EnviroNearby

Mason County, Michigan air quality history

Daily Air Quality Index summaries from EPA monitors in Mason, Michigan, 1989–2025.

89%
Good days in 2025
36
Median daily AQI, 2025
0
Days above 100 in 2025
213
Days with an AQI in 2025 (of 365)

In 2025, EPA reported a daily Air Quality Index (AQI) for Mason, Michigan on 213 days. 190 of those days (89%) were in the Good range, and the median daily AQI was 36. That median is lower than the typical monitored county in Michigan (median 39 across 26 counties), and lower than the typical monitored U.S. county (40). The main pollutant was ozone on 213 days. Across the years with fuller monitoring, the share of Good days rose: an average of 67% in 1993–1995 versus 86% in 2023–2025.

EPA did not report an AQI for every day of 2025 here (213 of 365 days). Figures describe monitored days only; seasonal monitoring (for example, ozone measured only in warmer months) can make a county look cleaner or dirtier than a full-year record would.

Years with fewer than 120 monitored days (1989, 1990, 1991, 1992) are shown in the table but left out of comparisons and trend statements.

How the days broke down each year

Each bar is one year. The colored segments show the share of monitored days that fell in each AQI category.

Share of monitored days in each AQI category, by year0%25%50%75%100%1989: Good 23 of 29 days1989: Moderate 6 of 29 days19891990: Good 36 of 44 days1990: Moderate 7 of 44 days1990: Unhealthy for Sensitive Groups 1 of 44 days1991: Good 69 of 111 days1991: Moderate 21 of 111 days1991: Unhealthy for Sensitive Groups 7 of 111 days1991: Unhealthy 8 of 111 days1991: Very Unhealthy 6 of 111 days1992: Good 42 of 46 days1992: Moderate 4 of 46 days1993: Good 141 of 202 days1993: Moderate 39 of 202 days1993: Unhealthy for Sensitive Groups 13 of 202 days1993: Unhealthy 9 of 202 days1994: Good 132 of 195 days1994: Moderate 41 of 195 days1994: Unhealthy for Sensitive Groups 20 of 195 days1994: Unhealthy 2 of 195 days19941995: Good 113 of 175 days1995: Moderate 32 of 175 days1995: Unhealthy for Sensitive Groups 18 of 175 days1995: Unhealthy 8 of 175 days1995: Very Unhealthy 4 of 175 days1996: Good 126 of 181 days1996: Moderate 35 of 181 days1996: Unhealthy for Sensitive Groups 15 of 181 days1996: Unhealthy 3 of 181 days1996: Very Unhealthy 2 of 181 days1997: Good 134 of 183 days1997: Moderate 32 of 183 days1997: Unhealthy for Sensitive Groups 12 of 183 days1997: Unhealthy 5 of 183 days1998: Good 121 of 182 days1998: Moderate 41 of 182 days1998: Unhealthy for Sensitive Groups 14 of 182 days1998: Unhealthy 6 of 182 days1999: Good 114 of 183 days1999: Moderate 37 of 183 days1999: Unhealthy for Sensitive Groups 20 of 183 days1999: Unhealthy 11 of 183 days1999: Very Unhealthy 1 of 183 days19992000: Good 145 of 183 days2000: Moderate 28 of 183 days2000: Unhealthy for Sensitive Groups 8 of 183 days2000: Unhealthy 1 of 183 days2000: Very Unhealthy 1 of 183 days2001: Good 128 of 183 days2001: Moderate 31 of 183 days2001: Unhealthy