Metro area air quality · 2025

Portland-South Portland, ME

Median AQI 40 (Good). 74.0% of monitored days rated Good across this metro's EPA monitoring network.

40
Median AQI
74.0%
Good-air days
1
Unhealthy days
54%
Cleaner than US metros

Portland-South Portland, ME posted a 2025 median AQI of 40 (Good) with 74.0% Good days

According to the U.S. EPA Air Quality System, the Portland-South Portland, ME CBSA recorded a median AQI of 40 across 365 monitored days in 2025, with 74.0% rated Good and 1 unhealthy or worse. This is an annual metro comparison, not a live advisory.

Data source EPA Air Quality System (AQS)

Cleaner than US metros
The Portland-South Portland, ME CBSA's 74.0% Good-day share places it cleaner than 54% of US metros in 2025.
Closest median-AQI peer
Altoona, PA (76.9% Good, median AQI 40) is the closest nationwide median-AQI peer to Portland-South Portland, ME.
2020–2025 Good-day shift
Portland-South Portland, ME's Good-day share moved +3.4 points from 2020 to 2025 (median AQI -2).
Median AQI
Median AQI: 40

Aggregated from EPA Air Quality System (2025), 365 monitored days. EPA AQS →

The verdict

Portland-South Portland, ME's peak reading 169 is in the upper max-AQI tercile among CBSA metros here. On the EPA scale the metro runs good, cleaner than the median US metro, with a median AQI of 40 and 74.0% of 365 monitored days in the Good band.

40
median AQI · Good
74.0%
days rated Good
54%
cleaner than US metros
169
worst single reading

1 day reached Unhealthy or worse across the metro's monitors in 2025.

Annual release change

What changed in the 2025 EPA annual release

2024 → 2025
  • Portland-South Portland, ME metro's Good-day share fell from 79.2% to 75.0% between the 2024 and 2025 EPA annual releases, a change of -4.2 percentage points.
  • Portland-South Portland, ME metro's peak single-day AQI rose from 129 to 169 between the 2024 and 2025 EPA annual releases, a change of +40 points.
  • Portland-South Portland, ME metro's Unhealthy-category days rose from 0 to 1 between the 2024 and 2025 EPA annual releases, a change of +1 days.

Compared against 365 monitored days in 2024 and 360 in 2025. These are arithmetic differences between two stored EPA annual summaries, not a cause claim or a live advisory.

National release changes Two-year CSV

Portland-South Portland, ME's peak reading 169 is in the upper max-AQI tercile among CBSA metros here. Signature read: peak AQI 169 (high tercile vs CBSA corpus). Aggregated EPA AQS monitors across this Core-Based Statistical Area logged a median Air Quality Index of 40 over 365 measured days in 2025 (Good / Moderate). The monitored-day split lands at 270 Good (74%), 89 Moderate (24%), and 5 Unhealthy-for-Sensitive-Groups (1%). The 90th-percentile AQI finished at 63; the highest single-day reading hit 169 (Unhealthy).

Across 2020–2025, Good-day share shifted +3.4 points and median AQI moved -2 (improving). The annual record contains 1 day at Unhealthy or worse. Metros with comparable AQI profiles include Altoona, PA (76.9% good, AQI 40); Beaver Dam, WI (72.3% good, AQI 40); Bismarck, ND (76.7% good, AQI 40).

Health effects of air pollution →

Air Quality Summary

Median AQI

40

Good

Good Days

74.0%

270 of 365 days

Max AQI

169

Unhealthy

Health Grade

Moderate

90th pct: 63

AQI Day Breakdown (2025)

Distribution of 365 monitored days across EPA AQI categories.

Good

270

74%

Moderate

89

24%

USG

5

1%

Unhealthy

1

0%

Very Unhealthy

0

0%

Hazardous

0

0%

Health Impact Assessment

Moderate

The Portland-South Portland, ME metro area's air quality is rated Moderate based on 74.0% good air days. There were 1 unhealthy days in 2025 when outdoor activity should be limited.

Air quality is acceptable most days, but 5 days reached "Unhealthy for Sensitive Groups" and peak AQI reached 169. Sensitive groups should monitor real-time conditions at AirNow.gov.

