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The U.S. '''National Ambient Air Quality Standards''' ('''NAAQS''', pronounced {{IPAc-en|ˈ|n|æ|k|s}} {{respell|naks}}) are limits on atmospheric concentration of six [[air pollution|pollutants]] that cause [[smog]], [[acid rain]], and other health hazards.<ref name=":1">{{Cite web|url=http://ohioepa.custhelp.com/app/answers/detail/a_id/907/~/definition-of-national-ambient-air-quality-standards-(naaqs)|title=Definition of National Ambient Air Quality Standards (NAAQS)|website=ohioepa.custhelp.com|language=en-US|access-date=2018-03-29}}</ref> Established by the [[United States Environmental Protection Agency]] (EPA) under authority of the [[Clean Air Act (United States)|Clean Air Act]] (42 U.S.C. 7401 et seq.), NAAQS is applied for outdoor air throughout the country.<ref name=":0">{{Cite web |url=http://tapseis.anl.gov/glossacro/dsp_wordpopup.cfm?word_id=444 |title=Trans-Alaska Pipeline System Renewal Environmental Impact Statement article |access-date=2009-10-12 |archive-url=https://web.archive.org/web/20171020191429/http://tapseis.anl.gov/glossacro/dsp_wordpopup.cfm?word_id=444 |archive-date=2017-10-20 |url-status=dead }}</ref>
The six '''criteria air pollutants''' (CAP), or '''criteria pollutants''', for which limits are set in the NAAQS are [[tropospheric ozone|ozone]] (O<sub>3</sub>), [[atmospheric particulate matter]], [[lead]], [[carbon monoxide]] (CO), [[sulfur oxide]]s (SO<sub>x</sub>), and [[nitrogen oxide]]s (NO<sub>x</sub>).<ref>{{Cite web|url=https://www.epa.gov/criteria-air-pollutants|title=Criteria Air Pollutants {{!}} US EPA|last=EPA,OAR|first=US|website=US EPA|date=9 April 2014|language=en|access-date=2018-01-18}}</ref> These are typically emitted from many sources in [[Industrial sector|industry]], [[mining]], [[transportation]], [[electricity generation]] and [[agriculture]]. In many cases they are the products of the [[combustion]] of [[fossil fuels]] or industrial processes.<ref>http://des.nh.gov/organization/commissioner/pip/factsheets/ard/documents/ard-41.pdf {{Bare URL PDF|date=January 2022}}</ref>
The [[National Emissions Standards for Hazardous Air Pollutants]] cover many other chemicals, and require the maximum achievable reduction that the EPA determines is feasible.
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#* PM<sub>2.5</sub>, fine particles: 2.5 μm in size or less. Particulate Matter (PM) was listed in the 1996 Criteria document issued by the EPA. In April 2001, the EPA created a Second External Review Draft of the Air Quality Criteria for PM, which addressed updated studies done on particulate matter and the modified pollutant standards done since the First External Review Draft. In May 2002, a Third External Review Draft was made, and the EPA revised PM requirements again. After issuing a fourth version of the document, the EPA issued the final version in October 2004.
# [[Lead]] (Pb): In the mid-1970s, lead was listed as a criteria air pollutant that required NAAQS regulation. In 1977, the EPA published a document which detailed the Air Quality Criteria for lead. This document was based on the scientific assessments of lead at the time. Based on this report (1977 Lead AQCD), the EPA established a "1.5 µg/m<sup>3</sup> (maximum quarterly calendar average) Pb NAAQS in 1978.<ref>{{Cite web|url=http://cfpub.epa.gov/ncea/cfm/recordisplay.cfm?deid=158823.|title = Air Quality Criteria for Lead (Final Report, 2006)}}</ref>" The Clean Air Act requires periodic review of NAAQS, and new scientific data published after 1977 made it necessary to revise the standards previously established in the 1977 Lead AQCD document. An Addendum to the document was published in 1986 and then again as a Supplement to the 1986 AQCD/Addendum in 1990. In 1990, a Lead Staff Paper was prepared by the EPA's Office of Air Quality Planning and Standards (OPQPS), which was based on information presented in the 1986 Lead/AQCD/Addendum and 1990 Supplement, in addition to other OAQPS sponsored lead exposure/risk analyses. In this paper, it was proposed that the Pb NAAQS be revised further and presented options for revision to the EPA. The EPA elected to not modify the Pb NAAQS further, but decided to instead focus on the 1991 U.S. EPA Strategy for Reducing Lead Exposure. The EPA concentrated on regulatory and remedial clean-up efforts to minimize Pb exposure from numerous non-air sources that caused more severe public health risks, and undertook actions to reduce air emissions.
