Median AQI
39
Good
Michigan · County air quality · 2025
Bay County, Michigan
Median AQI 39 (Good). 70.4% of monitored days rated Good, direct from EPA Air Quality System stations.
- 39
- Median AQI
- 70.4%
- Good-air days
- 0
- Unhealthy days
- 34%
- Cleaner than US counties
Bay County's 2025 annual median AQI is 39 (Good), with 70.4% Good days
According to the U.S. EPA Air Quality System, Bay County, Michigan posted a 2025 median AQI of 39 (Good) across 358 monitored days, with 70.4% rated Good and 0 unhealthy or worse. This is an annual comparison, not today's advisory.
Data source EPA Air Quality System (AQS)
- Cleaner than US
- Bay County's 70.4% Good-day share places it cleaner than 34% of US counties in 2025.
- Closest median-AQI peer
- Aiken County (90.6% Good, median AQI 39) is the closest nationwide median-AQI peer to Bay County.
- 2020–2025 trend
- Bay County's floor-qualifying Good-day share is worsening from 2020 to 2025 (89.0% → 70.4%).
- Median AQI
- Median AQI: 39
The verdict
According to EPA Air Quality System monitoring data, Bay County's air is good on balance, more polluted than the median US county, a median AQI of 39 with 70.4% of 358 monitored days in the Good band, driven mainly by PM2.5.
- 39
- median AQI · Good
- 70.4%
- days rated Good
- PM2.5
- main pollutant
- 34%
- cleaner than US counties
Not one day crossed into the Unhealthy tier; the worst single reading was AQI 139.
Annual release change
What changed in the 2025 EPA annual release
- Bay County's peak single-day AQI rose from 73 to 139 between the 2024 and 2025 EPA annual releases, a change of +66 points.
- Bay County's Good-day share fell from 76.3% to 70.4% between the 2024 and 2025 EPA annual releases, a change of -5.9 percentage points.
Data updated August 2026
- Good · 252 d · 70.4%
- Moderate · 103 d · 28.8%
- USG · 3 d · 0.8%
How did Bay County's air break down in 2025?
Across 358 monitored days, 252 fell in the Good band (AQI 0–50) and 0 reached Unhealthy or worse. The worst single day peaked at AQI 139 (USG).
EPA AQS →Air quality data from EPA Air Quality System (2025). 358 days monitored. Median AQI: 39 (Good).
Insights: Bay County, Michigan. EPA monitors recorded a median Air Quality Index of 39 across 358 measured days in 2025, earning a Good classification and a Moderate health grade. Of those days, 252 (70%) fell in the Good band (AQI 0–50), 103 (29%) registered Moderate readings, and 3 (1%) reached Unhealthy for Sensitive Groups. The 90th-percentile AQI settled at 60, while the worst single-day peak hit 139 (USG). Residents averaged about 21.1 clean-air days per 30-day window during the monitoring year.
The full pollutant mix ranks as PM2.5 (358 days). Across the 2020–2025 trend window, the share of monitored days rated Good shifted by -18.7 points and the median AQI moved +17 points, placing the multi-year trajectory at worsening. The worst single reading over the 2020–2025 window was AQI 139, usg but never Unhealthy. Nearby counties show comparable readings: Allegan (63.9% good, AQI 44); Benzie (91.1% good, AQI 35); Berrien (80.9% good, AQI 41); Cass (85.3% good, AQI 40); Clinton (90.2% good, AQI 39); Genesee (83.2% good, AQI 38).
Air Quality Summary
Good Days
70.4%
252 of 358 monitored days
-4.6pp vs 75% target
Max AQI Recorded
139
USG
Health Grade
Moderate
90th-pct AQI 60
Bay County, Good days share 70.4%
National avg
Share of monitored days at AQI 0-50 (Good band).
Bay County, Unhealthy-or-worse share 0.0%
National avg
Share of monitored days above AQI 150 (Unhealthy or worse). Bar maxes at 10% (worst-county ceiling).
AQI Day Breakdown (2025)
Distribution of 358 monitored days across EPA AQI categories.
Good
252
70%
Moderate
103
29%
USG
3
1%
Unhealthy
0
0%
Very Unhealthy
0
0%
Hazardous
0
0%
Pollutant Breakdown
Days where each pollutant was the primary driver of AQI readings in Bay County.
- PM2.5 · 358 days · 100.0%
Health Impact Assessment
Moderate
Bay County's air quality is rated Moderate based on 70.4% good air days. No unhealthy air days were recorded in 2025.
Air quality was acceptable on 70.4% of monitored days in 2025, with a median AQI of 39. Unusually sensitive people can check real-time conditions at AirNow.gov during high-pollution events.
The dominant pollutant is PM2.5, driving AQI on 358 of 358 monitored days.
5-Year Air Quality Trend
Air quality is worsening in Bay County. Highest Good-day share: 2020 (89.0%, 325 of 365 monitored days).
| Year | Good | Moderate | USG | Unhealthy | Median AQI | Max AQI |
|---|---|---|---|---|---|---|
| 2020 | 325 | 40 | 0 | 0 | 22 | 77 |
| 2021 | 290 | 73 | 0 | 0 | 28 | 86 |
| 2022 | 287 | 67 | 0 | 0 | 32 | 85 |
| 2023 | 229 | 128 | 3 | 3 | 42 | 192 |
| 2024 | 277 | 86 | 0 | 0 | 37 | 73 |
| 2025 | 252 | 103 | 3 | 0 | 39 | 139 |
How does Bay County compare?
