Bad Air Days in Belknap County, New Hampshire: under 1 a Year

under 1 bad air days a year

Rare: among the cleanest-air counties EPA monitors. Ranked 912 of 996 monitored counties, where 1 is worst.

What makes them bad: mostly particles: smoke and dust (100% particle days).

When the bad days come in Belknap

Most bad days here fall in July. Averaged over 2016 to 2025.

Is a purifier worth it here?

Bad air days are rare here. A standard furnace filter is enough; no purifier needed for outdoor air.
On the odd bad day, check AirNow and keep windows shut.

The worst days on file

In 2000 to 2009 this county averaged 9 bad days a year; in 2016 to 2025, under 1. The air has improved. The worst recent year was 2021, with 1 bad day on 365 monitored days.

Date	AQI	Main pollutant
2021-07-26	110	PM2.5
2023-07-18	105	PM2.5
2025-07-26	92	PM2.5
2023-07-17	91	PM2.5
2023-09-30	90	PM2.5

Most days, not the worst ones

Across 3,341 monitored days from 2016 to 2025, the air in Belknap County rated Good on 90% of them, more often than the 72% typical of monitored counties, and Moderate on 10%. The median day sat at AQI 33; one day in ten went above 50.

The pollutant setting the AQI on an ordinary day here:

ozone52%1,731 days
fine particles (PM2.5)48%1,610 days

Every recent year on file

Year	Days monitored	Good	Moderate	Over 100	Median AQI	Highest AQI
2025*	365	324	41	0	32	92
2024	365	311	54	0	35	78
2023	365	310	54	1	35	105
2022	358	339	19	0	32	68
2021	365	340	24	1	31	110
2020	359	342	17	0	32	65
2019	316	292	24	0	32	77
2018	283	257	26	0	33	80
2017	308	261	47	0	34	84
2016	257	230	27	0	35	84
2015	213	201	12	0	33	90
2014	213	201	12	0	32	90
2013	213	200	13	0	32	84
2012	210	179	31	0	37	97
2011	211	193	18	0	35	93
2010	213	186	24	3	38	119

*2025 may be a partial year in EPA's file.

Nearby counties on file

How this is counted

A bad air day is a day the county's daily AQI from EPA AirData went above 100, the level where EPA tells children, older adults and people with asthma or heart disease to cut back outdoors. Monitors do not run every day, so yearly counts are scaled to 365 days. This is the record, not today's air: for that, check AirNow. More on the method in how the county numbers are made, on the AQI 100 line in what counts as a bad air day.