# [[Carbon monoxide]] (CO): The EPA set the first NAAQS for carbon monoxide in 1971. The primary standard was set at 9 ppm averaged over an 8-hour period and 35 ppm over a 1-hour period.<ref>{{Cite web|url=http://www.epa.gov/airquality/carbonmonoxide/.|title = Air Quality Planning and Standards}}</ref> The majority of CO emitted into the ambient air is from mobile sources. The EPA has reviewed and assessed the current scientific literature with respect to CO in 1979, 1984, 1991, and 1994.<ref>http://www.epa.gov/ncea/pdfs/coaqcd.pdf. {{Bare URL PDF|date=January 2022}}</ref> After the review in 1984 the EPA decided to remove the secondary standard for CO due to lack of significant evidence of the adverse environmental impacts. On January 28, 2011 the EPA decided that the current NAAQS for CO were sufficient and proposed to keep the existing standards as they stood. The EPA is strengthening monitoring requirements for CO by calling for CO monitors to be placed in strategic locations near large urban areas. Specifically, the EPA has called for monitors to be placed and operational in CBSA's (core based statistical areas) with populations over 2.5 million by January 1, 2015; and in CBSA's with populations of 1 million or more by January 1, 2017. In addition they are requiring the collocation of CO monitors with NO<sub>2</sub> monitors in urban areas having a population of 1 million for more. As of May 2011 there were approximately 328 operational CO monitors in place nationwide. The EPA has provided some authority to the EPA Regional Administrators to oversee case-by-case requested exceptions and in determining the need for additional monitoring systems above the minimum required.<ref name="CO">{{Cite web|url=http://www.epa.gov/airquality/carbonmonoxide/pdfs/COFactSheet.pdf.|title=Air Quality Planning and Standards}}</ref> The EPA reports the national average concentration of CO has decreased by 82% since 1980.<ref>http://www.epa.gov/airtrends/carbon.html.</ref> The last nonattainment designation was deemed in attainment on September 27, 2010. Currently all areas in the US are in attainment.<ref name="CO"/>
# [[Sulfur oxide]]s (SO<sub>x</sub>): SO<sub>x</sub> refers to the oxides of sulfur, a highly reactive group of gases. SO<sub>2</sub> is of greatest interest and is used as the indicator for the entire SO<sub>x</sub> family. The EPA first set primary and secondary standards in 1971. Dual primary standards were set at 140 ppb averaged over a 24-hour period, and at 30 ppb averaged annually. The secondary standard was set at 500 ppb averaged over a 3-hour period, not to be exceeded more than once a year. The most recent review took place in 1996 during which the EPA considered implementing a new NAAQS for 5-minute peaks of SO<sub>2</sub> affecting sensitive populations such as asthmatics. The Agency did not establish this new NAAQS and kept the existing standards.<ref>{{Cite web|url=http://www3.epa.gov/airquality/sulfurdioxide/.|title = Sulfur Dioxide | Air & Radiation | US EPA}}</ref> In 2010 the EPA decided to replace the dual primary standards with a new 1-hour standard set at 75 ppb. On March 20, 2012 the EPA "took final action" to maintain the existing NAAQS as they stood.<ref>http://www3.epa.gov/airquality/sulfurdioxide/pdfs/20120320factsheet.pdf. {{Bare URL PDF|date=January 2022}}</ref> Only three monitoring sites have exceeded the current NAAQS for SO<sub>2</sub>, all of which are located in the Hawaii Volcanoes National Park. The violations occurred between 2007-2008 and the state of Hawaii suggested these should be exempt from regulatory actions due to an 'exceptional event' (volcanic activity). Since 1980 the national concentration of SO<sub>2</sub> in the ambient air has decreased by 83%.<ref>http://www.epa.gov/airtrends/sulfur.html.</ref> Annual average concentrations hover between 1-6 ppb. Currently all ACQR's are in attainment for SO<sub>2</sub>.<ref>{{Cite web|url=http://www.epa.gov/airquality/sulfurdioxide/basic.html.|title = Air Quality Planning and Standards}}</ref>
# [[Nitrogen oxide]]s (NO<sub>x</sub>): The EPA first set primary and secondary standards for the oxides of nitrogen in 1971. Among these are nitric oxide (NO), nitrous oxide (N<sub>2</sub>O), and nitrogen dioxide (NO<sub>2</sub>), all of which are covered in the NAAQS. NO<sub>2</sub> is the oxide measured and used as the indicator for the entire NO<sub>x</sub> family as it is of the most concern due to its quick formation and contribution to the formation of harmful ground level ozone.<ref name="airqualityNO">{{Cite web|url=http://www.epa.gov/airquality/nitrogenoxides/.|title = Air Quality Planning and Standards}}</ref> In 1971 the primary and secondary NAAQS for NO<sub>2</sub> were both set at an annual average of 0.053 ppm. The EPA reviewed this NAAQS in 1985 and 1996, and in both cases concluded that the existing standard was sufficient. The most recent review by the EPA occurred in 2010, resulting in a new 1-hour NO<sub>2</sub> primary standard set at 100 ppb; the annual average of 0.053 ppm remained the same. Also considered was a new 1-hour secondary standard of 100 ppb. This was the first time the EPA reviewed the environmental impacts separate from the health impacts for this group of criteria air pollutants<ref name="airqualityNO"/> Also, in 2010, the EPA decided to ensure compliance by strengthening monitoring requirements, calling for increased numbers of monitoring systems near large urban areas and major roadways. On March 20, 2012 the EPA "took final action" to maintain the existing NAAQS as they stand.<ref>Fact Sheet – Air Quality Designations for the 2010 Primary Nitrogen Dioxide (NO<sub>2</sub>) National Ambient Air Quality Standards http://www.epa.gov/airquality/nitrogenoxides/designations/pdfs/20120120FS.pdf.</ref> The national average of NO<sub>x</sub> concentrations has dropped by 52% since 1980.<ref>http://www.epa.gov/airtrends/nitrogen.html.</ref> The annual concentration for NO<sub>2</sub> is reported to be averaging around 10-20 ppb, and is expected to decrease further with new mobile source regulations.<ref>{{Cite web|url=http://www.epa.gov/airquality/nitrogenoxides/basic.html.|title = Air Quality Planning and Standards}}</ref> Currently all areas of the US are classified as in attainment.<ref name="airqualityNO"/>
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