California Air Resources Board

Exceptional Events (EE) are natural or unusual events that can overwhelm existing strategies designed to control man-made pollution. If such an event occurs, data that would cause an exceedance of a federal or state standard can be excluded when determining compliance with that standard.

Data from a concurred event is not used for area designation purposes or other select regulatory determinations. The data, however, remains in both the State and federal databases.

Examples of exceptional events include, but are not limited to, wildfires (and resulting smoke), high winds and dust, volcanic activities, stratospheric ozone intrusions, and fireworks.

Exceptional Events must meet the following criteria, as specified in the federal Exceptional Events Rule (80 FR 68216), before being approved by either CARB or U.S. EPA:

1. The event meets the statutory definition of an exceptional event;
2. There is a clear causal relationship between the event and the measured exceedance(s);
3. The event was not reasonably controllable or preventable;
4. The event was a human activity that is unlikely to recur at a particular location OR was a natural event; and
5. The public comment process and review procedures were followed.

Air districts may work both individually and cooperatively with other districts, CARB, and U.S. EPA to document events that have occurred within and across their boundaries. Final documentation is then submitted to CARB after completion of a 30-day public comment period. 

• If the event impacts a federal standard, and CARB concurs with the district(s) assessment, CARB will forward the documentation to U.S. EPA for further review and concurrence. 
• If the event only impacts a State standard, CARB will conduct the final review.

Additional Information

U.S. EPA Website - Treatment of Air Quality Data Influenced by Exceptional Events

Links to Area Designation Staff Reports (also known as Initial Statement of Reasons for Rulemaking)

• 2023
• 2022
• 2021
• 2020
• 2019
• 2018
• 2017
• 2016 Changes by Operation of Law (no staff report)
• 2015 Changes by Operation of Law (no staff report)
• 2014: No staff report (no changes to area designations)
• 2013
• 2012
• 2011
• 2010
• 2009: No staff report (no changes to area designations)
• 2008: No staff report (no changes to area designations)
• 2007 Changes by Operation of Law (no staff report)
• 2006
• 2005: No staff report (no changes to area designations)
• 2004
• 2003
• 2002
• 2001
• 2000
• 1999
• 1998
• 1997
• 1996

Staff Reports prior to 1996 are available upon request.

2025 Amendments

The California Air Resources Board (CARB) will consider proposed updates to the area designations for State ambient air quality standards. Information regarding these amendments, as well as Notice of Public Comment Period can be found on the CARB Regulatory Webpage.

2025 Workshop

On Wednesday, October 1, 2025, the California Air Resources Board (CARB) held a public webinar on proposed updates to the area designations for State ambient air quality standards. During the webinar, staff presented background information on the State standards, the designation process, and proposed amendments to the area designations. These amendments reflect updates based on the annual review of the latest complete monitoring data (2022 to 2024). 

• Meeting Notice
• Presentation
• Presentation Text

2024 Amendments

CARB adopted updates to the area designations for State ambient air quality standards at a public meeting on January 23, 2025.  Information regarding these amendments can be found on the CARB regulatory webpage. These amendments have been approved by the Office of Administrative Law (OAL) on September 9, 2025, with an effective date of January 1, 2026.

The Health and Safety Code (H&SC) section 39607(e) requires the California Air Resources Board (CARB) to establish and periodically review area designation criteria. These designation criteria provide the basis for CARB to designate areas of California as attainment, nonattainment, or unclassified for the State standards.

In addition, H&SC section 39608 requires CARB to use the designation criteria to designate areas of California and to annually review those area designations. CARB makes area designations for ten pollutants: ozone, suspended particulate matter (PM₁₀ and PM_2.5), carbon monoxide, nitrogen dioxide, sulfur dioxide, sulfates, lead, hydrogen sulfide, and visibility reducing particles.  CARB made the first area designations for State ambient air quality standards (State standards) in 1989.  H&SC 39608 also requires CARB to conduct an annual review of the area designations and update them as appropriate.  These updates are based on the three most recent calendar years of complete and validated air quality data.

