Bad Air Days in Harford County, Maryland: 7 a Year

7 bad air days a year

More bad air days than three quarters of monitored counties. Ranked 142 of 996 monitored counties, where 1 is worst.

What makes them bad: mostly ozone: summer smog (10% particle days).

Which months to watch

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

What actually helps here

Ozone drives many bad days here. Purifiers and filters do not remove ozone, so do not buy one for that. Ozone peaks on hot afternoons: exercise outside in the morning.

The record for Harford

In 2000 to 2009 this county averaged 43 bad days a year; in 2016 to 2025, 7. The air has improved. The worst recent year was 2016, with 12 bad days on 359 monitored days.

Date	AQI	Main pollutant
2023-06-08	200	PM2.5
2023-06-29	172	PM2.5
2023-06-07	164	PM2.5
2017-06-13	156	Ozone
2016-09-23	154	Ozone

An ordinary day in Harford

Across 3,573 monitored days from 2016 to 2025, the air in Harford County rated Good on 68% of them, less often than the 72% typical of monitored counties, and Moderate on 30%. The median day sat at AQI 44; one day in ten went above 67.

The pollutant setting the AQI on an ordinary day here:

ozone53%1,903 days
fine particles (PM2.5)47%1,670 days

Year by year

Year	Days monitored	Good	Moderate	Over 100	Over 150	Median AQI	Highest AQI
2025*	343	259	81	3	0	42	143
2024	363	262	95	6	1	42	154
2023	359	238	111	10	3	45	200
2022	360	262	98	0	0	43	97
2021	357	219	129	9	0	46	122
2020	357	286	70	1	0	41	119
2019	359	229	121	9	0	45	133
2018	358	247	101	10	0	42	150
2017	358	206	143	9	2	47	156
2016	359	211	136	12	1	48	154
2015	358	198	154	6	1	48	156
2014	357	161	193	3	0	52	133
2013	353	180	168	5	0	50	133
2012	360	153	185	22	5	54	201
2011	304	125	150	29	12	54	209
2010	263	133	89	41	17	50	205

*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.