Bad Air Days in Delaware County, Pennsylvania: 4 a Year

4 bad air days a year

About the middle of monitored US counties. Ranked 281 of 996 monitored counties, where 1 is worst.

What makes them bad: mostly ozone: summer smog (40% particle days).

The bad-air season here

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

What to buy, and what not to

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 Delaware

In 2000 to 2009 this county averaged 24 bad days a year; in 2016 to 2025, 4. The air has improved. The worst recent year was 2023, with 9 bad days on 365 monitored days.

Date	AQI	Main pollutant
2023-06-07	271	PM2.5
2023-06-08	222	PM2.5
2023-06-29	170	PM2.5
2023-06-06	156	PM2.5
2025-06-14	153	PM2.5

An ordinary day in Delaware

Across 3,653 monitored days from 2016 to 2025, the air in Delaware County rated Good on 49% of them, less often than the 72% typical of monitored counties, and Moderate on 50%. The median day sat at AQI 50; one day in ten went above 70.

The pollutant setting the AQI on an ordinary day here:

fine particles (PM2.5)72%2,621 days
ozone28%1,007 days
nitrogen dioxide1%25 days

Delaware County by year

Year	Days monitored	Good	Moderate	Over 100	Over 150	Median AQI	Highest AQI
2025*	365	238	124	3	1	44	153
2024	366	250	114	2	0	43	136
2023	365	187	169	9	4	50	271
2022	365	191	174	0	0	49	95
2021	365	182	180	3	0	51	142
2020	366	174	189	3	0	52	129
2019	365	180	178	7	0	51	133
2018	365	95	264	6	0	56	143
2017	365	158	205	2	0	52	119
2016	366	121	240	5	0	55	136
2015	365	120	236	9	0	55	150
2014	361	95	262	4	0	56	143
2013	365	139	221	5	0	54	119
2012	365	139	204	22	1	55	154
2011	363	122	227	14	1	56	151
2010	365	132	224	9	1	56	151

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

Pennsylvania 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.