Information regarding the statutory requirements, as well as maps of current area designations, can be found through the Resources tab on the left.

There are two federal primary standards for Lead (Pb) - a quarterly-averaged standard of 1.5 micrograms per cubic meter (ug/m³) (in certain areas) and a rolling 3-month average of 0.15 ug/m³.  The secondary lead standards are the same as the primary standards.  The only nonattainment area in California for lead is in the South Coast Air Basin.

Maps of current area designations can be found through the Resources tab on the left.  Information about specific lead nonattainment areas and the efforts to bring them into attainment of the lead standards can be found on the California State Implementation Plan (SIP) webpage.

Lead Primary and Secondary Standards

• May 2012 - CARB approved a revision to the State Implementation Plan (SIP) to address attainment of the federal lead standard in the South Coast Air Basin portion of Los Angeles County.
  • This is the only area in California designated as nonattainment for lead. The SIP revision for lead was submitted to U.S. EPA for approval by the June 30, 2012, deadline. Additional information about this item is found on CARB's South Coast Air Quality Management Plans webpage. CARB previously submitted a SIP revision addressing the infrastructure requirements for lead. The Lead Infrastructure SIP is also available on the web.
• November 2011 - U.S. EPA issued a second set of final area designations (FR Notice  November 22, 2011; effective December 31, 2011). 
• November 2010 - U.S. EPA issued an initial set of final area designations (FR Notice November 22, 2010; effective December 31, 2010).
• October 2009 - CARB staff submitted recommended area designations to U.S. EPA. The full submittal package and U.S. EPA's response are available on the U.S. EPA website.
• October 2008 - U.S. EPA revised the primary lead standard (FR Notice November 12, 2008; effective January 12, 2009).

Additional information about activities related to lead can be found on U.S. EPA's lead home page.

There is one federal primary standard for sulfur dioxide (SO₂) – a 1-hour standard of 75 parts per billion (ppb), revised in 2010 – and one federal secondary standard – an annual average standard of 10 ppb, revised in 2024.  All areas in California are designated attainment/unclassifiable for SO₂.

Maps of current area designations can be found through the Resources tab on the left.

Sulfur Dioxide Primary Standards

• December 2017 - U.S. EPA completed the third round of sulfur dioxide (SO₂) designations for California. All areas in California were designated as attainment/unclassifiable.
• February 2013 - U.S. EPA published notice in the Federal Register (FR) of proposed nonattainment designations for the 2010 primary federal sulfur dioxide (SO₂) standards.  No California areas were included in the proposal; all areas of the State remained undesignated. 
• August 2012 - U.S. EPA published notice extending the deadline for designating areas with respect to the 2010 primary federal SO₂ standard for up to 1 year.  U.S. EPA had until June 3, 2013, to complete the initial designations (FR Notice August 3, 2012).
• June 2011 - CARB staff submitted area designation recommendations on June 20, 2011.
  • Transmittal Letter
  • Enclosure 1: Staff Report
  • Appendix 1: Five Factor Analyses
  • Appendix 2: 2009 SO₂ 1-Hour Design Values
  • Enclosure 2: Area Boundary Descriptions
• June 2010 - U.S.EPA revised the primary SO₂ standards (FR Notice June 22, 2010; effective August 23, 2010).

Additional information about activities related to SO₂ can be found on U.S. EPA's SO2 home page.

Sulfur Dioxide Secondary Standard

• December 2025 - CARB staff submitted area designation recommendations on December 3, 2025.
  • Transmittal Letter
  • Enclosure 1: Staff Report
  • Appendix 1: Five Factor Analysis
  • Appendix 2: 2024 SO₂ Annual Design Values
  • Appendix 3: Area Boundary Descriptions
• December 2024 - U.S. EPA revised the secondary SO₂ standard (FR Notice December 27, 2024; effective January 27, 2025).
• March 2012 - U.S. EPA took final action to retain the existing SO₂ and NO₂ secondary standards (FR Notice April 3, 2012; effective June 4, 2012).

Additional information about activities related to the secondary standards can be found on U.S. EPA's NO₂/SO₂ secondary standards webpage.

