The three most polluted cities in the US this autumn are in one California valley

The most polluted cities in the US this autumn are in California's San Joaquin Valley. Good Air ranked 266 US cities by their average daily US AQI from 15 September to 19 October 2026, using only readings from government monitors published by the EPA's AirNow system. Hanford (75), Fresno (73) and Bakersfield (70) took the top three places, well ahead of Houston in fourth (61).

Those three numbers are in the Moderate band of the AQI, not the Unhealthy ones. This was a calm stretch for most of the country: after the summer wildfire season and before winter, the average US city scored 37, which is Good. That is what makes the valley stand out. Hanford and Fresno were the only two cities in our list whose air was worse than Good on every one of the 35 days.

These numbers are our own analysis.

Key findings

• The San Joaquin Valley holds the top three places. Hanford averaged 75, Fresno 73 and Bakersfield 70. A fourth valley city, Reedley, ranks 19th at 50.
• Hanford and Fresno were Moderate or worse on all 35 days, the only cities in our ranking to do so. Bakersfield was above 50 on 33 days. By contrast, 53 cities never had a day above 50.
• The valley got worse in October. Fresno's average rose from 67 in the second half of September to 79 from 1 to 19 October, and Hanford's from 71 to 79. The national average went from 38 to 37.
• It is particles, not ozone. The 10 monitors with the highest coarse-particle (PM10) levels among 429 in the country are all in the San Joaquin Valley. Hanford's averaged 95 µg/m³, against a national average of 20.
• Unhealthy days were rare everywhere. Only 10 of 266 cities had even one day with an average above 100, the line where air becomes Unhealthy for Sensitive Groups: 12 days out of 9,225 city-days.
• The cleanest air was in Laramie, Wyoming (11) and Concord, New Hampshire (15).

Top 10 most polluted cities in the US

The table shows the average of each city's daily US AQI from 15 September to 19 October 2026, and on how many of the 35 days the daily average was above 50 (Moderate or worse). Cities are ranked on unrounded values. On the US AQI, 0 to 50 is Good, 51 to 100 Moderate, and 101 to 150 Unhealthy for Sensitive Groups (AirNow, AQI basics, viewed 20 October 2026).

California has six of the top 10 and 11 of the top 20. El Cajon and San Diego are both in San Diego County but have separate monitors. Texas has four cities in the top 20.

For comparison, here is where some large cities landed:

For today's ranking rather than this five-week average, see the live US air quality ranking. For one city in detail, see our look at Seattle's air quality this fall.

Why the San Joaquin Valley leads

The valley's lead is not a single bad week. A valley city (Hanford, Fresno, Bakersfield or Reedley) had the highest daily average in the country on 13 of the 35 days. The three main cities together were close to 100 on 3 October, when they averaged 99.6.

AirNow's city readings give one AQI number per hour, set by whichever pollutant is highest, so they don't say what caused it. The individual monitors do. They show that the valley's problem this autumn is particle pollution, and especially coarse particles.

PM10 is particles up to 10 micrometres across, most of it dust. These are the highest average PM10 readings from 15 September to 19 October among the 429 US monitors in our archive that measure it:

All ten are in the San Joaquin Valley. Farm work is one source of that dust: the Almond Board of California says nut harvesting produces "significant PM10 emissions" (Almond Board of California, updated 1 June 2020). Our monitor data can't separate farm dust from road or soil dust.

Fine particles (PM2.5), which come largely from burning, are high too. Among 1,258 US monitors that measure PM2.5, four valley monitors rank 4th to 7th: Fresno Foundry Park (14.8 µg/m³), Corcoran (14.5), Hanford (14.4) and Fresno Garland (14.0). The median US monitor averaged 5.2 µg/m³.

Ozone, the main summer smog pollutant, mattered least. In the hourly data we hold for 12 to 19 October, ozone set Hanford's AQI in 1 of 190 hours. PM10 set it in 125 hours and PM2.5 in 64. In Fresno, PM2.5 led for 98 hours and PM10 for 80. Bakersfield's mix was PM10 102 hours, PM2.5 45 and ozone 43.

This fits the valley's long record. The American Lung Association's State of the Air 2026 report ranked Bakersfield the most polluted US city for year-round particle pollution, with Fresno 4th and Visalia tied for 5th, on data from 2022 to 2024 (American Lung Association, 21 April 2026).