How does the Portland-South Portland, ME metro compare?

How Portland-South Portland, ME compares

Good-day share better than typical
Portland-South Portland, ME 74.0%
Altoona, PA 76.9%
Typical US metro 70.6%
Median AQI better than typical
Portland-South Portland, ME 40
Altoona, PA 40
Typical US metro 41
Unhealthy days / yr about typical
Portland-South Portland, ME 1
Altoona, PA 0
Typical US metro 1

5-Year Air Quality Trend

Air quality is improving in the Portland-South Portland, ME metro. Highest Good-day share: 2022 (89.1%, 320 of 359 monitored days).

Year Good Moderate Unhealthy Median AQI
2020 262 104 0 42
2021 313 49 0 36
2022 320 39 0 37
2023 261 101 0 41
2024 289 76 0 39
2025 270 89 1 40

Metros with a similar median AQI

CBSA peers nationwide whose annual median AQI sits near Portland-South Portland, ME (40).

Altoona, PA

Median AQI 40 · 76.9% Good-day share

40

Beaver Dam, WI

Median AQI 40 · 72.3% Good-day share

40

Bismarck, ND

Median AQI 40 · 76.7% Good-day share

40

Cape Coral-Fort Myers, FL

Median AQI 40 · 76.4% Good-day share

40

College Station-Bryan, TX

Median AQI 40 · 68.5% Good-day share

40

Fallon, NV

Median AQI 40 · 85.2% Good-day share

40

Metros with similar unhealthy-day exposure

A second relationship type: metros whose unhealthy-or-worse day count is closest to Portland-South Portland, ME's 1 (independent of median AQI).

Ann Arbor, MI

1 unhealthy-or-worse days · median AQI 41

41

Bangor, ME

1 unhealthy-or-worse days · median AQI 33

33

Baraboo, WI

1 unhealthy-or-worse days · median AQI 35

35

Bartlesville, OK

1 unhealthy-or-worse days · median AQI 44

44

Beaumont-Port Arthur, TX

1 unhealthy-or-worse days · median AQI 52

52

Bellingham, WA

1 unhealthy-or-worse days · median AQI 31

31

Frequently Asked Questions

What is the 90th percentile AQI for Portland-South Portland, ME? ▼
The 90th percentile AQI for the Portland-South Portland, ME metro area is 63, meaning 90% of monitored days had an AQI at or below this level. The maximum AQI recorded was 169 (Unhealthy).
How many unhealthy air days does Portland-South Portland, ME have? ▼
In 2025, the Portland-South Portland, ME metro area recorded 1 unhealthy air days (1 Unhealthy, 0 Very Unhealthy, 0 Hazardous). The maximum AQI reached 169.
What is the air quality in the Portland-South Portland, ME metro area? ▼
The Portland-South Portland, ME metro area has a median Air Quality Index (AQI) of 40, rated "Good." 74.0% of monitored days (270 out of 365) had good air quality in 2025.
Is air quality in Portland-South Portland, ME getting better or worse? ▼
Air quality in the Portland-South Portland, ME metro is improving across floor-qualifying years (2020-2025); years with fewer than 100 monitored days are excluded from that direction. Highest qualifying Good-day share: 2022 at 89.1% (320 of 359 days). Lowest qualifying Good-day share: 2020 at 71.6% (262 of 366 days).
What does AQI 40 mean for health? ▼
An AQI of 40 is "Good", air quality is satisfactory and poses little or no health risk. This is the best category on the EPA's AQI scale.
How does Portland-South Portland, ME compare to other metro areas? ▼
The Portland-South Portland, ME metro has 74.0% good air days. Similar metros: Altoona, PA (76.9% good), Beaver Dam, WI (72.3% good), Bismarck, ND (76.7% good).

Data Sources

Primary Source: U.S. Environmental Protection Agency (EPA), Air Quality System (AQS), Annual AQI by CBSA dataset for Portland-South Portland, ME, 2025. See our methodology for how AQI, median, and coverage figures are computed.

Read our methodology , how this data is sourced, computed, and verified.

Source: U.S. EPA Air Quality System (AQS) Annual AQI by CBSA summary, Portland-South Portland, ME · 2020–2025