EnviroNearby

Merced County, California air quality history

Daily Air Quality Index summaries from EPA monitors in Merced, California, 1982–2025.

51%
Good days in 2025
50
Median daily AQI, 2025
0
Days above 100 in 2025
365
Days with an AQI in 2025 (of 365)

In 2025, EPA reported a daily Air Quality Index (AQI) for Merced, California on 365 days. 186 of those days (51%) were in the Good range, and the median daily AQI was 50. That median is higher than the typical monitored county in California (median 45.5 across 46 counties), and higher than the typical monitored U.S. county (40). The main pollutant was ozone on 228 days. Across the years with fuller monitoring, the share of Good days fell: an average of 57% in 1991–1993 versus 48% in 2023–2025.

Years with fewer than 120 monitored days (1985, 1986, 1987, 1988, 1989, 1990…) 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%1982: Good 58 of 69 days1982: Moderate 11 of 69 days19821985: Good 9 of 14 days1985: Moderate 5 of 14 days1986: Good 44 of 53 days1986: Moderate 9 of 53 days1987: Good 44 of 60 days1987: Moderate 16 of 60 days1988: Good 32 of 57 days1988: Moderate 25 of 57 days1989: Good 45 of 64 days1989: Moderate 19 of 64 days19891990: Good 38 of 62 days1990: Moderate 23 of 62 days1990: Unhealthy for Sensitive Groups 1 of 62 days1991: Good 81 of 130 days1991: Moderate 32 of 130 days1991: Unhealthy for Sensitive Groups 6 of 130 days1991: Unhealthy 10 of 130 days1991: Very Unhealthy 1 of 130 days1992: Good 178 of 331 days1992: Moderate 60 of 331 days1992: Unhealthy for Sensitive Groups 56 of 331 days1992: Unhealthy 35 of 331 days1992: Very Unhealthy 2 of 331 days1993: Good 204 of 363 days1993: Moderate 86 of 363 days1993: Unhealthy for Sensitive Groups 59 of 363 days1993: Unhealthy 13 of 363 days1993: Very Unhealthy 1 of 363 days1994: Good 192 of 364 days1994: Moderate 83 of 364 days1994: Unhealthy for Sensitive Groups 66 of 364 days1994: Unhealthy 22 of 364 days1994: Very Unhealthy 1 of 364 days19941995: Good 190 of 362 days1995: Moderate 79 of 362 days1995: Unhealthy for Sensitive Groups 59 of 362 days1995: Unhealthy 30 of 362 days1995: Very Unhealthy 4 of 362 days1996: Good 197 of 363 days1996: Moderate 66 of 363 days1996: Unhealthy for Sensitive Groups 58 of 363 days1996: Unhealthy 41 of 363 days1996: Very Unhealthy 1 of 363 days1997: Good 261 of 361 days1997: Moderate 90 of 361 days1997: Unhealthy for Sensitive Groups 9 of 361 days1997: Unhealthy 1 of 361 days1998: Good 220 of 362 days1998: Moderate 67 of 362 days1998: Unhealthy for Sensitive Groups 41 of 362 days1998: Unhealthy 26 of 362 days1998: Very Unhealthy 8 of 362 days1999: Good 152 of 365 days1999: Moderate 92 of 365 days1999: Unhealthy for Sensitive Groups 73 of 365 days1999: Unhealthy 45 of 365 days1999: Very Unhealthy 3 of 365 days19992000: Good 154 of 363 days2000: Moderate 102 of 363 days2000: Unhealthy for Sensitive Groups 68 of 363 days2000: Unhealthy 36 of 363 days2000: Very Unhealthy 3 of 363 days2001: Good 152 of 357 days2001: Moderate 116 of 357 days2001: Unhealthy for Sensitive Groups 62 of 357 days2001: Unhealthy 27 of 357 days2002: Good 137 of 363 days2002: Moderate 96 of 363 days2002: Unhealthy for Sensitive Groups 74 of 363 days2002: Unhealthy 53 of 363 days2002: Very Unhealthy 3 of 363 days2003: Good 124 of 364 days2003: Moderate 122 of 364 days2003: Unhealthy for Sensitive Groups 65 of 364 days2003: Unhealthy 49 of 364 days2003: Very Unhealthy 4 of 364 days2004: Good 156 of 365 days2004: Moderate 133 of 365 days2004: Unhealthy for Sensitive Groups 62 of 365 days2004: Unhealthy 13 of 365 days2004: Very Unhealthy 1 of 365 days20042005: Good 221 of 364 days2005: Moderate 99 of 364 days2005: Unhealthy for Sensitive Groups 42 of 364 days2005: Unhealthy 2 of 364 days2006: Good 213 of 364 days2006: Moderate 100 of 364 days2006: Unhealthy for Sensitive Groups 43 of 364 days2006: Unhealthy 8 of 364 days2007: Good 215 of 364 days2007: Moderate 107 of 364 days2007: Unhealthy for Sensitive Groups 37 of 364 days2007: Unhealthy 5 of 364 days2008: Good 171 of 364 days2008: Moderate 134 of 364 days2008: Unhealthy for Sensitive Groups 44 of 364 days2008: Unhealthy 12 of 364 days2008: Very Unhealthy 3 of 364 days2009: Good 182 of 361 days2009: Moderate 130 of 361 days2009: Unhealthy for