Explainers · 7 min read
Why is Fresno's air quality so bad? What our monitor data shows for Fresno, Hanford and Bakersfield
Why is Fresno air quality so bad? Hanford, Fresno and Bakersfield were the most polluted of 266 US cities, 15 Sep to 20 Oct 2026, at twice the US median.
By the Good Air team
In this article · 8 sections
Why is Fresno's air quality so bad? Mostly because of particles, and because the San Joaquin Valley holds on to them. From 15 September to 20 October 2026, Hanford (average US AQI 75), Fresno (73) and Bakersfield (70) were the three most polluted of the 266 US cities we track, about twice the median city's 37, and dust (PM10) and fine particles (PM2.5) set their AQI far more often than ozone.
That is our own analysis of EPA AirNow monitor data, extending our ranking of the most polluted US cities this autumn to 20 October, the latest full day in our archive. Below: what is in the air, why the valley traps it, the wood-burning season, and what people in Fresno, Clovis, Visalia, Hanford and Bakersfield can do.
How bad is the air in Fresno compared with the rest of the US?
On the US AQI, 0 to 50 is Good and 51 to 100 Moderate, acceptable except for people unusually sensitive to pollution (AirNow, AQI basics, viewed 22 October 2026). The valley sits mid-Moderate; the typical US city is Good.
| Period (2026) | Fresno | Hanford | Bakersfield | US median city | Cities ranked |
|---|---|---|---|---|---|
| 15 Sep to 20 Oct | 73 (2nd) | 75 (1st) | 70 (3rd) | 37 | 266 |
| 1 to 20 Oct | 77 (1st) | 77 (2nd) | 72 (3rd) | 36 | 271 |
| 14 to 20 Oct | 67 (3rd) | 69 (2nd) | 62 (6th) | 34 | 273 |
| 20 Oct only | 58 | 49 | 59 | 38 | 269 |
- Every day. Fresno was above 50 on all 36 days, Hanford on 35 and Bakersfield on 34. Houston, fourth, averaged 61 and Los Angeles, eighth, 56.
- Often double. The three cities' average was at least twice the US median on 17 of 36 days. On 3 October it was nearly 100, against 28.
- Latest week. From 14 to 20 October the three averaged 62 to 69, about double the median of 34. Only El Paso, Texas (74), ranked above Hanford.
Long-term data agrees. The American Lung Association's State of the Air 2026 report, on 2022 to 2024 data, ranked Bakersfield the most polluted US city for year-round particle pollution, Fresno 4th and Visalia tied for 5th. For ozone, Visalia ranked 2nd, Bakersfield 3rd and Fresno 5th (American Lung Association, 21 April 2026).
What is in Fresno's air: dust and fine particles
A city's AQI each hour is set by its highest pollutant. In the 209 hours from 5 pm on 12 October to 11 am on 21 October (Pacific time), PM10 set Fresno's AQI in 100 hours, PM2.5 in 97 and ozone in 12. In Hanford it was PM10 157, PM2.5 51 and ozone 1; in Bakersfield PM10 134, ozone 43 and PM2.5 32.
PM10 is coarse particles, mostly dust; PM2.5 is fine particles, from burning and from chemical reactions in the air. From 15 September to 20 October, the ten US monitors with the highest PM10 were all in the valley:
| Monitor | PM10 (µg/m³) | Rank of 425 |
|---|---|---|
| Hanford, S. Irwin Street | 94 | 1 |
| Corcoran | 92 | 2 |
| Fresno, Drummond Street | 84 | 3 |
| Bakersfield, Golden State Highway | 77 | 4 |
| Clovis, N. Villa Avenue | 66 | 6 |
| Median US monitor | 17.6 |
PM2.5 was high too. Of 1,256 US monitors, Fresno Foundry Park (14.7 µg/m³), Corcoran (14.4), Hanford (14.2) and Fresno Garland (13.9) ranked 4th to 7th and Clovis (12.8) 10th, against a median of 5.2. Visalia's monitor reported on only 21 of 36 days, so we left it out.
Our data can't tell farm dust from road or soil dust.
Why does the San Joaquin Valley trap pollution?
The valley is a bowl. Undark described it as hemmed in by mountain ranges on three sides, and explained that in winter a layer of warm air traps cooler, smoggy air close to the ground: a temperature inversion (Undark, 3 December 2018).
It also cited the California Air Resources Board: ammonium nitrate makes up more than half of the region's PM2.5 on its most polluted days. It forms when nitrogen oxides from truck exhaust combine with ammonia from cattle manure.
Winter adds smoke. The Valley Air District says residential wood burning is one of the valley's largest sources of wintertime PM2.5, and that inversion layers trap that pollution near the ground, where people breathe (The Bakersfield Californian, 30 October 2025). In January 2026, ABC30 reported that the weather pattern that brings morning fog is also a pollution trap, and that more homes light fireplaces as fog lingers and temperatures drop (ABC30, 16 January 2026).
How the valley's air changes through the year
- Summer: ozone. Ozone forms when nitrogen oxides and volatile organic compounds react in sunlight, and it is most likely to reach unhealthy levels on hot, sunny days (EPA, updated 18 February 2026). Visalia, Bakersfield and Fresno all rank in the top five US cities for ozone in the Lung Association's report.
