Portland, ME Air Quality in May
Across 2019–2025, Portland's May air quality had a median AQI of 40 (Good) over 217 monitored days in the Portland-South Portland, ME area. 0.0% of May days (0 of 217) reached Unhealthy for Sensitive Groups or worse, and ground-level ozone set the daily AQI on 77.4% of days. The cleanest May on record here was 2024 (median AQI 37, 0 days above 100) and the worst was 2019 (median 45, 0 days above 100, peak AQI 81).
- Good: 86.2%
- Moderate: 13.8%
- Unhealthy for Sensitive Groups: 0.0%
- Unhealthy or worse: 0.0%
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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How is the air in Portland in May?
Good on a typical day: median AQI 40 across 2019–2025, with 0.0% of May days above AQI 100. Ground-level ozone is the main driver.
- ✓86.2% of May days were Good (AQI 0–50), 13.8% Moderate (51–100).
- ✓0 days of 217 reached AQI 101+; the May peak was 84.
- ✓Best May: 2024; worst May: 2019.
Frequently asked questions
What is the air quality like in Portland in May?▾
Across 2019–2025, the median daily AQI in Portland in May was 40 (Good). 86.2% of days were Good, 13.8% Moderate, and 0.0% reached Unhealthy for Sensitive Groups or worse, based on 217 monitored days in EPA's Air Quality System.
Is May a good time for outdoor exercise in Portland?▾
A typical May day in Portland sits in the Good range (median AQI 40), so running, cycling and children's outdoor play are fine for everyone on most days; fewer than 1% of May days reached AQI 101+ in 2019–2025.
Which pollutant causes Portland's May air quality problems?▾
Ground-level ozone set the daily AQI on 77.4% of May days in Portland. The highest May AQI recorded in 2019–2025 was 84 (Moderate), in 2019.
Portland May air quality, year by year
Portland May air quality by year (Portland-South Portland, ME, EPA AQS daily AQI)
| Year | Days monitored | Median AQI | Days above 100 | Peak AQI | Main pollutant |
|---|---|---|---|---|---|
| 2019worst | 31 | 45 | 0 | 81 | PM2.5 |
| 2020 | 31 | 43 | 0 | 61 | O3 |
| 2021 | 31 | 38 | 0 | 84 | O3 |
| 2022 | 31 | 39 | 0 | 58 | O3 |
| 2023 | 31 | 43 | 0 | 77 | O3 |
| 2024cleanest | 31 | 37 | 0 | 74 | O3 |
| 2025 | 31 | 39 | 0 | 69 | O3 |
What May air means in Portland
A typical May day in Portland sits in the Good range (median AQI 40), so running, cycling and children's outdoor play are fine for everyone on most days; fewer than 1% of May days reached AQI 101+ in 2019–2025.
May's median AQI of 40 is in line with Portland's whole-year median of 40, so this period is neither a clean nor a dirty stretch for the area. Nationally, the median May AQI across every EPA metro area (104,064 metro-days, 2019–2025) is 44, so Portland runs below the national May norm; 0.0% of its May days topped AQI 100 versus 1.3% nationally (below average).
May is part of Portland's spring season (March, April, May); the spring page pools all three months.
Activity-specific thresholds: running, cycling, kids' sports.
Right now in Portland
The latest EPA AirNow observation puts Portland's AQI at 29 (O3). The figures above are multi-year month 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.