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 |
Primary and Secondary | PM2.5 | 24 hour | 65 µg/m3 | 98th percentile, averaged over 3 years |
| 1997 62 FR 38652 Jul 18, 1997 |
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 |
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 |
Primary and Secondary | PM10 | Annual | 50 µg/m3 | Annual arithmetic mean, averaged over 3 years |
| 2006 71 FR 61144 Oct 17, 2006 |
Primary and Secondary |
PM2.5 | 24 hour | 35 µg/m3 | 98th percentile, averaged over 3 years 6 |
| 2006 71 FR 61144 Oct 17, 2006 |
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 |
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 |
Primary | PM2.5 | Annual | 12.0 µg/m3 | Annual arithmetic mean, averaged over 3 years 2, 7 |
| 2012 78 FR 3085 Jan 15, 2013 |
Secondary | PM2.5 | Annual | 15.0 µg/m3 | Annual arithmetic mean, averaged over 3 years 2, 7 |
| 2012 78 FR 3085 Jan 15, 2013 |
Primary and Secondary | PM2.5 | 24 hour | 35 µg/m3 | 98th percentile, averaged over 3 years 6 |
| 2012 78 FR 3085 Jan 15, 2013 |
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 |
Primary and secondary standards retained, without revision. | ||||
| 2024 | Primary | PM2.5 | Annual | 9.0 µg/m3 | Annual arithmetic mean, averaged over 3 years 2, 7 |
|
2024 |
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).
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, 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).
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).
8. The EPA revoked the annual PM10 NAAQS in 2006.