Portland Best & Worst AQI Days: Air Quality Guide
The worst air quality day on record for Portland (Portland-South Portland, ME area) was August 12, 2025, when the daily AQI reached 169 (Unhealthy), set by ground-level ozone, in peak ozone season. Across 2,554 monitored days in 2019–2025, 1 of the 10 worst days was Unhealthy (AQI 151+) or worse; 8 of the 10 were driven by ground-level ozone and 2 by fine particulate matter (PM2.5), and 3 of them fell in 2021. The cleanest day was November 13, 2025 at AQI 13 (Good); the 10 best days span 5 different years.
Reduce Indoor Ozone in Portland
Today's main pollutant in Portland is ground-level ozone (AQI 29). 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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When were Portland's worst and best air quality days?
Worst: August 12, 2025, AQI 169 (Unhealthy, O3). Best: November 13, 2025, AQI 13 (Good). Based on 2,554 EPA-monitored days in the Portland-South Portland, ME area, 2019–2025.
- ✓1 of the 10 worst days were Unhealthy (AQI 151+) or worse; the 10th-worst was AQI 114 (Unhealthy for Sensitive Groups).
- ✓Ground-level ozone drove 8 of the 10 worst days.
- ✓2021 accounts for 3 of the 10 worst days.
Frequently asked questions
What was the worst air quality day in Portland?▾
August 12, 2025: the Portland-South Portland, ME area's daily AQI hit 169 (Unhealthy), driven by ground-level ozone, in peak ozone season. That is the highest of 2,554 monitored days in 2019–2025.
What was the best air quality day in Portland?▾
November 13, 2025, with a daily AQI of 13 (Good). The daily AQI is the highest reading from any monitor in the area, so even the cleanest day reflects the area's worst monitor.
What was Portland's worst air quality day in 2025?▾
August 12, 2025, AQI 169 (Unhealthy), set by ground-level ozone, in peak ozone season.
Which pollutant causes Portland's worst air days?▾
Ground-level ozone set the AQI on 8 of the 10 worst days; fine particulate matter (PM2.5) on 2. The tables above list the defining pollutant for every day.
Portland's 10 worst air quality days, 2019–2025
Highest daily AQI recorded for the Portland-South Portland, ME area (EPA AQS). The daily AQI is the highest reading from any monitor in the area that day.
| # | Date | AQI | Category | Defining pollutant |
|---|---|---|---|---|
| 1 | (2025 page) | 169 | Unhealthy | O3 — ground-level ozone, in peak ozone season |
| 2 | (2025 page) | 147 | Unhealthy for Sensitive Groups | O3 — ground-level ozone, in peak ozone season |
| 3 | (2024 page) | 129 | Unhealthy for Sensitive Groups | O3 — ground-level ozone, in peak ozone season |
| 4 | (2021 page) | 129 | Unhealthy for Sensitive Groups | O3 — ground-level ozone, in peak ozone season |
| 5 | (2021 page) | 129 | Unhealthy for Sensitive Groups | O3 — ground-level ozone, in peak ozone season |
| 6 | (2022 page) | 126 | Unhealthy for Sensitive Groups | O3 — ground-level ozone, in peak ozone season |
| 7 | (2021 page) | 122 | Unhealthy for Sensitive Groups | O3 — ground-level ozone, in peak ozone season |
| 8 | (2023 page) | 120 | Unhealthy for Sensitive Groups | PM2.5 — fine particulate matter (PM2.5), in wildfire smoke season |
| 9 | (2019 page) | 115 | Unhealthy for Sensitive Groups | O3 — ground-level ozone, in peak ozone season |
| 10 | (2025 page) | 114 | Unhealthy for Sensitive Groups | PM2.5 — fine particulate matter (PM2.5), in wildfire smoke season |
Worst day of each year in Portland
The single highest-AQI day in each year on record. Click a year for its full month-by-month statistics.
| Year | Worst day | AQI | Category | Defining pollutant |
|---|---|---|---|---|
| 2019 | 115 | Unhealthy for Sensitive Groups | O3 — ground-level ozone, in peak ozone season | |
| 2020 | 97 | Moderate | NO2 — nitrogen dioxide | |
| 2021 | 129 | Unhealthy for Sensitive Groups | O3 — ground-level ozone, in peak ozone season | |
| 2022 | 126 | Unhealthy for Sensitive Groups | O3 — ground-level ozone, in peak ozone season | |
| 2023 | 120 | Unhealthy for Sensitive Groups | PM2.5 — fine particulate matter (PM2.5), in wildfire smoke season | |
| 2024 | 129 | Unhealthy for Sensitive Groups | O3 — ground-level ozone, in peak ozone season | |
| 2025 | 169 | Unhealthy | O3 — ground-level ozone, in peak ozone season |
Portland's 10 best air quality days, 2019–2025
Lowest daily AQI recorded for the Portland-South Portland, ME area. Ties are listed most recent first.
| # | Date | AQI | Category | Defining pollutant |
|---|---|---|---|---|
| 1 | (2025 page) | 13 | Good | O3 — ground-level ozone |
| 2 | (2025 page) | 13 | Good | O3 — ground-level ozone |
| 3 | (2021 page) | 13 | Good | O3 — ground-level ozone |
| 4 | (2024 page) | 15 | Good | O3 — ground-level ozone, in peak ozone season |
| 5 | (2023 page) | 15 | Good | O3 — ground-level ozone |
| 6 | (2022 page) | 15 | Good | O3 — ground-level ozone |
| 7 | (2021 page) | 15 | Good | PM2.5 — fine particulate matter (PM2.5) |
| 8 | (2025 page) | 16 | Good | O3 — ground-level ozone |
| 9 | (2021 page) | 16 | Good | O3 — ground-level ozone, in peak ozone season |
| 10 | (2023 page) | 17 | Good | O3 — ground-level ozone |
Right now in Portland
The latest EPA AirNow observation puts Portland's AQI at 29 (O3). The days above are historical records, 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.