EnviroNearby

Merrimack County, New Hampshire air quality history

Daily Air Quality Index summaries from EPA monitors in Merrimack, New Hampshire, 1985–2025.

97%
Good days in 2025
37
Median daily AQI, 2025
0
Days above 100 in 2025
212
Days with an AQI in 2025 (of 365)

In 2025, EPA reported a daily Air Quality Index (AQI) for Merrimack, New Hampshire on 212 days. 205 of those days (97%) were in the Good range, and the median daily AQI was 37. That median is about the same as the typical monitored county in New Hampshire (median 37 across 7 counties), and lower than the typical monitored U.S. county (40). The main pollutant was ozone on 212 days. Across the years with fuller monitoring, the share of Good days rose: an average of 82% in 1986–1988 versus 95% in 2023–2025.

EPA did not report an AQI for every day of 2025 here (212 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 (1985, 1991) 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%1985: Good 81 of 87 days1985: Moderate 2 of 87 days1985: Unhealthy for Sensitive Groups 3 of 87 days1985: Unhealthy 1 of 87 days19851986: Good 119 of 132 days1986: Moderate 9 of 132 days1986: Unhealthy for Sensitive Groups 4 of 132 days1987: Good 187 of 218 days1987: Moderate 21 of 218 days1987: Unhealthy for Sensitive Groups 5 of 218 days1987: Unhealthy 5 of 218 days1988: Good 158 of 227 days1988: Moderate 38 of 227 days1988: Unhealthy for Sensitive Groups 17 of 227 days1988: Unhealthy 10 of 227 days1988: Very Unhealthy 4 of 227 days1989: Good 173 of 231 days1989: Moderate 49 of 231 days1989: Unhealthy for Sensitive Groups 7 of 231 days1989: Unhealthy 1 of 231 days1989: Very Unhealthy 1 of 231 days1990: Good 175 of 236 days1990: Moderate 42 of 236 days1990: Unhealthy for Sensitive Groups 17 of 236 days1990: Unhealthy 2 of 236 days19901991: Good 59 of 60 days1991: Moderate 1 of 60 days1992: Good 163 of 191 days1992: Moderate 21 of 191 days1992: Unhealthy for Sensitive Groups 7 of 191 days1993: Good 219 of 239 days1993: Moderate 13 of 239 days1993: Unhealthy for Sensitive Groups 5 of 239 days1993: Unhealthy 2 of 239 days1994: Good 192 of 211 days1994: Moderate 15 of 211 days1994: Unhealthy for Sensitive Groups 3 of 211 days1994: Unhealthy 1 of 211 days1995: Good 209 of 227 days1995: Moderate 16 of 227 days1995: Unhealthy for Sensitive Groups 2 of 227 days19951996: Good 212 of 231 days1996: Moderate 15 of 231 days1996: Unhealthy for Sensitive Groups 2 of 231 days1996: Unhealthy 2 of 231 days1997: Good 184 of 204 days1997: Moderate 15 of 204 days1997: Unhealthy for Sensitive Groups 4 of 204 days1997: Unhealthy 1 of 204 days1998: Good 209 of 229 days1998: Moderate 15 of 229 days1998: Unhealthy for Sensitive Groups 5 of 229 days1999: Good 181 of 247 days1999: Moderate 62 of 247 days1999: Unhealthy for Sensitive Groups 4 of 247 days2000: Good 208 of 256 days2000: Moderate 46 of 256 days2000: Unhealthy for Sensitive Groups 2 of 256 days20002001: Good 173 of 238 days2001: Moderate 59 of 238 days2001: Unhealthy for Sensitive Groups 5 of 238 days2001: Unhealthy 1 of 238 days2002: Good 171 of 242 days2002: Moderate 57 of 242 days2002: Unhealthy for Sensitive Groups 10 of 242 days2002: Unhealthy 4 of 242 days2003: Good 179 of 242 days2003: Moderate 60 of 242 days2003: Unhealthy for Sensitive Groups 3 of 242 days2004: Good 170 of 242 days2004: Moderate 65 of 242 days2004: Unhealthy for Sensitive Groups 6 of 242 days2004: Unhealthy 1 of 242 days2005: Good 164 of 241 days2005: Moderate 71 of 241 days2005: Unhealthy for Sensitive Groups 6 of 241 days20052006: Good 175 of 238 days2006: Moderate 60 of 238 days2006: Unhealthy for Sensitive Groups 3 of 238 days2007: Good 169 of 240 days2007: Moderate 65 of 240 days2007: Unhealthy for Sensitive Groups 5 of 240 days2007: Unhealthy 1 of 240 days2008: Good 180 of 241 days2008: Moderate 60 of 241 days2008: Unhealthy for Sensitive Groups 1 of 241 days2009: Good 191 of 241 days2009: Moderate 49 of 241 days2009: Unhealthy for Sensitive Groups 1 of 241 days2010: Good 173 of 238 days2010: Moderate 62 of 238 days2010: Unhealthy for Sensitive Groups 3 of 238 days20102011: Good 184 of 242 days2011: Moderate 58 of 242 days2012: Good 178 of 242 days2012: Moderate 63 of 242 days2012: Unhealthy for Sensitive Groups 1 of 242 days2013: Good 193 of 244 days2013: Moderate 51 of 244 days2014: Good 259 of 297 days2014: Moderate 37 of 297 days2014: Unhealthy for Sensitive Groups 1 of 297 days2015: Good 354 of 362 days2015: Moderate 7 of 362 days2015: Unhealthy for Sensitive Groups 1 of 362 days20152016: Good 344 of 358 days2016: Moderate 14 of 358 days2017: Good 352 of 364 days2017: Moderate 12 of 364 days2018: Good 203 of 213 days2018: Moderate 10 of 213 days2019: Good 206 of 210 days2019: Moderate 4 of 210 days2020: Good 209 of 213 days2020: Moderate 4 of 213 days20202021: Good 200 of 214 days2021: Moderate 13 of 214 days2021: Unhealthy for Sensitive Groups 1 of 214 days2022: Good 205 of 212 days2022: Moderate 7 of 212 days2023: Good 197 of 213 days2023: Moderate 16 of 213 days2024: Good 205 of 214 days2024: Moderate 9 of 214 days2025: Good 205 of 212 days2025: Moderate 7 of 212 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 year0501001501985: 90th percentile AQI 441986: 90th percentile AQI 501987: 90th percentile AQI 711988: 90th percentile AQI 1261989: 90th percentile AQI 771990: 90th percentile AQI 971991: 90th percentile AQI 341992: 90th percentile AQI 741993: 90th percentile AQI 481994: 90th percentile AQI 491995: 90th percentile AQI 471996: 90th percentile AQI 481997: 90th percentile AQI 501998: 90th percentile AQI 491999: 90th percentile AQI 702000: 90th percentile AQI 612001: 90th percentile AQI 662002: 90th percentile AQI 762003: 90th percentile AQI 682004: 90th percentile AQI 662005: 90th percentile AQI 722006: 90th percentile AQI 662007: 90th percentile AQI 762008: 90th percentile AQI 612009: 90th percentile AQI 582010: 90th percentile AQI 642011: 90th percentile AQI 612012: 90th percentile AQI 612013: 90th percentile AQI 592014: 90th percentile AQI 522015: 90th percentile AQI 452016: 90th percentile AQI 442017: 90th percentile AQI 452018: 90th percentile AQI 472019: 90th percentile AQI 442020: 90th percentile AQI 442021: 90th percentile AQI 472022: 90th percentile AQI 462023: 90th percentile AQI 482024: 90th percentile AQI 462025: 90th percentile AQI 461985: Median AQI 251986: Median AQI 331987: Median AQI 281988: Median AQI 401989: Median AQI 431990: Median AQI 411991: Median AQI 191992: Median AQI 301993: Median AQI 291994: Median AQI 281995: Median AQI 291996: Median AQI 311997: Median AQI 321998: Median AQI 321999: Median AQI 392000: Median AQI 322001: Median AQI 392002: Median AQI 392003: Median AQI 392004: Median AQI 412005: Median AQI 452006: Median AQI 402007: Median AQI 402008: Median AQI 412009: Median AQI 362010: Median AQI 402011: Median AQI 382012: Median AQI 422013: Median AQI 372014: Median AQI 332015: Median AQI 332016: Median AQI 312017: Median AQI 322018: Median AQI 382019: Median AQI 362020: Median AQI 382021: Median AQI 362022: Median AQI 362023: Median AQI 392024: Median AQI 372025: Median AQI 37198519901995200020052010201520202025
  • 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
Ozone212100%

