Days of Bad Air

Shade every state by how many unhealthy days its typical monitored county logged, and the country splits in two. The West and the desert Southwest carry nearly all the weight; a wide band of the interior and the eastern seaboard barely registers. Air quality in America is not a national condition - it is a regional one.

Hover a state for its figures

Fill = the state's mean bad-air days across its monitored counties, on a single-hue teal density ramp. Grey states have no reporting monitor - that is a data gap, not clean air. Hover a state for its figure; the table below is the source of truth.

The 8 worst states in a table (the map's source of truth)

State	Bad-air days / county	Counties	Top pollutant
California	21.9	52	Ozone
Arizona	17	13	Ozone
Texas	10.4	45	PM2.5
Idaho	9.1	19	PM2.5
Colorado	8.9	30	Ozone
Connecticut	8.3	8	Ozone
New Mexico	8.1	16	Ozone
North Dakota	7.8	9	Ozone

One row per county, ranked by days over an AQI of 100. Southern California and its inland valleys own the top of the board outright - eight of the worst ten counties sit downwind of Los Angeles, where sun, traffic, and mountains trap the smog. The bar is the count; the tag is the pollutant that drove the most of those days.

1. 01 Riverside CA 147 Ozone med 87
2. 02 San Bernardino CA 144 Ozone med 87
3. 03 Los Angeles CA 133 Ozone med 80
4. 04 Tulare CA 111 Ozone med 71
5. 05 Kern CA 94 Ozone med 72
6. 06 Fresno CA 79 Ozone med 67
7. 07 Maricopa AZ 73 Ozone med 77
8. 08 Pinal AZ 70 PM10 med 77
9. 09 El Paso TX 63 Ozone med 67
10. 10 Cameron TX 61 PM2.5 med 61
11. 11 San Diego CA 52 Ozone med 71
12. 12 Imperial CA 50 Ozone med 63
13. 13 Boise ID 49 PM2.5 med 48
14. 14 Harris TX 47 PM2.5 med 61
15. 15 Salt Lake UT 43 Ozone med 54

Bar length = bad-air days on a shared scale (max 147, Riverside). The tag names the pollutant behind the most of those days and is always text-labeled, so colour is never the sole signal. med = the county's median AQI across the year.

Not every bad-air county is bad the same way. Read left-to-right for the everyday load - the median day's AQI. Read bottom-to-top for how often the air turns unhealthy. And the bigger the bubble, the worse that county's single peak day. Riverside sits high and right: bad most days. A clean county with one giant bubble was simply caught under wildfire smoke once.

Each bubble is one county; area scales with its worst single day (peak AQI 169-593). Colour marks the dominant pollutant and is always text-labeled. Values below.

The 16 counties in a table (the chart's source of truth)

County	Median AQI	Bad-air days	Peak AQI	Top pollutant
Riverside, CA	87	147	213	Ozone
San Bernardino, CA	87	144	593	Ozone
Los Angeles, CA	80	133	230	Ozone
Tulare, CA	71	111	185	Ozone
Kern, CA	72	94	177	Ozone
Fresno, CA	67	79	169	Ozone
Maricopa, AZ	77	73	261	Ozone
Pinal, AZ	77	70	337	PM10

Here is the story the atlas hides. The typical monitored county went from about 15.2 bad-air days a year in 2000 to roughly 4 in 2024 - a 74% fall. Cleaner fuels, catalytic converters, and a wave of coal-plant retirements read here as one long slide down. The easy wins are spent, though: the curve flattens after the mid-2010s, and wildfire smoke now pushes the western numbers back up.

One marker per reporting year; the wash beneath the line is decoration, not a second series. Y axis = mean bad-air days across every monitored county that year. 1,104 counties reported in 2000; 996 in 2024.

The national average fell hard, but the cleanup was not shared. The East and the South cut their bad-air days by roughly 80 to 90 percent - an ozone problem that cleaner fuels and retired coal plants steadily solved. The West is the holdout: it came down too, but only about 40 percent, and still sits several times higher than any other region, because its air is ruled by wildfire smoke (PM2.5), not the ozone the rest of the country fixed. Same scale on every panel - the dashed line is the 2024 national average.

Small multiples on one shared vertical scale (0-32 days), so a taller line means dirtier air. The dashed line marks the 4-day national average for 2024; a region riding above it is still doing worse than the country as a whole.

A bad-air day is credited to whichever pollutant pushed the index highest. Two culprits do almost all the damage: summertime ozone (50.6%) and fine-particle PM2.5. Add coarse PM10 and particle pollution accounts for 48% of bad days; every other regulated pollutant, together, barely clears one percent.

One 100%-wide bar; segment width = that pollutant's share of the nation's bad-air days, with a 2px gap between slots. Other folds the two trace gases: NO2 0.9% and CO 0.2%. Colour is always paired with a text label.

The driver flips by region

Ozone needs sun and traffic; particles come from fire, woodsmoke, and winter inversions. So which pollutant leads is really a map of geography and season - the same bad-air day means something different in a Los Angeles July and a Salt Lake January.

Abstract counts become a calendar you can feel. Each arc is a run of days from Riverside County's 366 monitored days in 2024, coloured by the EPA's six official air-quality bands. Most of the ring is green and yellow - but 147 days crossed into unhealthy territory, and that slice is what this whole page is counting.

Each ring segment is proportional to that band's share of the year; the six colours are the EPA's canonical AQI scale, and every band carries its AQI range and day count in the key. The Riverside 2024 split is real: the county's actual Good/Moderate/USG/Unhealthy/Very Unhealthy/Hazardous day counts from the annual file.

Line up all 996 monitored counties by how many unhealthy days they logged and the inequality is stark. The median county had just 1 bad-air days all year. 705 of them had two or fewer. The burden lives in the thin tail on the right - the 42 counties with more than twenty bad days that the headlines are really about.

Each column counts the monitored counties whose 2024 bad-air-day total fell in that range; every count is labeled so the short tail stays readable. Columns deepen with severity on the same teal ramp as the atlas. Buckets are the real 2024 distribution, county by county.