There are two federal primary standards for nitrogen dioxide (NO₂) - a 1-hour standard of 100 parts per billion (ppb) and an annual average of 0.053 parts per million (ppm).  There is one federal secondary standard - an annual average of 0.053 ppm (same as the primary standard).  All areas in California are designated as attainment/unclassified for the federal NO₂ standards.

Maps of current area designations can be found through the Resources tab on the left.  Information about specific NO₂ nonattainment areas and the efforts to bring them into attainment of the NO₂ standards can be found on the California State Implementation Plan (SIP) webpage.

Nitrogen Dioxide Primary Standards

• December 2012 - CARB submitted the NO₂ Infrastructure SIP to U.S. EPA
• January 2012 - U.S. EPA issued final area designations  (Federal Register (FR) Notice February 17, 2012; effective February 29, 2012).
• January 2011 - CARB submitted recommended area designations to U.S. EPA. The full submittal package and U.S. EPA's response are available on the U.S. EPA website.
• January 2010 - U.S. EPA revised the primary nitrogen dioxide (NO₂) standards (FR Notice February 9, 2010; effective April 12, 2010).

Information about activities related to the NO₂ standards can be found on U.S. EPA's NO2 home page.

Nitrogen Dioxide Secondary Standards

• March 2012 - U.S. EPA retained the existing SO₂ and NO₂ secondary standards (FR Notice April 3, 2012; effective June 4, 2012).

Additional information about activities related to the secondary standards can be found on U.S. EPA's NO₂/SO₂ secondary standards webpage.

There are two federal primary standards for PM_2.5 (fine particulate matter) - an annual average standard of 9.0 ug/m³ and a 24-hour average standard of 35 ug/m³.  There are two federal secondary standards - an annual standard of 15 ug/m³ and a 24-hour standard of 35 ug/m³ (the same as the primary).  Detailed information regarding the federal particulate matter (PM) standards can be found on the U.S. EPA website. 

In February 2024, U.S. EPA lowered the federal primary PM_2.5 annual standard to 9.0 ug/m³ from the 12.0 ug/m³ standard set in 2012. The secondary annual standard remains at 15.0 ug/m³. States and Tribal Authorities will submit initial recommendations of areas that do not attain this standard (i.e., nonattainment areas) to U.S. EPA by February 2025, and U.S. EPA will finalize area designations by February 2026. The steps in this process can be found in the 2024 9.0 ug/m³ Annual PM_2.5 Standard section below.

Annual Standard.   In February 2024, U.S. EPA lowered the federal primary PM_2.5 annual standard from 12.0 ug/m³ to 9.0 ug/m³; the secondary annual standard remained at 15 ug/m³.  Current area designation information can be found on U.S. EPA's website.

24-Hour Standard.  In December 2006, U.S. EPA lowered the federal 24-hour PM_2.5 standard from 65 ug/m³ to 35 ug/m³; the secondary 24-hour standard is the same as the primary.  Current area designation information can be found on U.S. EPA's website.

Maps of current area designations for the 2012 annual and 2006 24-hour standards can be found through the Resources tab on the left. These maps will be updated to include the 2024 9.0 ug/m³ annual PM_2.5 standard when these designations have been finalized in early 2026. Information about specific PM_2.5 nonattainment areas and the efforts to bring them into attainment of the federal PM_2.5 standards can be found on the California State Implementation Plan (SIP) webpage.

2024 9.0 ug/m³ Annual PM_2.5 Standard

• November 2027 - Moderate Area State Implementation Plans (SIPs) due for areas designated as nonattainment
  • CARB supporting website: SIPs for the 9 ug/m³ PM2.5 Standard
• February 2026 - U.S. EPA will issue final area designations 
• February 2025 - CARB will submit initial recommendations for area designations 
• January 23, 2025 - CARB will hold a public hearing to consider initial area designation and boundary recommendations. Staff recommendations to be presented at the hearing are contained in the Staff Report.
• September and December 2024 - CARB held public workshops on the area designation process and the initial area designation recommendations
• February 2024 - U.S. EPA issued final revised PM2.5 NAAQS (effective May 6, 2024)
  • Federal Register Notice (89 FR 16202)

