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

1 bad air day a year

Fewer bad air days than most monitored US counties. Ranked 591 of 996 monitored counties, where 1 is worst.

What makes them bad: a mix of particles and ozone (47% particle days).

The bad-air season here

Most bad days here fall in July and May and June and April and August. Averaged over 2016 to 2025.

What to buy, and what not to

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.

Better or worse than it was

In 2000 to 2009 this county averaged 14 bad days a year; in 2016 to 2025, 1. The air has improved. The worst recent year was 2023, with 5 bad days on 365 monitored days.

Date	AQI	Main pollutant
2021-07-26	144	PM2.5
2025-07-26	123	PM2.5
2023-09-30	119	PM2.5
2023-04-14	112	Ozone
2023-07-01	109	PM2.5

An ordinary day in Hillsborough

Across 3,653 monitored days from 2016 to 2025, the air in Hillsborough 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 37; one day in ten went above 51.

The pollutant setting the AQI on an ordinary day here:

ozone89%3,260 days
fine particles (PM2.5)11%386 days

Year by year

Year	Days monitored	Good	Moderate	Over 100	Over 150	Median AQI	Highest AQI
2025*	365	310	51	4	0	38	123
2024	366	328	38	0	0	37	97
2023	365	305	55	5	0	39	119
2022	365	329	36	0	0	39	100
2021	365	328	35	2	0	37	144
2020	366	351	15	0	0	37	90
2019	365	345	20	0	0	37	80
2018	365	327	38	0	0	37	97
2017	365	336	28	1	0	37	101
2016	366	324	39	3	0	37	108
2015	365	289	74	2	0	41	129
2014	365	300	64	1	0	41	129
2013	365	280	84	1	0	40	112
2012	366	296	64	6	0	40	150
2011	365	296	66	3	1	39	161
2010	365	287	70	8	0	39	140

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

Elsewhere in New Hampshire

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, and on why a purifier does not touch ozone in smoke vs ozone.