for Sensitive Groups 16 of 183 days2001: Unhealthy 8 of 183 days2002: Good 113 of 183 days2002: Moderate 38 of 183 days2002: Unhealthy for Sensitive Groups 23 of 183 days2002: Unhealthy 8 of 183 days2002: Very Unhealthy 1 of 183 days2003: Good 131 of 183 days2003: Moderate 35 of 183 days2003: Unhealthy for Sensitive Groups 13 of 183 days2003: Unhealthy 4 of 183 days2004: Good 135 of 178 days2004: Moderate 39 of 178 days2004: Unhealthy for Sensitive Groups 4 of 178 days20042005: Good 125 of 175 days2005: Moderate 38 of 175 days2005: Unhealthy for Sensitive Groups 10 of 175 days2005: Unhealthy 2 of 175 days2006: Good 128 of 158 days2006: Moderate 22 of 158 days2006: Unhealthy for Sensitive Groups 6 of 158 days2006: Unhealthy 2 of 158 days2007: Good 132 of 183 days2007: Moderate 33 of 183 days2007: Unhealthy for Sensitive Groups 16 of 183 days2007: Unhealthy 2 of 183 days2008: Good 135 of 183 days2008: Moderate 46 of 183 days2008: Unhealthy for Sensitive Groups 2 of 183 days2009: Good 161 of 183 days2009: Moderate 20 of 183 days2009: Unhealthy for Sensitive Groups 2 of 183 days20092010: Good 154 of 183 days2010: Moderate 27 of 183 days2010: Unhealthy for Sensitive Groups 2 of 183 days2011: Good 159 of 183 days2011: Moderate 20 of 183 days2011: Unhealthy for Sensitive Groups 3 of 183 days2011: Unhealthy 1 of 183 days2012: Good 131 of 182 days2012: Moderate 29 of 182 days2012: Unhealthy for Sensitive Groups 19 of 182 days2012: Unhealthy 3 of 182 days2013: Good 141 of 181 days2013: Moderate 38 of 181 days2013: Unhealthy for Sensitive Groups 2 of 181 days2014: Good 156 of 183 days2014: Moderate 25 of 183 days2014: Unhealthy for Sensitive Groups 2 of 183 days20142015: Good 159 of 183 days2015: Moderate 23 of 183 days2015: Unhealthy for Sensitive Groups 1 of 183 days2016: Good 157 of 180 days2016: Moderate 17 of 180 days2016: Unhealthy for Sensitive Groups 6 of 180 days2017: Good 217 of 245 days2017: Moderate 27 of 245 days2017: Unhealthy for Sensitive Groups 1 of 245 days2018: Good 226 of 252 days2018: Moderate 25 of 252 days2018: Unhealthy for Sensitive Groups 1 of 252 days2019: Good 244 of 254 days2019: Moderate 10 of 254 days20192020: Good 229 of 251 days2020: Moderate 20 of 251 days2020: Unhealthy for Sensitive Groups 1 of 251 days2020: Unhealthy 1 of 251 days2021: Good 206 of 243 days2021: Moderate 34 of 243 days2021: Unhealthy for Sensitive Groups 3 of 243 days2022: Good 219 of 236 days2022: Moderate 15 of 236 days2022: Unhealthy for Sensitive Groups 1 of 236 days2022: Unhealthy 1 of 236 days2023: Good 178 of 221 days2023: Moderate 41 of 221 days2023: Unhealthy for Sensitive Groups 2 of 221 days2024: Good 218 of 244 days2024: Moderate 26 of 244 days20242025: Good 190 of 213 days2025: Moderate 23 of 213 days2025
  • Good
  • Moderate
  • Unhealthy for Sensitive Groups
  • Unhealthy
  • Very Unhealthy
  • Hazardous

Typical and high-end AQI over time

The median is the middle day of the year. The 90th percentile is the level that only the highest 10% of days exceeded.