Year-by-year data

Annual AQI summary by year
YearDays with AQIGoodModerateUnhealthy for Sensitive GroupsUnhealthyVery UnhealthyHazardousMedian AQI90th pct.Max AQIMain pollutant
202521220570000374690Ozone
202421420590000374674Ozone
20232131971600003948100Ozone
202221220570000364684Ozone
20212142001310003647101Ozone
202021320940000384474Ozone
201921020640000364461Ozone
2018213203100000384787Ozone
2017364352120000324590Ozone
2016358344140000314484Ozone
2015362354710003345105Ozone
20142972593710003352101Ozone
2013244193510000375997Ozone
20122421786310004261101Ozone
2011242184580000386193Ozone
20102381736230004064126Ozone
20092411914910003658105Ozone
20082411806010004161102Ozone
20072401696551004076154Ozone
20062381756030004066112Ozone
20052411647160004572126Ozone
20042421706561004166166Ozone
20032421796030003968119Ozone
200224217157104003976164Ozone
20012381735951003966192Ozone
20002562084620003261119Ozone
19992471816240003970138Ozone
19982292091550003249147Ozone
19972041841541003250182Ozone
19962312121522003148156Ozone
19952272091620002947105Ozone
19942111921531002849161Ozone
19932392191352002948169Ozone
19921911632170003074133Ozone
1991605910000193453PM10
199023617542172004197185Ozone
19892311734971104377201Ozone
19882271583817104040126222Ozone
19872181872155002871161Ozone
1986132119940003350147Ozone
19858781231002544179Ozone

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 New Hampshire with a similar median AQI in 2025

All monitored counties in New Hampshire

Source: U.S. EPA AirData, “Annual AQI by County” files for 1985–2025 (EPA download page). EPA file dates: 2026-06-26. Imported into EnviroNearby November 5, 2026. How we use this data · Report a problem