Bad Air Days in Frederick County, Maryland: 2 a Year

2 bad air days a year

About the middle of monitored US counties. Ranked 483 of 996 monitored counties, where 1 is worst.

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

The bad-air season here

Most bad days here fall in June and July and May. 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 36 bad days a year; in 2016 to 2025, 2. The air has improved. The worst recent year was 2023, with 6 bad days on 232 monitored days.

Date	AQI	Main pollutant
2023-06-29	136	Ozone
2021-05-20	126	Ozone
2023-06-01	126	Ozone
2016-06-01	122	Ozone
2016-07-21	115	Ozone

The air on a normal day

Across 2,371 monitored days from 2016 to 2025, the air in Frederick County rated Good on 82% of them, more often than the 72% typical of monitored counties, and Moderate on 18%. The median day sat at AQI 42; one day in ten went above 61.

The pollutant setting the AQI on an ordinary day here:

ozone100%2,371 days

Year by year

Year	Days monitored	Good	Moderate	Over 100	Over 150	Median AQI	Highest AQI
2025*	237	203	34	0	0	41	93
2024	243	192	51	0	0	43	100
2023	232	167	59	6	0	44	136
2022	236	212	23	1	0	41	115
2021	244	207	36	1	0	41	126
2020	237	217	20	0	0	40	90
2019	243	198	44	1	0	44	112
2018	245	204	41	0	0	41	100
2017	243	190	51	2	0	42	105
2016	211	151	57	3	0	43	122
2015	209	160	48	1	0	44	119
2014	208	171	37	0	0	42	80
2013	208	171	36	1	0	43	119
2012	198	120	64	14	1	47	174
2011	214	146	59	9	0	44	150
2010	214	118	78	18	1	49	169

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

Elsewhere in Maryland

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.