The desert spikes, and why we don't headline them

Some of the highest single readings in our data came from desert cities. Phoenix logged an hourly US AQI of 867 on 15 September, Yuma 807 on 11 October and El Paso 608 on 16 October. Hazardous starts at 301, so these look alarming. We don't rank on them, for three reasons.

First, they are short. El Paso's AQI was above 400 in seven of the hours between about 11 pm on 15 October and 7 am on 16 October, local time, with PM10 the cause each hour. That matches a windstorm: the National Weather Service had a high wind warning in force for El Paso County from 6 pm on 15 October to noon on 16 October, with a blowing dust advisory until 9 am (KVIA, 16 October). Tucson, Yuma and San Luis also had PM10 spikes on 16 and 17 October. Second, we could not match the Phoenix and Yuma readings to a reported dust storm, so some may be sensor faults. Third, Yuma and San Luis reported identical hourly values on 16 and 17 October, so they shared a monitor on those days and are not two separate events.

Leaving out every day with an hour above 300 barely changes the top 10. Phoenix drops from 57 to 54 and from 7th to 10th. El Paso falls from 11th (55) to 27th (50), and Yuma from 30th to 92nd. Tucson and San Luis were never near the top.

The 12 days above 100 were spread over 10 cities: Hanford (2 October), Bakersfield (3 October), Reedley (1 October), El Cajon (17 October), Wilmington, North Carolina (9 October), Phoenix (15 September), El Paso and Tucson (16 October), San Luis (16 October) and Yuma (11, 16 and 17 October). El Cajon's day came during the Santa Ana winds, with PM10 again the cause; we covered that event in our Santa Ana winds report.

Top 10 cleanest cities in the US

Idaho Falls misses the list by less than 0.1. None of the ten had a day above 100, and Laramie, Concord, Lynchburg, Burlington, Juneau and Parkersburg never had a day above 50.

One caution applies to this list. A city's AQI is set by the highest pollutant measured there, so a city that measures fewer pollutants can look cleaner. In our hourly data for 12 to 19 October, Laramie, Juneau, Lubbock and Butte reported only PM2.5, and Lynchburg, Sioux City and Parkersburg only ozone. Nine of the ten most polluted cities also reported PM10, which can push the AQI up on dusty days. The valley's lead doesn't depend on this: its PM2.5 alone is among the highest in the country, as shown above.

What comes next

All of this happened in what is usually a quiet season, after the summer wildfire season and before winter. In the San Joaquin Valley, winter brings smoke from fireplaces and wood stoves, which the valley air district limits on bad days under its Check Before You Burn programme. That smoke adds to fine particles, not dust, so the valley's mix could shift from PM10 towards PM2.5. We will update this ranking in January with winter data.

If smoke does reach your area, our wildfire smoke safety guide explains masks, purifiers and when to stay inside.

How we measured

• Data. The hourly US AQI that the EPA's AirNow system publishes for each city, collected by Good Air, plus AirNow's individual monitor readings for PM2.5 and PM10. Government monitors only: no low-cost sensors, satellite estimates or model output.
• City value. For each day, the mean of that day's hourly AQI values, counted only if the city had at least 12 hourly readings. A city counts only if it had at least 30 of the 35 days: 266 of the 283 US cities in our archive qualified. The city figure is then the mean of its daily values. This is not the same as EPA's official daily AQI, which uses 24-hour and 8-hour averages, so it can differ a little from AirNow's daily figures.
• Period. 15 September to 19 October 2026, days in UTC. Our US archive starts partway through 14 September, so that day had too few readings to count.
• Outliers. We kept every hour in the ranking and checked it without the days that had an hour above 300. The top 10 cities stay the same.
• Shared monitors. Some neighbouring cities share readings. Yuma and San Luis did on 16 and 17 October; Cape Girardeau, Missouri and Carbondale, Illinois had identical values on 34 of 35 days. Little Rock and Conway, Arkansas (13th) matched on 12 of 35 days. No two cities in either top 10 table share all their readings.
• Monitors. The PM10 and PM2.5 figures are means of daily averages at each monitor with at least 30 valid days. Monitor names are AirNow's.