Portland, ME Air Quality in 2021
In 2021, Portland's air quality (Portland-South Portland, ME area) had a median daily AQI of 36 (Good) across 365 monitored days. 3 days (0.8%) reached Unhealthy for Sensitive Groups or worse, 313 were Good, and the year's peak AQI was 129 (Unhealthy for Sensitive Groups). Ground-level ozone set the daily AQI on 267 of those days. The cleanest month of 2021 was October (median AQI 29, 0 days above 100) and the worst was June (median 46, 2 days above 100, peak 129).
- Good: 85.8%
- Moderate: 13.4%
- Unhealthy for Sensitive Groups: 0.8%
- Unhealthy or worse: 0.0%
Reduce Indoor Ozone in Portland
Today's main pollutant in Portland is ground-level ozone (AQI 27). Important: standard HEPA filters do NOT remove ozone — only a purifier with a substantial activated-carbon bed adsorbs it. The biggest wins are cutting indoor sources (gas appliances, certain "ionizer" purifiers) and ventilating when outdoor ozone is low (early morning). A high-carbon unit helps with the gases and VOCs that linger indoors.
Recommended: Levoit Core 300 — True HEPA + activated carbon · CADR ~141 CFM · rooms up to 219 sq ft
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How was Portland's air quality in 2021?
Good on a typical day: median AQI 36, with 3 days above AQI 100 out of 365 monitored — the 4th cleanest of 7 years on record for the area.
- ✓Peak AQI in 2021: 129 (Unhealthy for Sensitive Groups).
- ✓Ground-level ozone was the main pollutant on 267 days.
- ✓Cleanest month: October; worst: June.
Frequently asked questions
What was the average AQI in Portland in 2021?▾
EPA's Air Quality System puts Portland's (Portland-South Portland, ME) median daily AQI for 2021 at 36, which is Good. 3 days were Unhealthy for Sensitive Groups or worse and the year's highest AQI was 129.
Which month had the worst air quality in Portland in 2021?▾
The cleanest month of 2021 was October (median AQI 29, 0 days above 100) and the worst was June (median 46, 2 days above 100, peak 129).
Was 2021 a good or bad year for Portland's air?▾
Among the 7 years EPA has published for the area (2019–2025), 2021 ranks 4th cleanest by days above AQI 100. 2021's median of 36 is below the area's typical annual median of 40.
Portland month by month in 2021
Portland air quality month by month, 2021 (Portland-South Portland, ME, EPA AQS daily AQI)
| Month | Days monitored | Median AQI | Days above 100 | Peak AQI | Main pollutant |
|---|---|---|---|---|---|
| January | 31 | 35 | 0 | 65 | O3 |
| February | 28 | 41 | 0 | 58 | O3 |
| March | 31 | 43 | 0 | 71 | O3 |
| April | 30 | 40 | 0 | 67 | O3 |
| May | 31 | 38 | 0 | 84 | O3 |
| Juneworst | 30 | 46 | 2 | 129 | O3 |
| July | 31 | 31 | 0 | 92 | O3 |
| August | 31 | 40 | 1 | 129 | O3 |
| September | 30 | 31 | 0 | 49 | O3 |
| Octobercleanest | 31 | 29 | 0 | 46 | O3 |
| November | 30 | 31 | 0 | 56 | O3 |
| December | 31 | 33 | 0 | 54 | O3 |
How 2021 compares
Among the 7 years EPA has published for the area (2019–2025), 2021 ranks 4th cleanest by days above AQI 100.
2021's median of 36 is below the area's typical annual median of 40.
Activity-specific thresholds: running, cycling, kids' sports.
Right now in Portland
The latest EPA AirNow observation puts Portland's AQI at 27 (O3). The figures above are multi-year annual statistics, not today's reading.
View current Portland air quality →Source: EPA Air Quality System (AQS) pre-generated Daily AQI by CBSA files — daily AQI by CBSA (Portland-South Portland, ME), 2019–2025. Daily AQI per CBSA is the highest AQI reported by any monitor in the area that day; the defining parameter is the pollutant that set it. Month and season statistics pool every such day in the period across all years listed. Retrieved 2026-09-09.