marginal” nonattainment and range through “extreme” nonattainment.

The CDPHE Air Pollution Control Division, Colorado Air Quality Control Commission, Regional Air Quality Council, and local partners continue to evaluate and implement control strategies aimed at reducing ozone pollution. In particular, this work focuses on reducing air pollution emissions known as “ozone precursors.” Ozone precursors include nitrogen oxides and volatile organic compounds. These air pollutants can form ground-level ozone when they react in the heat and sunlight. Vehicles that run on gas, other equipment that runs on gas or other fossil fuels, and industry operations are all large sources of ozone precursors in Colorado.

Nonattainment designations under the 2008 federal ozone pollution standard

In 2022, the EPA reclassified the Denver metro/north Front Range from a “serious” to a “severe” nonattainment area under the 2008 ozone pollution standard. Northern Weld County is classified as in attainment for the 2008 standard.
The EPA will next evaluate Colorado’s nonattainment status with the 2008 standard in 2027. The attainment deadline for the 2008 standard is July 20, 2027, based on ozone pollution data between 2024 and 2026.

Nonattainment designations under the 2015 federal ozone pollution standard

In July 2024, the EPA approved the state’s voluntary request to update its nonattainment classification for the 2015 standard. This update reclassified the Denver metro/northern Front Range, including northern Weld County, from a “moderate” to a “serious'' nonattainment area. The state voluntarily requested this update based on air monitoring data collected between 2021 and 2023, which showed the area would not meet the EPA’s 2015 standard for ozone pollution.

In November 2025, the Colorado Air Quality Control Commission approved updates to the State Implementation Plan for the 2015 federal ozone pollution standard. The commission also made updates to state air quality regulations 3, 7, 11, 25, 26, and air quality standards to reduce ozone precursor emissions.

Colorado announced plans to submit a request to EPA for a voluntary reclassification from a “serious” to a “severe” nonattainment area under the 2015 standard.

Voluntary reclassification provides administrative relief for both state and federal agencies. This allows the division to continue to develop and implement strategies to reduce ground-level ozone pollution.

Following voluntary reclassification, the EPA will next evaluate Colorado’s nonattainment status with the 2015 standard in 2032. The attainment deadline for the 2015 standard is Aug. 3, 2032, based on ozone pollution data between 2029 and 2031.

Learn more about the history of ozone and past planning efforts in Colorado.

Learn more about ground-level ozone and how it may impact your health.

State Implementation Plans and other actions to address ozone pollution

Because of nonattainment with the federal ozone pollution standards set in 2008 and 2015, Colorado must prepare and submit State Implementation Plans to the EPA. These plans show how Colorado will reduce ozone pollution in the nonattainment areas. The CDPHE Air Pollution Control Division occasionally proposes updates to the plans. These updates must be considered through rulemaking hearings before the Colorado Air Quality Control Commission. If the commission approves, the state submits the updated plans to the EPA.

In December 2022, the commission approved the latest State Implementation Plan for “moderate” nonattainment with the 2015 standard. In November 2025, the commission also updated the plan to reflect the “serious” classification for the 2015 standard.

In December 2023, the commission approved the latest State Implementation Plan for “severe” nonattainment with the 2008 standard.

Colorado’s State Implementation Plans are important tools in reducing ozone pollution. However, the plans do not necessarily include all actions that may help address ozone pollution. Furthermore, regulations or policies that focus on reducing greenhouse gas emissions may also in turn reduce ozone precursor emissions. Actions the state has taken since 2023 include:

The Nitrogen Oxides (NOx) Reduction Program for Upstream Oil and Gas Operators, which Colorado initiated in March 2023 at the direction of Governor Jared Polis. The committee will complete its recommendations by the end of 2025. In November 2025, the Air Quality Control Commission extended this program, expanding leak detection and repair program requirements for oil and gas operations, including storage and transferring processes, as well as minimizing emissions from maintaining certain oil and gas equipment during the summer months.
The Advanced Clean Trucks and Low Nitrogen Oxide (NOx) Omnibus Rules, which Colorado adopted in April 2023.
The Colorado Clean Cars Standard, which Colorado adopted in October 2023.
The Greenhouse Gas Emissions and Energy Management for Manufacturing Phase 2 (GEMM 2) Program, which Colorado adopted in October 2023.
The stationary engine standards for oil and gas operations, which Colorado adopted in December 2023.
The lawn and garden equipment requirements, which Colorado adopted in February 2024.
The Greenhouse Gas Intensity Program for Upstream Oil and Gas Operators, which Colorado last updated in May 2024.
The Cumulative Impacts and Enhanced Standards and Practices Rules, which the Colorado Energy and Carbon Management Commission (ECMC) adopted in October 2024.
The Midstream Fuel Combustion Equipment Rule, which Colorado adopted in December 2024.
Updates to the Vehicle Inspection and Maintenance Program, which Colorado adopted in January 2025.
The Pneumatic Controller and Pump Retrofit Program, which Colorado adopted in February 2025.

Public participation opportunities

Colorado will continue to explore more ways to reduce ozone pollution and update its State Implementation Plans. This process includes continued opportunities for engagement and feedback. New opportunities to get involved will be posted here as they become available.

Ozone 101 public information sessions

The CDPHE Air Pollution Control Division has recently hosted 101 public information sessions for community members to learn about ground-level ozone in an accessible and plain language format.

Virtual meeting: Tuesday, April 29, 2025.

Recording | Presentation

Virtual meeting: Tuesday, May 6, 2025.

Recording | Presentation

Ozone public meetings

The division held the first of two public meetings in spring 2025 to share proposed updates to the 2025 State Implementation Plan and other efforts to reduce ground-level ozone pollution across Colorado’s Front Range ozone nonattainment area.

Virtual meeting: Saturday, May 17, 2025.

Recording | Presentation

Virtual meeting: Wednesday, June 4, 2025..

Recording | Presentation

The division held public meetings last fall to share information on how Colorado plans to continue reducing ground-level ozone pollution across Colorado’s Front Range. The division discussed strategy proposals for further reducing ozone pollution and updating Colorado’s State Implementation Plan.

Virtual meeting: Tuesday, Sept. 24, 2024.