Bad Air Days in Montgomery County, Maryland: 1 a Year

1 bad air day a year

Fewer bad air days than most monitored US counties. Ranked 629 of 996 monitored counties, where 1 is worst.

What makes them bad: mostly ozone: summer smog (25% particle days).

When the bad days come in Montgomery

Most bad days here fall in June and May. 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.

Better or worse than it was

In 2000 to 2009 this county averaged 28 bad days a year; in 2016 to 2025, 1. The air has improved. The worst recent year was 2023, with 5 bad days on 357 monitored days.

Date	AQI	Main pollutant
2023-06-29	165	PM2.5
2023-06-08	162	PM2.5
2023-06-07	152	PM2.5
2021-05-20	119	Ozone
2022-06-30	119	Ozone

The air on a normal day

Across 3,556 monitored days from 2016 to 2025, the air in Montgomery County rated Good on 75% of them, close to the 72% typical of monitored counties, and Moderate on 25%. The median day sat at AQI 41; one day in ten went above 59.

The pollutant setting the AQI on an ordinary day here:

ozone52%1,838 days
fine particles (PM2.5)48%1,718 days

Montgomery County by year

Year	Days monitored	Good	Moderate	Over 100	Over 150	Median AQI	Highest AQI
2025*	360	299	61	0	0	39	90
2024	356	271	85	0	0	40	100
2023	357	291	61	5	3	42	165
2022	355	241	113	1	0	43	119
2021	361	251	107	3	0	42	119
2020	355	310	45	0	0	37	77
2019	361	284	77	0	0	41	88
2018	347	208	138	1	0	46	112
2017	359	268	89	2	0	43	105
2016	345	248	97	0	0	42	100
2015	345	160	180	5	0	52	143
2014	355	196	159	0	0	48	90
2013	354	230	123	1	0	45	105
2012	360	162	191	7	1	52	154
2011	335	139	182	14	1	54	156
2010	362	150	188	24	0	53	136

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

Maryland compared

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.