The Clean Air Act regulates ozone as a criteria pollutant. The U.S. Environmental Protection Agency (EPA) sets National Ambient Air Quality Standards for ozone (O3). These standards include both primary standards to protect public health and secondary standards to protect the environment. EPA reviews pollutant standards every five years. Sometimes, a review results in a tightened standard. The first ozone standard was published in 1979 and based on one-hour average concentrations. In 1997, EPA revised the ozone standard based on eight-hour average concentrations. In 2008, and again in 2015, EPA revised the ozone standard. Both revisions lowered the standard level, but were still based on eight-hour average concentrations.

The ozone standards below include the state implementation plan elements developed for each of the standards and the relevant documents associated with the boundary designations. If you are looking for a document that is not available on our website, please submit a Open Records/ Sunshine Law Request.

2015 Standard

8-hr Ozone Standard = 70 parts per billion (ppb)

2015 Standard Boundary Recommendations Actions

MoDNR Submittal Date MoDNR Action/Document EPA Date EPA Action/Document
Feb. 16, 2018

Revision to Area Boundary Designation Recommendation for the 2015 Ozone Standard

June 14, 2021

Revised Air Quality Designations for the 2015 Ozone National Ambient Air Quality Standards (Jefferson County) [86 FR 31438]

June 4, 2018

Additional Air Quality Designations for the 2015 Ozone National Ambient Air Quality Standards (remaining area) [83 FR 25776]
Effective: Aug. 3, 2018

Sept. 22, 2017 MoDNR Letter to EPA Requesting No Action on 2015 Ozone Recommendations April 30, 2018 EPA Letter to Governor Regarding Area Designations for the 2015 Ground-level Ozone National Ambient Air Quality Standards - Completes area designations for the 2015 ozone standard
Dec. 20, 2017 EPA Response to Recommendations on Designations for the Revised 2015 National Ambient Air Quality Standards for Ozone - 120-day letter for the second round designations under the 2015 ozone standard
Sept. 30, 2016

2015 Ozone Standard Area Boundary Designation Recommendation

Nov. 16, 2017

Air Quality Designations for the 2015 Ozone National Ambient Air Quality Standards (NAAQS) (most areas) [82 FR 54232]
Effective: Jan. 16, 2018

Nov. 6, 2017 EPA Letter to Governor Regarding 2015 Ground-level Ozone Standards Met - Initial round designations for the 2015 ozone standard

2015 Standard Infrastructure SIP Actions

MoDNR Submittal Date MoDNR Action/Document EPA Date EPA Action/Document
Nov. 1, 2022 Supplement to the Interstate Transport Provisions for the 2015 Ozone Standard Pending Pending
June 17, 2022 MoDNR Comment Letter on EPA Proposed Rule: Federal Implementation Plan Addressing Regional Ozone Transport for the 2015 Ozone National Ambient Air Quality Standard  April 6, 2022 Federal Implementation Plan Addressing Regional Ozone Transport for the 2015 Ozone National Ambient Air Quality Standard [87 FR 20036]
April 22, 2022 MoDNR Comment Letter on EPA Proposed Rule: Air Plan Disapproval; Missouri Interstate Transport of Air Pollution for the 2015 8-Hour Ozone National Ambient Air Quality Standards Feb. 2, 2022 Air Plan Disapproval; Missouri Interstate Transport of Air Pollution for the 2015 8-Hour Ozone National Ambient Air Quality Standards [87 FR 9533]
Nov. 8, 2021 MoDNR Comment Letter on Federal Register Action: Proposed Consent Decree, Clean Air Action Citizen Suit Oct. 15, 2021 Proposed Consent Decree, Clean Air Act Citizen Suit [86 FR 57423]
June 10, 2019

Interstate Transport Provisions for the 2015 Ozone Standard

Feb. 13, 2022

Air Plan Disapprovals; Interstate Transport of Air Pollution for the 2015 8-Hour Ozone National Ambient Air Quality Standards [88 FR 9336]

April 11, 2019 Infrastructure Elements for the 2015 Ozone Standard Sept. 30, 2019

Air Plan Approval; Missouri; Infrastructure State Implementation Plan Requirements for the 2015 Ozone National Ambient Air Quality Standard [84 FR 51413]

2015 Standard St. Louis Area Actions

MoDNR Submittal Date MoDNR Action/Document EPA Date EPA Action/Document
Sept. 6, 2023

MoDNR Submittal of St. Louis Moderate Nonattainment Area Plan for the 2015 Ozone Standard

Pending Pending
April 21, 2023 MoDNR Comment Letter on Federal Register Action: Draft Guidance on the Preparation of State Implementation Plan Provisions March 23, 2023 Draft Guidance on the Preparation of State Implementation Plan Provisions that Address the Nonattainment Area Contingency Measure Requirements for Ozone and Particulate Matter [88 FR 15571]
    Oct. 7, 2022 Determinations of Attainment by the Attainment Date, Extensions of the Attainment Date, and Reclassification of Areas Classified as Marginal for the 2015 Ozone National Ambient Air Quality Standards [87 FR 60897]
April 8, 2022 Maintenance Plan for the St. Louis Nonattainment Area for the 2015 Ozone Standard Pending Pending
April 8, 2022 Redesignation Request for the St. Louis Nonattainment Area for the 2015 Ozone Standard Pending Pending
Sept. 8, 2021

