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Air Quality Program

National and State Ambient Air Quality Standards

EPA sets National Ambient Air Quality Standards (NAAQS) for criteria pollutants. States are required to meet the national standards. A state can set more stringent ambient air quality standards within the state. Washington state adopts current federal NAAQS in state regulations (Chapter 173-476 WAC, Ambient Air Quality Standards). The federal Clean Air Act requires EPA to review NAAQS every 5 years to make sure the standards protect human health and the environment. State regulations are updated when EPA revises or establishes a new standard.

The federal Clean Air Act requires EPA to set the standards based on scientific evidence. The standards must protect sensitive populations including children, elderly, and people with existing heart and lung disease. EPA cannot consider the cost of the pollution controls when setting a standard. It is strictly health-based. However, EPA can take into account the cost of control strategies in implementing a standard.

EPA sets two types of NAAQS:

  1. Primary standards set limits to protect public health, including the health of sensitive populations such as asthmatics, children, and the elderly.
  2. Secondary standards set limits to protect public welfare, including protection against decreased visibility, damage to animals, crops, vegetation, and buildings.

Each standard is measured in one of three ways:

  • Parts per million (ppm) by volume
  • Parts per billion (ppb) by volume
  • Micrograms per cubic meter of air (µg/m3)

NAAQS are set under Section 109 of the Clean Air Act. Regulatory information about NAAQS is contained in the Code of Federal Regulations (40 CFR Part 501). When EPA establishes or revises a standard, Ecology and EPA evaluate the areas of the state to determine their air quality status. See Air Quality Designations page for more information.

Current NAAQS are listed on EPA's web site.

Ambient air quality standards

Ambient air quality standards, adopted by the Washington State, are listed in the table below. This table provides an overview. See Chapter 173-476 WAC for more detail.

Pollutant Averaging Time Level Remarks Measurement Method Interpretation Method
Particle Pollution PM-10 24-hour 150 µg/m3 Not to be exceeded more than once per year averaged over 3 years 40 C.F.R. Part 50, Appendix J 40 C.F.R. Part 50, Appendix K
PM-2.5 Annual 12.0 µg/m3 Annual mean, averaged over 3 years 40 C.F.R. Part 50, Appendix L 40 C.F.R. Part 50, Appendix N
24-hour 35 µg/m3 98th percentile, averaged over 3 years
Lead Rolling 3-month average 0.15 µg/m3 Not to be exceeded 40 C.F.R. Part 50, Appendix G 40 C.F.R. Part 50, Appendix R
Sulfur Dioxide Annual* 0.02 ppmv Not to be exceeded in a calendar year 40 C.F.R. Part 50, Appendix A-1 or A-2 WAC 173-476-130(3)
24-hour* 0.14 ppmv Not to be exceeded more than once per year
3-hour 0.5 ppmv Not to be exceeded more than once per year
1-hour 75 ppbv 99th percentile of 1-hour daily maximum concentrations, averaged over 3 years
Nitrogen Dioxide Annual 53 ppbv Annual Mean 40 C.F.R. Part 50, Appendix F 40 C.F.R. Part 50, Appendix S
1-hour 100 ppbv 98th percentile of 1-hour daily maximum concentrations, averaged over 3 years
Ozone 8-hour 0.075 ppmv Annual fourth-highest daily maximum 8-hr concentration, averaged over 3 years 40 C.F.R. Part 50, Appendix D 40 C.F.R. Part 50, Appendix P
Carbon Monoxide 8-hour 9 ppmv Not to be exceeded more than once per year 40 C.F.R. Part 50, Appendix C WAC 173-476-160(3)
1-hour 35 ppmv

*Sunset provision. Annual and 24-hour SO2 standards have a “sunset provision.“ They will no longer apply to those areas that have been in attainment status (designated by EPA) for the one-hour SO2 standard for one year. (See WAC 173-476-130 and 40 C.F.R. 50.17 for additional details.)

History of NAAQS

1970s - 2011 Ozone

The one-hour ozone standard, originally set at 0.08 parts per million (ppm) in 1971, was revised in 1979 to 0.12 ppm. In Washington, most of King, Pierce, and part of Snohomish counties and Vancouver (part of Portland-Vancouver area) were designated as nonattainment for this standard. These areas met the standard and were redesignated back into attainment in 1993.

In July 1997, after a thorough review of the current science relating to the adverse health effects of ozone pollution, EPA replaced the one-hour standard with an eight-hour standard of 0.08 parts per million. EPA also changed the form of the standard from no more than one exceedance per year to the three-year average of the 4th maximum concentration.

The new standard was challenged in court. In February 2001, the U.S. Supreme Court upheld the standard, unanimously rejecting arguments that EPA must balance compliance costs against the health benefits when setting ambient air standards. (Whitman v. American Trucking Associations, Inc., 531 U.S. 457, 471 (2001)). The justices also ruled against the arguments that EPA exceeded its lawmaking authority when it set the revised standards in 1997. However, the Supreme Court ruled that EPA's implementation guidance for the revised ozone standard was unlawful and remanded it to EPA. It ruled that EPA must develop a reasonable approach to implementing the Clean Air Act requirements for the standard. This court case reaffirmed the right of EPA to set air quality standards based only on the impact on public health.

