Timeline of Particulate Matter (PM) National Ambient Air Quality Standards (NAAQS)
| Final Decision/Action | Primary/ Secondary | Indicator | Averaging Time | Level1 2 | Form |
|---|---|---|---|---|---|
1971 36 FR 8186 Apr 30, 1971 | Primary | TSP | 24 hours | 260 µg/m3 | Not to be exceeded more than once per year |
| Secondary | 150 µg/m3 | Not to be exceeded more than once per year | |||
| Primary | 1 year | 75 µg/m3 | Annual geometric mean | ||
| Secondary | 60 µg/m3 | Annual geometric mean | |||
1987 | Primary and Secondary | PM10 | 24 hours | 150 µg/m3 | Not to be exceeded more than once per year on average over a 3-year period |
| 1 year | 50 µg/m3 | Annual arithmetic mean, averaged over 3 years | |||
1997 62 FR 38652, July 18, 1997 (pdf) (1,878 KB) | Primary and Secondary | PM2.5 | 24 hours | 65 µg/m3 | Annual 98th percentile, averaged over 3 years |
| 1 year | 15.0 µg/m3 | Annual arithmetic mean, averaged over 3 years 3 | |||
| Primary and Secondary | PM10 | 24 hours | 150 µg/m3 | 99th percentile, averaged over 3 years | |
| Retained annual PM10 standard, without revision | |||||
1999 Court decision vacated 1997 PM10 standards revisions4 | Primary and Secondary | PM2.5 | PM2.5 standards were upheld. | ||
| Primary and Secondary | PM10 | 24 hours | 150 µg/m3 | Not to be exceeded more than once per year on average over a 3-year period | |
| 1 year | 50 µg/m3 | Annual arithmetic mean, averaged over 3 years | |||
2006 71 FR 61144, Oct 17, 2006 (pdf) (768 KB) | Primary and Secondary | PM2.5 | 24 hours | 35 µg/m3 | Annual 98th percentile, averaged over 3 years |
| 1 year | 15.0 µg/m3 | Annual arithmetic mean, averaged over 3 years5 | |||
| Primary and Secondary | PM10 | 24 hours | Retained, without revision | ||
| Revoked annual standards | |||||
2013 | Primary and Secondary | PM2.5 | 24 hours | Retained the 24-hour PM2.5 standards, without revision | |
| Primary | 1 year | 12.0 µg/m3 | Annual arithmetic mean, averaged over 3 years6 | ||
| Secondary | 15.0 µg/m3 | Annual arithmetic mean, averaged over 3 years6 | |||
| Primary and Secondary | PM10 | 24 hours | Retained 24-hr PM10 standards, without revision | ||
2020 85 FR 82684, Dec 18, 2020 (pdf) (640 KB) | Primary and secondary standards retained, without revision. | ||||
2024 89 FR 16202, March 6, 2024 (pdf) (4.8 MB) | Primary | PM2.5 | 1 year | 9.0 µg/m3 | Annual arithmetic mean, averaged over 3 years |
| All other primary and secondary PM2.5 and PM10 standards, retained without revision. | |||||
2024 89 FR 105692, Dec 27, 2024 (pdf) (1,510 KB) | Secondary PM2.5 and PM10 standards, retained without revision. | ||||
| Abbreviations: TSP = Total Suspended Particles, PM10 = mass median diameter at/below 10 microns, PM2.5 = mass median diameter at/below 2.5 microns, µg/m3 = micrograms per cubic meter of air | |||||
1. The level of the annual PM2.5 standards is defined to one decimal place (e.g., 15.0 µg/m3). For example, a 3-year average of annual means equal to 15.04 µg/m3 would round to 15.0 µg/m3 and, thus, meet a standard of 15.0 µg/m3 and a 3-year average of 15.05 µg/m3 would round to 15.1 µg/m3 and, hence, violate a standard of 15.0 µg/m3 (40 CFR part 50 Appendix N (pdf) (365 KB)).
2. The level of the 24-hour PM2.5 standards is an integer (zero decimal places). For example, a 3-year average 98th percentile concentration of 35.49 µg/m3 would round to 35 µg/m3 and thus meet a 24-hour standard of 35 µg/m3 and a 3-year average of 35.50 µg/m3 would round to 36 µg/m3 and, hence, violate a 24-hour standard of 35 µg/m3 (40 CFR part 50 Appendix N (pdf) (365 KB)).
3. The level of the annual PM2.5 standards was to be compared to measurements made at sites that represent “community-wide air quality” recording the highest level, or, if specific requirements were satisfied, to average measurements from multiple community-wide air quality monitoring sites (“spatial averaging”).
4. In May 1999, the 1997 PM10 standards were vacated by decision of the U.S. Court of Appeals for the District of Columbia Circuit, leaving the pre-existing 1987 standards in place. Pursuant to the court’s decision, EPA removed the vacated 1997 PM10 standards from the regulations (CFR) (69 FR 45592, July 30, 2004) and deleted the regulatory provision (at 40 CFR 50.6(d)) that controlled the transition from the pre- existing 1987 PM10 standards to the 1997 PM10 standards (65 FR 80776, December 22, 2000). The pre-existing 1987 PM10 standards remained in place. Id. at 80777. See 69 FR 45592 (pdf) (173 KB), July 30, 2004.
5. The EPA tightened the constraints on the spatial averaging criteria by further limiting the conditions under which some areas may average measurements from multiple community-oriented monitors to determine compliance (see 71 FR 61165 - 61167 (pdf) (768 KB)).
6. The EPA removed the spatial averaging provisions as part of the form of the annual primary and secondary PM2.5 standards (see 78 FR 3088-3089, Jan 15, 2013).