Bad Air Days in Napa County, California: 8 a Year

8 bad air days a year

More bad air days than three quarters of monitored counties. Ranked 117 of 996 monitored counties, where 1 is worst.

What makes them bad: mostly particles: smoke and dust (88% particle days).

Which months to watch

Most bad days here fall in October and November and September. Averaged over 2016 to 2025.

Is a purifier worth it here?

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 record for Napa

In 2000 to 2009 this county averaged 3 bad days a year; in 2016 to 2025, 8. It has got worse. The worst recent year was 2020, with 15 bad days on 365 monitored days.

Date	AQI	Main pollutant
2017-10-13	274	PM2.5
2017-10-10	246	PM2.5
2020-10-02	224	PM2.5
2018-11-16	195	PM2.5
2020-10-01	191	PM2.5

Most days, not the worst ones

Across 1,963 monitored days from 2016 to 2021, the air in Napa County rated Good on 63% of them, less often than the 72% typical of monitored counties, and Moderate on 35%. The median day sat at AQI 44; one day in ten went above 63.

The pollutant setting the AQI on an ordinary day here:

fine particles (PM2.5)64%1,263 days
ozone35%688 days

Napa County by year

Year	Days monitored	Good	Moderate	Over 100	Over 150	Median AQI	Highest AQI
2021	140	108	32	0	0	41	77
2020	365	259	91	15	8	39	224
2019	363	295	66	2	0	37	119
2018	364	210	142	12	8	46	195
2017	365	137	214	14	7	53	274
2016	366	219	147	0	0	44	90
2015	365	158	206	1	0	53	108
2014	365	111	254	0	0	55	90
2013	365	150	212	3	0	53	119
2012	366	224	142	0	0	44	85
2011	365	192	171	2	0	49	120
2010	365	215	148	2	1	46	159

California 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.