Bad Air Days in Nevada County, California: 27 a Year

27 bad air days a year

Among the worst tenth of monitored US counties. Ranked 21 of 996 monitored counties, where 1 is worst.

What makes them bad: mostly ozone: summer smog (24% particle days).

The bad-air season here

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

What actually helps 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.
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 record for Nevada

In 2000 to 2009 this county averaged 71 bad days a year; in 2016 to 2025, 27. The air has improved. The worst recent year was 2017, with 81 bad days on 365 monitored days.

Date	AQI	Main pollutant
2021-08-23	326	PM2.5
2021-08-22	305	PM2.5
2022-09-10	293	PM2.5
2020-09-12	277	PM2.5
2020-09-13	256	PM2.5

An ordinary day in Nevada

Across 3,641 monitored days from 2016 to 2025, the air in Nevada County rated Good on 65% of them, less often than the 72% typical of monitored counties, and Moderate on 27%. The median day sat at AQI 45; one day in ten went above 85.

The pollutant setting the AQI on an ordinary day here:

ozone66%2,408 days
fine particles (PM2.5)34%1,233 days

Every recent year on file

Year	Days monitored	Good	Moderate	Over 100	Over 150	Median AQI	Highest AQI
2025*	362	289	72	1	0	42	108
2024	365	313	50	2	0	36	132
2023	363	282	81	0	0	42	97
2022	365	185	161	19	8	50	293
2021	365	183	129	53	19	50	326
2020	366	223	111	32	17	46	277
2019	361	251	105	5	0	45	122
2018	363	231	104	28	9	45	218
2017	365	194	90	81	19	49	185
2016	366	232	89	45	5	45	179
2015	364	213	120	31	2	48	166
2014	365	211	120	34	1	47	168
2013	362	202	139	21	0	48	140
2012	366	226	112	28	2	43	164
2011	365	233	111	21	1	44	154
2010	365	242	101	22	1	44	154

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

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, and on why a purifier does not touch ozone in smoke vs ozone.