Bad Air Days in Knox County, Maine: 1 a Year

1 bad air day a year

Fewer bad air days than most monitored US counties. Ranked 671 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 June and May and July. Averaged over 2016 to 2025.

What actually helps 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 record for Knox

In 2000 to 2009 this county averaged 13 bad days a year; in 2016 to 2025, 1. The air has improved. The worst recent year was 2017, with 3 bad days on 215 monitored days.

Date	AQI	Main pollutant
2017-05-18	143	Ozone
2017-06-12	136	Ozone
2016-07-22	122	Ozone
2021-06-06	119	Ozone
2018-05-02	112	Ozone

Most days, not the worst ones

Across 2,414 monitored days from 2016 to 2025, the air in Knox County rated Good on 96% of them, more often than the 72% typical of monitored counties, and Moderate on 4%. The median day sat at AQI 34; one day in ten went above 45.

The pollutant setting the AQI on an ordinary day here:

ozone100%2,414 days

Knox County by year

Year	Days monitored	Good	Moderate	Over 100	Over 150	Median AQI	Highest AQI
2025*	359	351	8	0	0	33	77
2024	355	343	12	0	0	33	84
2023	301	296	5	0	0	33	87
2022	212	206	6	0	0	35	80
2021	211	192	17	2	0	35	119
2020	148	142	6	0	0	31	71
2019	213	204	9	0	0	36	84
2018	219	209	9	1	0	36	112
2017	215	205	7	3	0	35	143
2016	181	171	9	1	0	33	122
2015	178	164	13	1	0	36	119
2014	205	194	11	0	0	34	87
2013	208	188	15	5	0	38	140
2012	177	165	12	0	0	34	97
2011	181	166	12	3	0	34	115
2010	212	191	18	3	1	40	166

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