2012 12.0 ug/m³ Annual PM_2.5 Standard

• January 2015 - U.S. EPA issued final area designations (effective April 15, 2015)
  • Federal Register Notice
• August 2014 - U.S. EPA responded to CARB area designations recommendations
  • Letter and Response (issued August 19, 2014)
• November 2013 - CARB submitted initial recommendations for area designations
  • Letter to U.S. EPA (November 25, 2013)
  • Enclosure - Staff Report:  PM_2.5 Area Recommendations for the Revised Federal PM_2.5 Annual Standard
• November 2013 - CARB held a public hearing (November 21, 2013) to consider the proposed recommendations
  • Notice of Public Hearing
  • Staff Report

2006 35 ug/m³ 24-Hour PM_2.5 Standard

• July 2018 - U.S. EPA redesignated the Chico Planning Area to attainment (for further information, see the Butte County Air Quality Management Plans page on CARB's website)
  • Federal Register Notice
• December 2014 - U.S. EPA redesignated the Yuba City-Marysville Planning Area to attainment (for further information, see the Feather River Air Quality Management Plans page on CARB's website)
  • Federal Register Notice
• February 2011 - U.S. EPA issued revised area designations
  • Federal Register Notice 
• November 2009 - U.S. EPA issued final area designations (effective December 14, 2009)
  • Federal Register Notice
  • Map showing 24-hour PM_2.5 Nonattainment Areas (located on U.S. EPA website)
• October 2008 - CARB submitted additional analysis for area designation recommendations
  • Letter (signed October 15, 2008)
    • Enclosure 1 - Technical Support - PM_2.5 Designations Recommendations
• August 2008 - U.S. EPA responded to CARB area designation recommendations
  • Letter and Response (issued August 18, 2008)
• December 2007 - CARB submitted initial recommendations for area designations
  • Letter (issued December 17, 2007)
  • Enclosure 1 - Initial Recommendations for Area Designations Under the Revised Federal PM_2.5 Standard
  • Enclosure 2 - Staff Report: Nonattainment Area Designation for the Revised Federal PM_2.5 Standard
  • Enclosure 3 - Information to Support Recommendations for Federal PM_2.5 Nonattainment Area Boundaries
  • Enclosure 4 - Boundary Descriptions for Recommended Nonattainment Areas under the Federal PM_2.5 Standard
  • Enclosure 5 - PM_2.5 Monitoring Data Summary, 2004-2006
• December 2007 - CARB held a public hearing (December 6-7, 2007)
  • Hearing Notice 
  • Staff Report
  • Potential Exceptional Events: U.S. EPA guidance allows for the exclusion from regulatory determinations of air quality data affected by exceptional events. Since U.S. EPA based their final nonattainment area designations on 2005-2007 PM_2.5 air quality monitoring data, CARB submitted 2007 calendar year exceptional events documentation. This document addresses monitoring days which may have been impacted by wildfires late in 2007 for areas in which the data may affect their designation status.

1997 65 ug/m³ 24-Hour PM_2.5 Standard and 15 ug/m³ Annual PM_2.5 Standard

• April 2005 - U.S. EPA finalized PM_2.5 area designations
  • Based on 2002-2004 PM_2.5 air quality monitoring data, the South Coast Air Basin and the San Joaquin Valley Air Basin were designated nonattainment for both the 24-hour and annual PM_2.5 standards.
• February 2004 - CARB submitted recommendations for PM_2.5 area designations
  • Letter (issued February 11, 2004)
  • Enclosure 1 - Initial Recommendations for Area Designations under the Federal PM_2.5 Standard
  • Enclosure 2 - Information to Support Recommendations for Federal PM_2.5 Nonattainment Area Boundaries
  • Enclosure 3 - Boundary Descriptions for Recommended Nonattainment Areas under the Federal PM_2.5 Standard
  • Enclosure 4 - PM_2.5 Monitoring Data Summary (2000-2002)

There is one federal primary standard for ozone - an 8-hour average standard of 0.070 parts per million (ppm); the secondary standard is the same as the primary.  Detailed information about activities related to the federal ozone standards and designations can be found on the U.S. EPA website.  Maps of current area designations can be found through the Resources tab on the left. 