Marginal Nonattainment Area Plan for the Missouri Portion of the St. Louis Nonattainment Area Under the 2015 Ozone Standard

Implementation Impact Report: Marginal Nonattainment Area Plan for the Missouri Portion of the St. Louis Nonattainment Area for the 2015 Ozone Standard

Feb. 7, 2023 Air Plan Approval; MO; Marginal Nonattainment Plan for the St. Louis Area for the 2015 8-Hour Ozone Standard [88 FR 7885]

2008 Standard

8-hr Ozone Standard = 75 parts per billion (ppb)

2008 Standard Boundary Recommendations Actions

MoDNR Submittal Date MoDNR Action/Document EPA Date EPA Action/Document
Dec. 8, 2011

Revised 2008 Ozone Nonattainment Designation Recommendation

May 21, 2012

Air Quality Designations for the 2008 Ozone National Ambient Air Quality Standards [77 FR 30088]
Effective: July 20, 2012

March 11, 2009

Nonattainment Area Recommendations for the 2008 Ozone National Ambient Air Quality Standard

Summary for Revised Missouri Recommendation: 8-hour Ozone Nonattainment Designations-2008 NAAQS

Dec. 8, 2011

EPA Response to Recommendations on the Revised 2008 Ozone National Ambient Air Quality Standards - 120-day letter

2008 Standard Infrastructure SIP Actions

MoDNR Submittal Date MoDNR Action/Document EPA Date EPA Action/Document
July 1, 2013

Section 110 Infrastructure Requirements for the 2008 Ozone National Ambient Air Quality Standard (NAAQS)

Sept. 24, 2017

Air Plan Approval; Missouri; Regional Haze Plan and Prong 4 (Visibility) for the 2012 PM2.5, 2010 NO2, 2010 SO2, and 2008 Ozone NAAQS [83 FR 48242]

March 22, 2018

Approval of Implementation Plans; State of Missouri; Elements of the Infrastructure State Implementation Plan Requirements for the 2008 Ozone, 2010 Nitrogen Dioxide, 2010 Sulfur Dioxide, and 2012 Fine Particulate Matter National Ambient Air Quality Standards (NAAQS) [83 FR 12496]

Oct. 26, 2016 Cross-State Air Pollution Rule Update for the 2008 Ozone NAAQS [81 FR 74504]

2008 Standard St. Louis Area Actions

MoDNR Submittal Date MoDNR Action/Document EPA Date EPA Action/Document
Nov. 12, 2019 Maintenance Plan Revision for the St. Louis (Missouri) Maintenance Area under the 1979, 1997, and 2008 Ozone Standards Sept. 30, 2021

Air Plan Approval; Approval of Missouri Air Quality Implementation Plans; Revisions to St. Louis 2008 8-Hour Ozone Maintenance Plan [86 FR 54098]
Effective: Nov. 1, 2021

Feb. 16, 2018 Technical Correction to the Redesignation Request and Maintenance Plan for the St. Louis (Missouri) 2008 Ozone Standard Nonattainment Area Sept. 20, 2018

Approval of Missouri Air Quality Implementation Plans; Redesignation of the Missouri Portion of the St. Louis-St. Charles-Farmington, MO-IL 2008 Ozone Area to Attainment [83 FR 47572]

July 27, 2017

2008 Ozone Redesignation Request Maintenance Plan Discussion of Emissions

Nov. 21, 2016 MoDNR Clarification Letter Regarding Redesignation Request and Maintenance Plan for the St. Louis (Missouri) 2008 Ozone Standard Nonattainment Area
Sept. 12, 2016

Redesignation Request and Maintenance Plan for the St. Louis (Missouri) 2008 Ozone Standard Nonattainment Area

Sept. 4, 2014

Marginal Area Plan for the Missouri Portion St. Louis Nonattainment Area for the 2008 8-hour Ground-Level Ozone National Ambient Air Quality Standard

Feb. 25, 2016

Approval and Promulgation of Air Quality Implementation Plans; State of Missouri; Emissions Inventory and Emissions Statement for the Missouri Portion of the St. Louis MO-IL Ozone Nonattainment Area [81 FR 9346]

Aug. 16, 2013 Early Progress Plan for the Missouri Portion of the St. Louis Nonattainment Area for the 2008 Ozone National Ambient Air Quality Standard Jan. 14, 2016

Approval of Missouri’s Air Quality Implementation Plans; Early Progress Plan of the St. Louis Nonattainment Area for the 2008 Ozone National Ambient Air Quality Standard [81 FR 1887]