EnviroNearby

Frederick County, Maryland air quality history

Daily Air Quality Index summaries from EPA monitors in Frederick, Maryland, 1980–2025.

86%
Good days in 2025
41
Median daily AQI, 2025
0
Days above 100 in 2025
237
Days with an AQI in 2025 (of 365)

In 2025, EPA reported a daily Air Quality Index (AQI) for Frederick, Maryland on 237 days. 203 of those days (86%) were in the Good range, and the median daily AQI was 41. That median is about the same as the typical monitored county in Maryland (median 39.5 across 16 counties), and about the same as the typical monitored U.S. county (40). The main pollutant was ozone on 237 days. Across the years with fuller monitoring, the share of Good days rose: an average of 73% in 1980–1982 versus 79% in 2023–2025.

EPA did not report an AQI for every day of 2025 here (237 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 (1991, 1992, 1993, 1994, 1995, 1998…) 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%1980: Good 105 of 159 days1980: Moderate 34 of 159 days1980: Unhealthy for Sensitive Groups 16 of 159 days1980: Unhealthy 2 of 159 days1980: Very Unhealthy 2 of 159 days19801981: Good 235 of 292 days1981: Moderate 37 of 292 days1981: Unhealthy for Sensitive Groups 14 of 292 days1981: Unhealthy 4 of 292 days1981: Very Unhealthy 2 of 292 days1982: Good 219 of 306 days1982: Moderate 58 of 306 days1982: Unhealthy for Sensitive Groups 21 of 306 days1982: Unhealthy 8 of 306 days1983: Good 230 of 317 days1983: Moderate 39 of 317 days1983: Unhealthy for Sensitive Groups 30 of 317 days1983: Unhealthy 16 of 317 days1983: Very Unhealthy 2 of 317 days1984: Good 296 of 316 days1984: Moderate 15 of 316 days1984: Unhealthy for Sensitive Groups 5 of 316 days1985: Good 106 of 120 days1985: Moderate 6 of 120 days1985: Unhealthy for Sensitive Groups 6 of 120 days1985: Unhealthy 2 of 120 days19851989: Good 44 of 58 days1989: Moderate 14 of 58 days1990: Good 48 of 56 days1990: Moderate 8 of 56 days1991: Good 47 of 55 days1991: Moderate 8 of 55 days1992: Good 60 of 60 days1993: Good 51 of 58 days1993: Moderate 7 of 58 days19931994: Good 55 of 58 days1994: Moderate 3 of 58 days1995: Good 32 of 35 days1995: Moderate 3 of 35 days1998: Good 53 of 114 days1998: Moderate 28 of 114 days1998: Unhealthy for Sensitive Groups 26 of 114 days1998: Unhealthy 6 of 114 days1998: Very Unhealthy 1 of 114 days1999: Good 96 of 196 days1999: Moderate 46 of 196 days1999: Unhealthy for Sensitive Groups 38 of 196 days1999: Unhealthy 15 of 196 days1999: Very Unhealthy 1 of 196 days2000: Good 136 of 209 days2000: Moderate 57 of 209 days2000: Unhealthy for Sensitive Groups 12 of 209 days2000: Unhealthy 4 of 209 days20002001: Good 102 of 209 days2001: Moderate 64 of 209 days2001: Unhealthy for Sensitive Groups 30 of 209 days2001: Unhealthy 13 of 209 days2002: Good 121 of 213 days2002: Moderate 54 of 213 days2002: Unhealthy for Sensitive Groups 26 of 213 days2002: Unhealthy 12 of 213 days2003: Good 158 of 214 days2003: Moderate 46 of 214 days2003: Unhealthy for Sensitive Groups 7 of 214 days2003: Unhealthy 2 of 214 days2003: Very Unhealthy 1 of 214 days2004: Good 140 of 203 days2004: Moderate 51 of 203 days2004: Unhealthy for Sensitive Groups 11 of 203 days2004: Unhealthy 1 of 203 days2005: Good 123 of 203 days2005: Moderate 59 of 203 days2005: Unhealthy for Sensitive Groups 20 of 203 days2005: Unhealthy 1 of 203 days20052006: Good 146 of 214 days2006: Moderate 50 of 214 days2006: Unhealthy for Sensitive Groups 16 of 214 days2006: Unhealthy 2 of 214 days2007: Good 122 of 214 days2007: Moderate 64 of 214 days2007: Unhealthy for Sensitive Groups 23 of 214 days2007: Unhealthy 5 of 214 days2008: Good 138 of 211 days2008: Moderate 56 of 211 days2008: Unhealthy for Sensitive Groups 17 of 211 days2009: Good 173 of 208 days2009: Moderate 32 of 208 days2009: Unhealthy for Sensitive Groups 3 of 208 days2010: Good 118 of 214 days2010: Moderate 78 of 214 days2010: Unhealthy for Sensitive Groups 17 of 214 days2010: Unhealthy 1 of 214 days20102011: Good 146 of 214 days2011: Moderate 59 of 214 days2011: Unhealthy for Sensitive Groups 9 of 214 days2012: Good 120 of 198 days2012: Moderate 64 of 198 days2012: Unhealthy for Sensitive Groups 13 of 198 days2012: Unhealthy 1 of 198 days2013: Good 171 of 208 days2013: Moderate 36 of 208 days2013: Unhealthy for Sensitive Groups 1 of 208 days2014: Good 171 of 208 days2014: Moderate 37 of 208 days2015: Good 160 of 209 days2015: Moderate 48 of 209 days2015: Unhealthy for Sensitive Groups 1 of 209 days20152016: Good 151 of 211 days2016: Moderate 57 of 211 days2016: Unhealthy for Sensitive Groups 3 of 211 days2017: Good 190 of 243 days2017: Moderate 51 of 243 days2017: Unhealthy for Sensitive Groups 2 of 243 days2018: Good 204 of 245 days2018: Moderate 41 of 245 days2019: Good 198 of 243 days2019: Moderate 44 of 243 days2019: Unhealthy for Sensitive Groups 1 of 243 days2020: Good 217 of 237 days2020: Moderate 20 of 237 days20202021: Good 207 of 244 days2021: Moderate 36 of 244 days2021: Unhealthy for Sensitive Groups 1 of 244 days2022: Good 212 of 236 days2022: Moderate 23 of 236 days2022: Unhealthy for Sensitive Groups 1 of 236 days2023: Good 167 of 232 days2023: Moderate 59 of 232 days2023: Unhealthy for Sensitive Groups 6 of 232 days2024: Good 192 of 243 days2024: Moderate 51 of 243 days2025: Good 203 of 237 days2025: Moderate 34 of 237 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 year0501001501980: 90th percentile AQI 1081981: 90th percentile AQI 901982: 90th percentile AQI 1001983: 90th percentile AQI 1221984: 90th percentile AQI 441985: 90th percentile AQI 771989: 90th percentile AQI 571990: 90th percentile AQI 521991: 90th percentile AQI 521992: 90th percentile AQI 381993: 90th percentile AQI 521994: 90th percentile AQI 491995: 90th percentile AQI 411998: 90th percentile AQI 1471999: 90th percentile AQI 1472000: 90th percentile AQI 932001: 90th percentile AQI 1332002: 90th percentile AQI 1292003: 90th percentile AQI 802004: 90th percentile AQI 872005: 90th percentile AQI 1012006: 90th percentile AQI 972007: 90th percentile AQI 1122008: 90th percentile AQI 902009: 90th percentile AQI 542010: 90th percentile AQI 932011: 90th percentile AQI 842012: 90th percentile AQI 902013: 90th percentile AQI 582014: 90th percentile AQI 582015: 90th percentile AQI 672016: 90th percentile AQI 742017: 90th percentile AQI 642018: 90th percentile AQI 612019: 90th percentile AQI 582020: 90th percentile AQI 492021: 90th percentile AQI 582022: 90th percentile AQI 512023: 90th percentile AQI 772024: 90th percentile AQI 642025: 90th percentile AQI 581980: Median AQI 351981: Median AQI 301982: Median AQI 371983: Median AQI 341984: Median AQI 191985: Median AQI 291989: Median AQI 371990: Median AQI 311991: Median AQI 301992: Median AQI 271993: Median AQI 281994: Median AQI 231995: Median AQI 281998: Median AQI 611999: Median AQI 512000: Median AQI 412001: Median AQI 512002: Median AQI 472003: Median AQI 412004: Median AQI 442005: Median AQI 462006: Median AQI 442007: Median AQI 482008: Median AQI 442009: Median AQI 402010: Median AQI 492011: Median AQI 442012: Median AQI 472013: Median AQI 432014: Median AQI 422015: Median AQI 442016: Median AQI 432017: Median AQI 422018: Median AQI 412019: Median AQI 442020: Median AQI 402021: Median AQI 412022: Median AQI 412023: Median AQI 442024: Median AQI 432025: Median AQI 41198019851993200020052010201520202025
  • 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
Ozone237100%