Sensitive Groups 49 of 361 days20092010: Good 106 of 365 days2010: Moderate 220 of 365 days2010: Unhealthy for Sensitive Groups 33 of 365 days2010: Unhealthy 6 of 365 days2011: Good 70 of 362 days2011: Moderate 232 of 362 days2011: Unhealthy for Sensitive Groups 58 of 362 days2011: Unhealthy 2 of 362 days2012: Good 136 of 366 days2012: Moderate 196 of 366 days2012: Unhealthy for Sensitive Groups 33 of 366 days2012: Unhealthy 1 of 366 days2013: Good 105 of 364 days2013: Moderate 211 of 364 days2013: Unhealthy for Sensitive Groups 43 of 364 days2013: Unhealthy 5 of 364 days2014: Good 120 of 364 days2014: Moderate 186 of 364 days2014: Unhealthy for Sensitive Groups 56 of 364 days2014: Unhealthy 2 of 364 days20142015: Good 107 of 365 days2015: Moderate 213 of 365 days2015: Unhealthy for Sensitive Groups 41 of 365 days2015: Unhealthy 4 of 365 days2016: Good 115 of 366 days2016: Moderate 216 of 366 days2016: Unhealthy for Sensitive Groups 33 of 366 days2016: Unhealthy 2 of 366 days2017: Good 127 of 364 days2017: Moderate 203 of 364 days2017: Unhealthy for Sensitive Groups 31 of 364 days2017: Unhealthy 3 of 364 days2018: Good 110 of 365 days2018: Moderate 214 of 365 days2018: Unhealthy for Sensitive Groups 32 of 365 days2018: Unhealthy 9 of 365 days2019: Good 176 of 365 days2019: Moderate 182 of 365 days2019: Unhealthy for Sensitive Groups 7 of 365 days20192020: Good 131 of 366 days2020: Moderate 203 of 366 days2020: Unhealthy for Sensitive Groups 17 of 366 days2020: Unhealthy 15 of 366 days2021: Good 155 of 365 days2021: Moderate 177 of 365 days2021: Unhealthy for Sensitive Groups 31 of 365 days2021: Unhealthy 2 of 365 days2022: Good 140 of 365 days2022: Moderate 212 of 365 days2022: Unhealthy for Sensitive Groups 13 of 365 days2023: Good 171 of 365 days2023: Moderate 179 of 365 days2023: Unhealthy for Sensitive Groups 15 of 365 days2024: Good 168 of 366 days2024: Moderate 163 of 366 days2024: Unhealthy for Sensitive Groups 34 of 366 days2024: Unhealthy 1 of 366 days20242025: Good 186 of 365 days2025: Moderate 179 of 365 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 year0501001502001982: 90th percentile AQI 571985: 90th percentile AQI 671986: 90th percentile AQI 541987: 90th percentile AQI 601988: 90th percentile AQI 681989: 90th percentile AQI 761990: 90th percentile AQI 711991: 90th percentile AQI 1401992: 90th percentile AQI 1511993: 90th percentile AQI 1221994: 90th percentile AQI 1361995: 90th percentile AQI 1471996: 90th percentile AQI 1541997: 90th percentile AQI 711998: 90th percentile AQI 1431999: 90th percentile AQI 1592000: 90th percentile AQI 1512001: 90th percentile AQI 1432002: 90th percentile AQI 1612003: 90th percentile AQI 1592004: 90th percentile AQI 1262005: 90th percentile AQI 1082006: 90th percentile AQI 1122007: 90th percentile AQI 1052008: 90th percentile AQI 1192009: 90th percentile AQI 1052010: 90th percentile AQI 1012011: 90th percentile AQI 1152012: 90th percentile AQI 972013: 90th percentile AQI 1122014: 90th percentile AQI 1132015: 90th percentile AQI 1052016: 90th percentile AQI 1002017: 90th percentile AQI 1002018: 90th percentile AQI 1012019: 90th percentile AQI 802020: 90th percentile AQI 962021: 90th percentile AQI 1002022: 90th percentile AQI 872023: 90th percentile AQI 872024: 90th percentile AQI 1002025: 90th percentile AQI 771982: Median AQI 381985: Median AQI 481986: Median AQI 341987: Median AQI 371988: Median AQI 421989: Median AQI 391990: Median AQI 391991: Median AQI 441992: Median AQI 471993: Median AQI 471994: Median AQI 491995: Median AQI 491996: Median AQI 461997: Median AQI 441998: Median AQI 451999: Median AQI 712000: Median AQI 582001: Median AQI 632002: Median AQI 722003: Median AQI 712004: Median AQI 602005: Median AQI 442006: Median AQI 452007: Median AQI 462008: Median AQI 542009: Median AQI 502010: Median AQI 592011: Median AQI 672012: Median AQI 572013: Median AQI 622014: Median AQI 612015: Median AQI 612016: Median AQI 582017: Median AQI 582018: Median AQI 562019: Median AQI 512020: Median AQI 572021: Median AQI 542022: Median AQI 562023: Median AQI 512024: Median AQI 522025: Median AQI 501982198919941999200420092014201920242025
  • 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
Ozone22862%
Fine particles (PM2.5)13136%
Coarse particles (PM10)62%

