Bad Air Days in Monterey County, California: 4 a Year

4 bad air days a year

About the middle of monitored US counties. Ranked 310 of 996 monitored counties, where 1 is worst.

What makes them bad: mostly particles: smoke and dust (100% particle days).

When the bad days come in Monterey

Most bad days here fall in August and September and November. 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.

The worst days on file

In 2000 to 2009 this county averaged 1 bad days a year; in 2016 to 2025, 4. It has got worse. The worst recent year was 2020, with 15 bad days on 366 monitored days.

Date	AQI	Main pollutant
2020-08-21	275	PM2.5
2020-08-20	235	PM2.5
2020-08-19	224	PM2.5
2020-08-22	198	PM2.5
2016-09-18	185	PM2.5

The air on a normal day

Across 3,653 monitored days from 2016 to 2025, the air in Monterey County rated Good on 77% of them, more often than the 72% typical of monitored counties, and Moderate on 22%. The median day sat at AQI 40; one day in ten went above 56.

The pollutant setting the AQI on an ordinary day here:

ozone49%1,804 days
fine particles (PM2.5)45%1,651 days
coarse dust (PM10)5%196 days

Year by year

Year	Days monitored	Good	Moderate	Over 100	Over 150	Median AQI	Highest AQI
2025*	365	314	51	0	0	38	62
2024	366	292	74	0	0	39	71
2023	365	274	90	1	0	42	115
2022	365	299	66	0	0	39	80
2021	365	261	104	0	0	41	80
2020	366	259	92	15	11	39	275
2019	365	303	61	1	0	39	144
2018	365	258	100	7	2	42	157
2017	365	265	99	1	0	41	121
2016	366	290	65	11	8	37	185
2015	365	285	79	1	0	41	120
2014	365	285	80	0	0	40	100
2013	365	251	114	0	0	44	97
2012	366	285	81	0	0	41	84
2011	365	293	72	0	0	41	77
2010	365	292	73	0	0	40	100

*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.