How Bay County compares
Good-day share worse than typical
Bay County 70.4%
Aiken County, South Carolina 90.6%
Typical US county 76.0%
Median AQI about typical
Bay County 39
Aiken County, South Carolina 39
Typical US county 39
Unhealthy days / yr better than typical
Bay County 0
Aiken County, South Carolina 0
Typical US county 1
Counties with a similar median AQI
Nationwide peers within about the same annual median AQI as Bay County (39), not geographic neighbors.
Aiken County, South Carolina
Median AQI 39 · 90.6% Good-day share
Albany County, New York
Median AQI 39 · 68.5% Good-day share
Anderson County, Tennessee
Median AQI 39 · 94.3% Good-day share
Berkeley County, West Virginia
Median AQI 39 · 83.6% Good-day share
Boyd County, Kentucky
Median AQI 39 · 80.0% Good-day share
Bullitt County, Kentucky
Median AQI 39 · 88.4% Good-day share
Same primary pollutant: PM2.5
PM2.5 led the AQI on the most monitored days here. These peers share that driver and the closest Good-day share (a second relationship type beyond median-AQI neighbors).
Boise County, Idaho
70.4% Good-day share · median AQI 42
Pottawattamie County, Iowa
70.3% Good-day share · median AQI 37
Tippecanoe County, Indiana
70.3% Good-day share · median AQI 39
Hancock County, Mississippi
70.1% Good-day share · median AQI 43
Johnson County, Iowa
70.1% Good-day share · median AQI 37
Macomb County, Michigan
70.1% Good-day share · median AQI 40
Michigan counties on the map
Geographic containment only, other monitored counties in Michigan, independent of AQI or pollutant match.
What Bay County's air means for you
The air here stays in the breathable range year-round, with PM2.5 the main thing to watch.
- Sensitive groups (asthma, heart conditions, children, older adults) should check the AQI on high-PM2.5 days, the guide explains what a median of 39 means for exposure. Health guidance
- See how Bay County ranks against every monitored U.S. county on Good-day share. County rankings
- Compare Bay County with its Michigan neighbours before drawing a conclusion about the region. All of Michigan
Based on 358 EPA-monitored days in 2025; figures reflect only days the monitors reported. Source: EPA Air Quality System annual summaries.
Keep exploring the annual AQI record
Air-quality reading list
Frequently Asked Questions
What is the air quality in Bay County, Michigan? ▼
Bay County has a median Air Quality Index (AQI) of 39, rated "Good." 70.4% of monitored days (252 out of 358) had good air quality in 2025.
What is the main pollutant in Bay County? ▼
The primary pollutant in Bay County is Fine Particles (PM2.5), which was the main AQI driver on 358 days (100% of monitored days). Penetrates deep into lungs and bloodstream. Linked to heart disease and premature death.
How many unhealthy air days does Bay County have? ▼
In 2025, Bay County recorded 0 unhealthy air days (0 Unhealthy, 0 Very Unhealthy, 0 Hazardous). The maximum AQI reached 139.
Is the air quality in Bay County getting better or worse? ▼
Air quality in Bay County is worsening across floor-qualifying years (2020-2025); years with fewer than 100 monitored days are excluded from that direction. Highest qualifying Good-day share: 2020 at 89.0% (325 of 365 days). Lowest qualifying Good-day share: 2023 at 63.1% (229 of 363 days).
What does AQI 39 mean for health? ▼
Bay County's median AQI of 39 falls in the "Good" category. See the AQI category guide for what each range means for health.
How does Bay County compare to other counties in Michigan? ▼
Bay County has 70.4% good air days. Nearby counties in Michigan: Allegan (63.9% good), Benzie (91.1% good), Berrien (80.9% good), Cass (85.3% good), Clinton (90.2% good), Genesee (83.2% good).
What is the 90th percentile AQI in Bay County? ▼
The 90th percentile AQI for Bay County is 60, meaning 90% of monitored days had an AQI at or below this level. The maximum AQI recorded was 139 (USG).
Data Sources
Primary Source: U.S. Environmental Protection Agency (EPA), Air Quality System (AQS), Annual AQI by County dataset, 2020–2025 (aqs.epa.gov). For current conditions (not shown here — this page is historical annual data), see EPA AirNow (airnow.gov). See our methodology for how AQI, median, and coverage figures are computed.
Informational only, not professional advice. Data from EPA AQS. See our disclaimer. Spotted a figure that looks wrong? Report a correction.
Source: U.S. EPA Air Quality System (AQS) Annual AQI by county summary files for Bay County, Michigan · 2020–2025
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Every figure on PlainAirData is rendered directly from EPA AQS air-quality monitoring data, no number is typed in by an editor. This page draws directly on EPA AQS air-quality monitoring data, no figure is typed in by an editor. See our editorial standards & corrections policy, the methodology behind these numbers, or report a data error.