In March 2008, EPA revised the eight hour ozone standard. The primary (public health) and secondary (public welfare) standards were set identically at 0.075 ppm (75 parts per billion). After this standard was challenged in court, EPA requested permission to reconsider the 2008 standard. This standard was not as protective as recommended by EPA?s panel of scientific advisors (Clean Air Scientific Advisory Committee). In September 2009, EPA announced that they plan to reconsider the 2008 standard. In January 2010, EPA proposed that the new standard is set within the range of 0.060 ppm to 0.070 ppm. However, in September 2011, the Administration issued a statement requesting EPA to withdraw the draft Ozone National Ambient Air Quality Standards and continue to implement the standard that was set in 2008. The President said "Work is already underway to update a 2006 review of the science that will result in the reconsideration of the ozone standard in 2013. Ultimately, I did not support asking state and local governments to begin implementing a new standard that will soon be reconsidered." When the new standard is issued, the 2008 standard will be revoked. EPA's current scientific and policy review to support the next revision of the ozone standard is underway. For more information, visit EPA's website.

1971 - 2010 Nitrogen Dioxide (NO2)

EPA first established standards for NO2 in 1971, setting both a primary standard (to protect health) and a secondary standard (to protect the public welfare) at 53 ppb, averaged annually. Prior to 2010, the Agency reviewed the standards twice, but chose not to revise them.

On January 22, 2010, EPA strengthened the standard by setting a new health-based 1-hour standard of 100 ppb. EPA also retained, with no change, the current annual average NO2 standard of 53 ppb.

1971 - 2010 Sulfur Dioxide (SO2)

On April 30, 1971, EPA promulgated primary and secondary NAAQS for sulfur oxides, measured as SO2. The primary standards were 0.14 ppm, averaged over a period of 24 hours and not to be exceeded more than once per year, and 0.030 ppm annual arithmetic mean. The secondary standard was set at 0.50 ppm averaged over a period of 3 hours and not to be exceeded more than once per year.

After periodic reviews of additional scientific information, EPA announced first in 1986 and then in 1996, its decision not to revise the NAAQS for SO2. In 2010, EPA replaced both the 24-hour and annual standards with a new short-term standard based on the 3-year average of the 99th percentile of the yearly distribution of 1-hour daily maximum SO2 concentrations. EPA set the level of this new standard at 75 ppb.

1971 - 2013 Particulate Matter (PM10, PM2.5, and TSP)

The nation's air quality standards for total suspended particulates (TSP) were first established in 1971 and were not significantly revised until 1987, when EPA established the PM10 NAAQS, changing the indicator of the standards to regulate respirable particles less than or equal to 10 microns in diameter. In Washington, parts of Seattle, Kent, Tacoma, Thurston County, Wallula, Spokane, and Yakima were all designated nonattainment for the 1987 PM10 standard in 1990. The areas met the standard and were redesignated as attainment areas between 1999 and 2005. Until 1997, EPA regulated PM10 with a 24-hour standard and an annual standard. As a result of the 1997 review, EPA revoked the annual standard and retained the 24-hour standard of 150µg/m3.

In 1997, EPA decided to separate particles for regulatory purposes into PM2.5 and PM10 because of the differing health effects. The primary and secondary NAAQS for PM2.5 were set at 65 µg/m3 averaged over a 24-hour period. An annual concentration of 15 µg/m3 based of the three-year average of the annual arithmetic mean PM2.5 concentrations from one or more community-oriented monitors.

In 2006, EPA revised PM2.5 primary and secondary standards. Both primary and secondary standards are identical and were set at 35 µg/m3 for a 24-hour average and retained the annual level at 15 µg/m3. Parts of Tacoma-Pierce County were designated nonattainment for the 2006 24-hour PM2.5 standard in 2009. EPA determined that the Tacoma-Pierce County nonattainment area met the 2006 24-hour PM2.5 standard based on 2009-2011 monitoring data and approved Ecology's request for a clean data determination in 2012. With regard to primary standards for PM10, EPA retained the 24-hour PM10 of 150 µg/m3 and revoked the annual PM10 standard.

In 2012, EPA revised the suite of standards for PM. EPA revised the annual PM2.5 standard by lowering the level to 12.0 µg/m3 and retained the 24-hour PM2.5 standard at a level of 35 µg/m3. EPA revised the Air Quality Index (AQI) for PM2.5 to be consistent with the revised primary PM2.5 standards. EPA also retained the current 24-hour PM10 standard of 150 µg/m3. The new standards went in effect in March, 2013.

1971 - 2011 Carbon Monoxide

Identical primary (health-based) and secondary (welfare-based) NAAQS for carbon monoxide (CO) were set in 1971 at 9 ppm, 8-hour average, and at 35 ppm, 1-hour average, neither to be exceeded more than once a year. In Washington, parts of Seattle, Tacoma, Vancouver, Spokane and Yakima were designated nonattainment for the 1971 CO standard. The areas met the standard and were redesignated attainment between 1996 and 2004. In 1985, EPA announced the decision not to revise the primary CO NAAQS and at the same time to revoke the secondary standard.

After the most recent review of the CO NAAQS, EPA proposed to retain the current primary standards. After review of the air quality criteria, EPA further concludes that no secondary standard should be set for CO at this time. EPA is also making changes to the ambient air monitoring requirements for CO, including those related to network design, and is updating, without substantive change, aspects of the Federal reference method.

1978 - 2009 Lead

In 1978, EPA set a primary and secondary NAAQS for lead (Pb) at a level of 1.5 micrograms lead per cubic meter of air (µg/m3), averaged over a calendar quarter. The standard was reviewed in the 1980s. Based on this review, EPA decided not to revise the 1978 standard. In a parallel effort, EPA developed the broad, multi-program and multimedia U.S. Strategy for Reducing Lead Exposure.

In 2004, EPA initiated a review of the air quality criteria for Pb. Based on its review, EPA significantly strengthened the standard in 2009. Both primary and secondary standards are identical and set to the level of 0.15 µg/m3, which is 1/10 of the previous standard.

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