Year-by-year data

Annual AQI summary by year
YearDays with AQIGoodModerateUnhealthy for Sensitive GroupsUnhealthyVery UnhealthyHazardousMedian AQI90th pct.Max AQIMain pollutant
2025237203340000415893Ozone
20242431925100004364100Ozone
20232321675960004477136Ozone
20222362122310004151115Ozone
20212442073610004158126Ozone
2020237217200000404990Ozone
20192431984410004458112Ozone
20182452044100004161100Ozone
20172431905120004264105Ozone
20162111515730004374122Ozone
20152091604810004467119Ozone
2014208171370000425880Ozone
20132081713610004358119Ozone
201219812064131004790174Ozone
20112141465990004484150Ozone
201021411878171004993169Ozone
20092081733230004054108Ozone
200821113856170004490150Ozone
2007214122642350048112159Ozone
200621414650162004497156Ozone
2005203123592010046101151Ozone
200420314051111004487174Ozone
20032141584672104180205Ozone
20022131215426120047129182Ozone
20012091026430130051133197Ozone
200020913657124004193187Ozone
1999196964638151051147212Ozone
199811453282661061147208Ozone
1995353230000284154PM10
1994585530000234959PM10
1993585170000285261PM10
1992606000000273846PM10
1991554780000305265PM10
1990564880000315265PM10
19895844140000375780PM10
1985120106662002977169Ozone
19843162961550001944147Ozone
19833172303930162034122203Ozone
1982306219582180037100197Ozone
198129223537144203090216Ozone
1980159105341622035108215Ozone

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 Maryland with a similar median AQI in 2025

All monitored counties in Maryland

Source: U.S. EPA AirData, “Annual AQI by County” files for 1980–2025 (EPA download page). EPA file dates: 2026-06-26. Imported into EnviroNearby November 5, 2026. How we use this data · Report a problem