Bad Air Days in Coos County, New Hampshire: 2 a Year

2 bad air days a year

About the middle of monitored US counties. Ranked 493 of 996 monitored counties, where 1 is worst.

What makes them bad: mostly ozone: summer smog (0% particle days).

Which months to watch

Most bad days here fall in April and May and March and June. 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 16 bad days a year; in 2016 to 2025, 2. The air has improved. The worst recent year was 2025, with 4 bad days on 345 monitored days.

Date	AQI	Main pollutant
2017-03-18	174	Ozone
2023-04-15	136	Ozone
2017-03-17	133	Ozone
2021-08-16	133	Ozone
2021-08-17	129	Ozone

Most days, not the worst ones

Across 3,475 monitored days from 2016 to 2025, the air in Coos County rated Good on 87% of them, more often than the 72% typical of monitored counties, and Moderate on 12%. The median day sat at AQI 43; one day in ten went above 55.

The pollutant setting the AQI on an ordinary day here:

ozone96%3,343 days
fine particles (PM2.5)3%107 days
carbon monoxide1%25 days

Every recent year on file

Year	Days monitored	Good	Moderate	Over 100	Over 150	Median AQI	Highest AQI
2025*	345	281	60	4	0	44	119
2024	365	296	69	0	0	44	90
2023	364	318	42	4	0	43	136
2022	359	350	9	0	0	40	64
2021	360	325	33	2	0	42	133
2020	366	339	27	0	0	43	74
2019	324	313	11	0	0	40	64
2018	271	222	46	3	0	44	115
2017	356	290	63	3	1	43	174
2016	365	303	58	4	0	44	119
2015	354	285	62	7	0	45	140
2014	365	311	54	0	0	43	97
2013	365	278	85	2	0	44	119
2012	365	267	94	4	0	45	108
2011	356	266	88	2	0	44	101
2010	362	294	63	5	0	44	129

*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.