Year-by-year data

Annual AQI summary by year
YearDays with AQIGoodModerateUnhealthy for Sensitive GroupsUnhealthyVery UnhealthyHazardousMedian AQI90th pct.Max AQIMain pollutant
202536518617900005077100Ozone
20243661681633410052100151Ozone
2023365171179150005187129Ozone
2022365140212130005687143Ozone
20213651551773120054100166Ozone
20203661312031715005796194PM2.5
201936517618270005180119Ozone
20183651102143290056101178PM2.5
20173641272033130058100161PM2.5
20163661152163320058100151PM2.5
20153651072134140061105159Ozone
20143641201865620061113157Ozone
20133641052114350062112165PM2.5
2012366136196331005797151Ozone
2011362702325820067115156PM2.5
20103651062203360059101177PM2.5
20093611821304900050105145Ozone
200836417113444123054119216Ozone
20073642151073750046105177Ozone
20063642131004380045112172Ozone
2005364221994220044108169Ozone
200436515613362131060126204Ozone
200336412412265494071159205Ozone
20023631379674533072161221Ozone
200135715211662270063143200Ozone
200036315410268363058151207Ozone
19993651529273453071159213Ozone
19983622206741268045143225Ozone
19973612619091004471174Ozone
19963631976658411046154212Ozone
19953621907959304049147209Ozone
19943641928366221049136202Ozone
19933632048659131047122205Ozone
19923311786056352047151202Ozone
199113081326101044140206PM10
199062382310003971129PM10
19896445190000397697PM10
19885732250000426880PM10
19876044160000376075PM10
1986534490000345492PM10
198514950000486772PM10
19826958110000385768NO2

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 California with a similar median AQI in 2025

All monitored counties in California

Source: U.S. EPA AirData, “Annual AQI by County” files for 1982–2025 (EPA download page). EPA file dates: 2026-06-26. Imported into EnviroNearby November 5, 2026. How we use this data · Report a problem