Bad Air Days in Maricopa County, Arizona: 89 a Year

89 bad air days a year

Among the worst tenth of monitored US counties. Ranked 6 of 996 monitored counties, where 1 is worst.

What makes them bad: mostly ozone: summer smog (13% particle days).

Which months to watch

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

What to buy, and what not to

Particles (wildfire smoke, dust) cause enough bad days here to matter. A HEPA purifier sized to your bedroom removes them; run it with windows shut on bad days.
Upgrade the furnace or AC filter to MERV 13 so the whole house is filtered, not just one room.
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.
Sign up for EnviroFlash ozone alerts for this area during the worst months.

The worst days on file

In 2000 to 2009 this county averaged 81 bad days a year; in 2016 to 2025, 89. It has got worse. The worst recent year was 2021, with 225 bad days on 365 monitored days.

Date	AQI	Main pollutant
2025-01-01	366	PM2.5
2025-09-03	329	PM10
2021-01-01	297	PM2.5
2020-08-19	282	Ozone
2018-01-01	274	PM2.5

Most days, not the worst ones

Across 3,653 monitored days from 2016 to 2025, the air in Maricopa County rated Good on 9% of them, less often than the 72% typical of monitored counties, and Moderate on 66%. The median day sat at AQI 81; one day in ten went above 132.

The pollutant setting the AQI on an ordinary day here:

ozone57%2,095 days
fine particles (PM2.5)22%811 days
coarse dust (PM10)19%711 days
nitrogen dioxide1%36 days

Every recent year on file

Year	Days monitored	Good	Moderate	Over 100	Over 150	Median AQI	Highest AQI
2025*	365	32	295	38	6	73	366
2024	366	33	260	73	8	77	261
2023	365	41	256	68	7	72	183
2022	365	27	242	96	23	75	264
2021	365	12	128	225	134	122	297
2020	366	11	154	201	141	112	282
2019	365	50	277	38	0	67	147
2018	365	49	252	64	7	71	274
2017	365	38	272	55	5	71	173
2016	366	47	282	37	1	67	227
2015	365	62	267	36	0	66	147
2014	365	32	288	45	3	69	246
2013	365	36	281	48	7	68	187
2012	366	33	265	68	7	71	268
2011	365	27	273	65	17	70	572
2010	365	86	250	29	2	64	170

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

Elsewhere in Arizona

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.