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    Most Recent Revision of PM Standard February 2024

    Background - TSP and PM10

    In 1971, EPA promulgated the original primary and secondary NAAQS (National Ambient Air Quality Standard) for particulate matter underIn 1971, EPA promulgated the original primary and secondary NAAQS (National Ambient Air Quality Standard) for particulate matter under Section 109 of the Clean Air Act. At that time, the reference method for collecting particulate samples for comparison to the standard was the "high-volume" sampler. This type of sampler collects particulate matter up to a size of 45 micrometers (µm) in diameter, called "total suspended particulate" (TSP). Micrometers are often also referred to as microns.

    In 1987, recognizing the risks of adverse health effects associated with smaller particles that are more likely to penetrate deeper into the respiratory system, EPA created a new particulate matter standard, based upon particles with a diameter of 10 microns or less (PM10). PM10 is also referred to as inhalable coarse particles. This then new size-specific indicator collected particulate matter with a nominal diameter of 10 microns or less. DEP designed and implemented an additional monitoring network to specifically measure PM10. At the same time, EPA abolished the TSP standard.

    Particulate Matter Standard Revised July 1997 - PM2.5

    EPA then concluded that the continued use of PM10 as the sole indicator for the particulate matter standard would not provide the most effective protection from the detrimental health effects of particulate matter. It was found that particulate matter with a diameter of 2.5 microns or less, posed the greatest risk to public health. These particles, also called fine particulate matter, could lodge themselves deeply in the lungs and remain for long periods. In July 1997, EPA completed its review of evidence on exposure to ambient fine particulate matter and revised the PM10 NAAQS by creating a PM2.5 standard as follows:

    • Created a different size-specific indicator for particulate matter, collecting particles with a nominal diameter of 2.5 microns or less.
    • Created an annual PM2.5 standard of 15 µg/m3.
    • Created a 24-hour PM2.5 standard of 65 µg/m3.
    • Retained the 24-hour PM10 standard of 150 µg/m3.

    Numerous legal challenges delayed implementation of the PM2.5 standards. In February 2001, the United States Supreme Court upheld EPA's authority under the Clean Air Act to set national ambient air quality standards for the protection of public health. In March 2002, the D.C. Circuit court rejected all of the remaining challenges to the standard. With the resolution of these legal issues, EPA began developing new strategies for implementation of the PM2.5 standards. In December 2004, EPA designated areas as attainment or non-attainment for the revised standard. For more information, see this page.

    Particulate Matter Standard Revised September 2006 - PM2.5

    The PM10 and PM2.5 standards were revised by EPA as follows:

    • Retained PM2.5 annual standard of 15 µg/m3.
    • Changed PM2.5 24-hour standard from 65 µg/m3 to 35 µg/m3.
    • Retained PM10 24-hour standard of 150 µg/m3.
    • Revoked PM10 annual standard

    EPA stated that the reason for revoking the annual standard for PM10 is that available evidence does not show a definitive link between long-term exposure to current PM levels and health problems. EPA has set the secondary standards (protective of public welfare) for particulate matter pollution at the same value as the new primary standards (protective of public health). 

    In October 2009, EPA designated areas as attainment or non-attainment for this revised standard. For more information, see this page.

    Particulate Matter Standard Revised December 2012 - PM2.5

    After the most recent review of the impacts of particulate matter pollution, EPA has strengthened the standards to better protect public health. The PM10 and PM2.5 standards were revised as follows:

    • Changed PM2.5 annual primary standard of 15 µg/m3 to 12 µg/m3
    • Retained PM2.5 annual secondary standard of 15 µg/m3
    • Retained PM2.5 24-hour standard of 35 µg/m3
    • Retained PM10 24-hour of 150 µg/m3

    EPA stated that the revision to the annual primary PM2.5 standard was necessary to provide increased protection against health effects associated with long- and short-term exposures. In addition, the secondary PM2.5 standards were retained since they were determined to already provide sufficient protection from visibility impairment and non-visibility related welfare effects.

    In December 2014, EPA designated areas as nonattainment or unclassifiable/attainment for this revised standard.

    Particulate Matter Standard Revised February 2024 - PM2.5

    EPA has strengthened the annual PM2.5 standard to better protect public health. The PM10 and PM2.5 standards were revised as follows:

    • Changed PM2.5 annual primary standard of 12 µg/m3 to 9 µg/m3
    • Retained PM2.5 annual secondary standard of 15 µg/m3
    • Retained PM2.5 24-hour standard of 35 µg/m3
    • Retained PM10 24-hour of 150 µg/m3

    EPA noted that the revision to the annual primary PM2.5 standard was necessary to protect against harmful and costly health impacts, consistent with the available health science. All other PM standards were retained. Finally, EPA revised the Air Quality Index and made changes to the monitoring network.

    In December 2014, EPA designated areas as nonattainment or unclassifiable/attainment for this revised standard.

    EPA Information