Information about specific nonattainment areas and the efforts to bring them into attainment of the federal standards can be found on the California State Implementation Plan webpage. 

2015 0.070 ppm 8-Hour Ozone Standard

• July 2018 - U.S. EPA finalized area designations
• October 2016- CARB submits recommended area designations to U.S. EPA
  • CARB Staff Report
• October 2015 - U.S. EPA lowered the federal 8-hour primary and secondary ozone standard from 0.075 ppm to 0.070 ppm

More information on the 2015 standard, including a number of fact sheets and the U.S. EPA implementation memo, can be found on the U.S. EPA Ground-Level Ozone 2015 NAAQS Regulatory Actions website.

2008 0.075 ppm 8-Hour Ozone Standard

• April 2012 - U.S. EPA issued final area designations and classifications.
  • The designations included two new nonattainment areas -- Eastern San Luis Obispo County and the Tuscan Buttes area of Tehama County (Federal Register (FR) Notice for area designations published May 21, 2012;  FR Notice May 21, 2012; effective July 20, 2012).  
• March 2009 - CARB submits recommended area designations to U.S. EPA
  • Letter of submittal to U.S. EPA
  • Enclosure 1: Summary of recommended area designations
  • Enclosure 2: Revised staff report and attachments
  • Enclosure 3: Supporting information for recommended nonattainment areas
  • Enclosure 4: Nonattainment area boundary descriptions
  • Enclosure 5: Ozone air quality summary data by site
• March 2008 - U.S. EPA revised the 8-hour ozone standard (FR Notice March 27, 2008; effective May 27, 2008).

More information on the 2008 standard, including a number of fact sheets and the U.S. EPA implementation memo, can be found on the U.S. EPA Ground-Level Ozone 2008 NAAQS Regulatory Actions website.  Information on current area designation status for this standard can be found on the U.S. EPA Green Book 8-Hour Ozone (2008) Area Information webpage.

1997 0.08 ppm 8-Hour Ozone Standard

The 1997 standard was revoked in April 2015.

• December 2012 - U.S. EPA determined the following seven (7) areas had attained the 1997 federal 8-hour ozone standard (FR Notice December 3, 2012; effective January 2, 2013):  
  • Amador and Calaveras Counties (Central Mountain Counties)
  • Chico (Butte County)
  • Eastern Kern County (Mojave Desert Air Basin portion of County)
  • Mariposa and Tuolumne Counties (Southern Mountain Counties)
  • Western Nevada County
  • Sutter County (Sutter Buttes), and
  • Ventura County (excluding the Channel Islands -- Anacapa and San Nicolas islands)
• April 2012 - U.S. EPA reclassified Subpart 1 areas under the 1997 8-hour ozone standard (FR Notice May 14, 2012; effective June 13, 2012).
• June 2004 - EPA issued final nonattainment area designations.
• February 2004 - CARB submitted updated recommended area designations
  • Letter of submittal to U.S. EPA
  • Appendix: Additional information to support U.S. EPA's reconsideration of proposed area boundaries and designations
  • Enclosure 1: Updated recommendations for area designations based on 2001-2003 data
  • Enclosure 2: Boundary descriptions for recommended nonattainment areas
  • Enclosure 3: Cover only of correspondence from local officials on area designations
• July 2003 - CARB updated recommended area designations to U.S. EPA.
  • Letter of submittal to U.S. EPA
  • Enclosure 1: Areas in California that violate the 8-hour federal ozone standard based on 2000-2002 data
  • Enclosure 2: 2000-2002 8-hour ozone data for California air monitoring sites
• July 2000 - CARB submitted recommended area designations to U.S. EPA.

Information on current area designation status for this standard can be found on the U.S. EPA Green Book 8-Hour Ozone (1997) Area Information webpage.