Timeline of Particulate Matter (PM) National Ambient Air Quality Standards (NAAQS)
| Final Rule/Decision | Primary/Secondary | Indicator1 | Averaging Time | Level2 | Form |
|---|---|---|---|---|---|
| 1971 36 FR 8186 Apr 30, 1971 |
Primary | TSP | 24 hour | 260 µg/m3 | Not to be exceeded more than once per year |
| 1971 36 FR 8186 Apr 30, 1971 |
Primary | TSP | Annual | 75 µg/m3 | Annual geometric mean |
| 1971 36 FR 8186 Apr 30, 1971 |
Secondary | TSP | 24 hour | 150 µg/m3 | Not to be exceeded more than once per year |
| 1971 36 FR 8186 Apr 30, 1971 |
Secondary | TSP | Annual | 60 µg/m3 | Annual geometric mean |
| 1987 52 FR 24634 Jul 1, 1987 |
Primary and Secondary | PM10 | 24 hour | 150 µg/m3 | Not to be exceeded more than once per year on average over a 3-year period |
| 1987 52 FR 24634 Jul 1, 1987 |
Primary and Secondary | PM10 | Annual | 50 µg/m3 | Annual arithmetic mean, averaged over 3 years |
| 1997 62 FR 38652 Jul 18, 1997 (pdf) (1,878 KB) |
Primary and Secondary | PM2.5 | 24 hour | 65 µg/m3 | 98th percentile, averaged over 3 years |
| 1997 62 FR 38652 Jul 18, 1997 (pdf) (1,878 KB) |
Primary and Secondary | PM2.5 | Annual | 15.0 µg/m3 | Annual arithmetic mean, averaged over 3 years 3, 4 |
| 1997 62 FR 38652 Jul 18, 1997 (pdf) (1,878 KB) |
Primary and Secondary | PM10 | 24 hour | 150 µg/m3 | Initially promulgated 99th percentile, averaged over 3 years; when 1997 standards for PM10 were vacated, the form of 1987 standards remained in place (not to be exceeded more than once per year on average over a 3-year period) 5 |
| 1997 62 FR 38652 Jul 18, 1997 (pdf) (1,878 KB) |
Primary and Secondary | PM10 | Annual | 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 hour | 35 µg/m3 | 98th percentile, averaged over 3 years 6 |
| 2006 71 FR 61144 Oct 17, 2006 (pdf) (768 KB) |
Primary and Secondary | PM2.5 | Annual | 15.0 µg/m3 | Annual arithmetic mean, averaged over 3 years 2, 7 |
| 2006 71 FR 61144 Oct 17, 2006 (pdf) (768 KB) |
Primary and Secondary | PM10 | 24 hour | 150 µg/m3 | Not to be exceeded more than once per year on average over a 3-year period |
| 2012 78 FR 3085 Jan 15, 2013 (pdf) (4 MB) |
Primary | PM2.5 | Annual | 12.0 µg/m3 | Annual arithmetic mean, averaged over 3 years 2, 7 |
| 2012 78 FR 3085 Jan 15, 2013 (pdf) (4 MB) |
Secondary | PM2.5 | Annual | 15.0 µg/m3 | Annual arithmetic mean, averaged over 3 years 2, 7 |
| 2012 78 FR 3085 Jan 15, 2013 (pdf) (4 MB) |
Primary and Secondary | PM2.5 | 24 hour | 35 µg/m3 | 98th percentile, averaged over 3 years 6 |
| 2012 78 FR 3085 Jan 15, 2013 (pdf) (4 MB) |
Primary and Secondary | PM10 | 24 hour 8 | 150 µg/m3 | Not to be exceeded more than once per year on average over a 3-year period |
|
2020 85 FR 82684 Dec 18, 2020 (pdf) (640 KB) |
Primary and secondary standards retained, without revision. | ||||
|
2024 89 FR 16202 Mar 6, 2024 (pdf) (4.8 MB) |
Primary | PM2.5 | Annual | 9.0 µg/m3 | Annual arithmetic mean, averaged over 3 years 2, 7 |
|
2024 89 FR 16202 Mar 6, 2024 (pdf) (4.8 MB) 89 FR 105692 Dec 27, 2024 (pdf) (1,510 KB) |
Secondary PM2.5 standards, and primary and secondary PM10 standards, retained without revision. | ||||
1. TSP = Total Suspended Particles
2. Units of measure are micrograms per cubic meter of air (µg/m3).
3. The level of the annual standard is defined to one decimal place (i.e., 15.0 µg/m3) as determined by rounding. For example, a 3-year average annual mean of 15.04 µg/m3 would round to 15.0 µg/m3 and, thus, meet the annual standard and a 3-year average of 15.05 µg/m3 would round to 15.1 µg/m3 and, hence, violate the annual standard (40 CFR part 50 Appendix N (pdf) (365 KB)).
4. The level of the standard 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”).
5. See 69 FR 45592 (pdf) (173 KB), July 30, 2004.
6. The level of the 24-hour standard is defined as an integer (zero decimal places) as determined by rounding. For example, a 3-year average 98th percentile concentration of 35.49 µg/m3 would round to 35 µg/m3 and thus meet the 24-hour standard and a 3-year average of 35.50 µg/m3 would round to 36 and, hence, violate the 24-hour standard (40 CFR part 50 Appendix N (pdf) (365 KB)).
7. 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)).
8. The EPA revoked the annual PM10 NAAQS in 2006.