Median and 90th-percentile daily AQI, by year0501001502001989: 90th percentile AQI 661990: 90th percentile AQI 551991: 90th percentile AQI 1611992: 90th percentile AQI 501993: 90th percentile AQI 1151994: 90th percentile AQI 1081995: 90th percentile AQI 1221996: 90th percentile AQI 1051997: 90th percentile AQI 971998: 90th percentile AQI 1081999: 90th percentile AQI 1402000: 90th percentile AQI 842001: 90th percentile AQI 1192002: 90th percentile AQI 1222003: 90th percentile AQI 932004: 90th percentile AQI 842005: 90th percentile AQI 932006: 90th percentile AQI 772007: 90th percentile AQI 1002008: 90th percentile AQI 742009: 90th percentile AQI 582010: 90th percentile AQI 612011: 90th percentile AQI 612012: 90th percentile AQI 1052013: 90th percentile AQI 672014: 90th percentile AQI 642015: 90th percentile AQI 542016: 90th percentile AQI 642017: 90th percentile AQI 512018: 90th percentile AQI 512019: 90th percentile AQI 462020: 90th percentile AQI 492021: 90th percentile AQI 542022: 90th percentile AQI 492023: 90th percentile AQI 672024: 90th percentile AQI 512025: 90th percentile AQI 511989: Median AQI 301990: Median AQI 231991: Median AQI 421992: Median AQI 231993: Median AQI 451994: Median AQI 421995: Median AQI 451996: Median AQI 441997: Median AQI 441998: Median AQI 441999: Median AQI 442000: Median AQI 402001: Median AQI 422002: Median AQI 432003: Median AQI 432004: Median AQI 382005: Median AQI 412006: Median AQI 392007: Median AQI 422008: Median AQI 442009: Median AQI 382010: Median AQI 392011: Median AQI 382012: Median AQI 422013: Median AQI 402014: Median AQI 392015: Median AQI 392016: Median AQI 382017: Median AQI 382018: Median AQI 372019: Median AQI 352020: Median AQI 372021: Median AQI 382022: Median AQI 362023: Median AQI 402024: Median AQI 372025: Median AQI 36198919941999200420092014201920242025
  • Median daily AQI
  • 90th-percentile daily AQI

Main pollutant by day, 2025

On each monitored day, the AQI reflects whichever measured pollutant had the highest index. This counts how many days each pollutant set the AQI. A pollutant that was not measured here cannot appear.

PollutantDays as main pollutantShare of monitored days
Ozone213100%

Year-by-year data

Annual AQI summary by year
YearDays with AQIGoodModerateUnhealthy for Sensitive GroupsUnhealthyVery UnhealthyHazardousMedian AQI90th pct.Max AQIMain pollutant
2025213190230000365187Ozone
2024244218260000375197Ozone
20232211784120004067143Ozone
20222362191511003649161Ozone
20212432063430003854115Ozone
20202512292011003749159Ozone
2019254244100000354664Ozone
20182522262510003751133Ozone
20172452172710003851108Ozone
20161801571760003864129Ozone
20151831592310003954105Ozone
20141831562520003964115Ozone
20131811413820004067112Ozone
2012182131291930042105156Ozone
20111831592031003861172Ozone
20101831542720003961126Ozone
20091831612020003858133Ozone
20081831354620004474143Ozone
2007183132331620042100177Ozone
20061581282262003977197Ozone
200517512538102004193187Ozone
20041781353940003884136Ozone
200318313135134004393187Ozone
2002183113382381043122201Ozone
2001183128311680042119187Ozone
20001831452881104084212Ozone
19991831143720111044140215Ozone
1998182121411460044108182Ozone
199718313432125004497195Ozone
1996181126351532044105240Ozone
1995175113321884045122216Ozone
1994195132412020042108182Ozone
1993202141391390045115200Ozone
1992464240000235064PM10
19911116921786042161222Ozone
19904436710002355112PM10
1989292360000306679PM10

What this data can and can’t tell you

It is a county-level summary. EPA reports one AQI per day for the county, based on the monitors operating there. Air at a particular street, school or home can differ, especially near roads, industry or wildfire smoke.

It is not a health assessment. The AQI is EPA’s scale for communicating daily air quality. A county’s history can’t tell you what the health risk is for any particular person. For daily conditions and health guidance, see AirNow.

Rules change over time. EPA has revised AQI breakpoints, most recently for fine particles in 2024, so some year-to-year change can reflect the scale rather than the air.

Counties in Michigan with a similar median AQI in 2025

All monitored counties in Michigan

Source: U.S. EPA AirData, “Annual AQI by County” files for 1989–2025 (EPA download page). EPA file dates: 2026-06-26. Imported into EnviroNearby November 5, 2026. How we use this data · Report a problem