Daily Air Quality Index summaries from EPA monitors in Napa, California, 1980–2021.
77%
Good days in 2021
41
Median daily AQI, 2021
0
Days above 100 in 2021
140
Days with an AQI in 2021 (of 365)
In 2021, EPA reported a daily Air Quality Index (AQI) for Napa, California on 140 days. 108 of those days (77%) were in the Good range, and the median daily AQI was 41. That median is lower than the typical monitored county in California (median 49 across 53 counties), and about the same as the typical monitored U.S. county (41). The main pollutant was ozone on 74 days. Across the years with fuller monitoring, the share of Good days fell: an average of 84% in 1980–1982 versus 76% in 2019–2021.
EPA did not report an AQI for every day of 2021 here (140 of 365 days). Figures describe monitored days only; seasonal monitoring (for example, ozone measured only in warmer months) can make a county look cleaner or dirtier than a full-year record would.
The most recent year with data for this county is 2021; EPA's files for 2022–2025 contain no row for it, which usually means its monitors stopped reporting.
How the days broke down each year
Each bar is one year. The colored segments show the share of monitored days that fell in each AQI category.
Good
Moderate
Unhealthy for Sensitive Groups
Unhealthy
Very Unhealthy
Hazardous
Typical and high-end AQI over time
The median is the middle day of the year. The 90th percentile is the level that only the highest 10% of days exceeded.
Median daily AQI
90th-percentile daily AQI
Main pollutant by day, 2021
On each monitored day, the AQI reflects whichever measured pollutant had the highest index. This counts how many days each pollutant set the AQI. A pollutant that was not measured here cannot appear.
Pollutant
Days as main pollutant
Share of monitored days
Ozone
74
53%
Fine particles (PM2.5)
65
46%
Carbon monoxide (CO)
1
1%
Year-by-year data
Annual AQI summary by year
Year
Days with AQI
Good
Moderate
Unhealthy for Sensitive Groups
Unhealthy
Very Unhealthy
Hazardous
Median AQI
90th pct.
Max AQI
Main pollutant
2021
140
108
32
0
0
0
0
41
56
77
Ozone
2020
365
259
91
7
7
1
0
39
66
224
PM2.5
2019
363
295
66
2
0
0
0
37
54
119
Ozone
2018
364
210
142
4
8
0
0
46
65
195
PM2.5
2017
365
137
214
7
5
2
0
53
71
274
PM2.5
2016
366
219
147
0
0
0
0
44
61
90
PM2.5
2015
365
158
206
1
0
0
0
53
72
108
PM2.5
2014
365
111
254
0
0
0
0
55
69
90
PM2.5
2013
365
150
212
3
0
0
0
53
73
119
PM2.5
2012
366
224
142
0
0
0
0
44
66
85
PM2.5
2011
365
192
171
2
0
0
0
49
69
120
PM2.5
2010
365
215
148
1
1
0
0
46
64
159
PM2.5
2009
365
211
151
3
0
0
0
46
69
122
PM2.5
2008
366
94
263
9
0
0
0
55
75
122
PM2.5
2007
365
124
235
6
0
0
0
55
79
120
PM2.5
2006
365
349
14
2
0
0
0
34
44
105
Ozone
2005
365
361
4
0
0
0
0
31
41
90
Ozone
2004
366
355
9
2
0
0
0
31
40
105
Ozone
2003
365
350
12
3
0
0
0
31
43
143
Ozone
2002
365
353
10
2
0
0
0
32
44
140
Ozone
2001
365
350
14
1
0
0
0
31
44
126
Ozone
2000
366
357
9
0
0
0
0
31
41
77
Ozone
1999
365
340
19
5
1
0
0
32
45
161
Ozone
1998
365
345
17
2
1
0
0
31
43
185
Ozone
1997
365
356
8
1
0
0
0
31
40
101
Ozone
1996
366
345
18
3
0
0
0
32
43
115
Ozone
1995
365
343
14
7
1
0
0
31
44
177
Ozone
1994
365
347
15
3
0
0
0
31
46
115
Ozone
1993
365
336
25
4
0
0
0
35
48
143
Ozone
1992
364
335
29
0
0
0
0
32
47
100
Ozone
1991
365
313
49
3
0
0
0
35
57
115
Ozone
1990
357
313
43
1
0
0
0
33
57
105
Ozone
1989
365
308
54
3
0
0
0
34
57
150
Ozone
1988
366
304
56
5
1
0
0
37
57
156
Ozone
1987
365
314
44
7
0
0
0
38
57
133
Ozone
1986
365
316
49
0
0
0
0
37
57
90
Ozone
1985
365
324
38
3
0
0
0
35
52
150
Ozone
1984
366
312
45
7
2
0
0
34
60
161
Ozone
1983
362
328
28
6
0
0
0
31
49
136
Ozone
1982
365
318
46
1
0
0
0
38
54
122
Ozone
1981
364
325
39
0
0
0
0
34
52
90
Ozone
1980
366
275
84
6
1
0
0
38
68
156
NO2
What this data can and can’t tell you
It is a county-level summary. EPA reports one AQI per day for the county, based on the monitors operating there. Air at a particular street, school or home can differ, especially near roads, industry or wildfire smoke.
It is not a health assessment. The AQI is EPA’s scale for communicating daily air quality. A county’s history can’t tell you what the health risk is for any particular person. For daily conditions and health guidance, see AirNow.
Rules change over time. EPA has revised AQI breakpoints, most recently for fine particles in 2024, so some year-to-year change can reflect the scale rather than the air.