- Fire season: wildfire smoke. In July 2024, smoke from fires in the Sierra National Forest left haze across Fresno, and an inversion was expected to hold it near the ground (ABC30, 2 July 2024). In September 2025, a district air quality alert advised communities to stay indoors because of wildfire smoke (ABC30, 4 September 2025).
- Autumn: dust, as our numbers above show.
- Winter: fine particles and wood smoke, trapped by inversions and fog. We will check whether the mix shifts from PM10 to PM2.5 when we update our US ranking in January.
Check Before You Burn: the wood-burning season
Check Before You Burn is the Valley Air District's wood smoke programme (Rule 4901). The district's page says it is in effect from 1 November through the end of February, with a daily declaration for each county (Valley Air District, Residential Wood Smoke Reduction Program, archived 30 December 2024). The 2025–26 season, the programme's 23rd, started on Saturday 1 November 2025 (The Bakersfield Californian, 30 October 2025).
It covers every indoor and outdoor home device that burns wood, pellets or manufactured logs. Natural gas and propane devices are exempt.
| Daily status | What it means |
|---|---|
| No Burning | Burning any solid fuel is prohibited from midnight to midnight. Violations can result in fines. |
| No Burning Unless Registered | Only a clean-burning device registered with the district may be used, indoors. All unregistered devices are prohibited. |
| Burning Discouraged | Allowed, but burn only clean, seasoned, dry wood, never trash or plastics. |
- Exemptions. Homes without natural gas service, or where wood is the only heat, can request one.
- Fines. In 2025 a first violation cost $100, halved by attending the district's smoke school (The Bakersfield Californian, 30 October 2025).
- How to check. Call 1-800-SMOG-INFO (1-800-766-4463), use the district's Valley Air app, or see the Check Before You Burn page on valleyair.org.
We could not confirm the district's announcement for the 2026–27 season or any rule changes: valleyair.org did not respond to our requests on 22 October 2026. Check the district's page, app or phone line before you light a fire.
When is the air worst during the day?
In our hourly data for 12 to 21 October (Pacific time), Fresno's AQI averaged 73 from 10 pm to midnight and 74 at 1 pm, and was lowest at 6 pm (58). Hanford peaked at 80 at 11 am, driven by dust: its PM10 averaged 117 µg/m³ at noon against 75 at 6 pm. Bakersfield peaked at 72 at 9 am and was lowest at 5 am (53).
Fine particles peaked at night: at Fresno Foundry Park, PM2.5 averaged 17.9 µg/m³ at 1 am, about twice its 4 to 5 pm level (9.4). So in mid-October, late afternoon was the cleanest time in Fresno and Hanford. Nine days is a small sample; winter may differ.
What to do on bad air days in the valley
- Check first. Our Fresno, Hanford and Bakersfield pages show the live AQI and AirNow's daily forecast for tomorrow, issued by the Valley Air District. For Clovis and Visalia, use Fresno and Hanford, the nearest. See also the US ranking and California weather alerts.
- Limit time outdoors. For smoke, the EPA calls it the most effective protection (EPA, updated 24 July 2026). Move exercise to cleaner hours.
- N95 if you must be outside. The EPA says a properly fitting N95 or P100 respirator helps during long periods outdoors in smoky air; it must seal over nose and chin.
- Clean one room's air. Portable air cleaners filter a single room, and cutting indoor sources comes first (EPA, updated 3 August 2026). In 2024 the district also advised changing air conditioning and car cabin filters (ABC30, 2 July 2024). See our air purifier size guide.
- Don't add smoke. From November, check the burn status daily.
This is general advice; people with asthma or heart or lung disease should follow their doctor's plan.
How we measured
- Data. Hourly city AQI and monitor PM2.5 and PM10 from the EPA's AirNow system, collected by Good Air. Government monitors only.
- Cities. A day counts with at least 12 hourly readings; its value is their mean. Cities needed 31 of 36 days (266 qualified), 18 of 20 for 1 to 20 October and 6 of 7 for the last week. These are not EPA's official daily figures.
- Time. Days in UTC; 20 October is the last complete day. Hourly patterns use 00:00 UTC on 13 October to 18:00 UTC on 21 October, in Pacific time. Monitors needed 31 valid days.
Frequently asked questions
Why is Fresno air quality so bad?
Fresno sits in the San Joaquin Valley, a bowl hemmed in by mountains where inversions trap pollution near the ground. In autumn 2026 the main problem was particles: dust (PM10) and fine particles (PM2.5) set Fresno's AQI far more often than ozone. From 15 September to 20 October 2026, Fresno averaged a US AQI of 73, second worst of 266 US cities.
Is Fresno the most polluted city in the US?
Among the 266 US cities we tracked from 15 September to 20 October 2026, Hanford was the most polluted (average US AQI 75), with Fresno second (73) and Bakersfield third (70). Fresno ranked first from 1 to 20 October, at 77. In the American Lung Association's 2026 report, Bakersfield ranked worst for year-round particle pollution and Fresno 4th.
When is the air quality worst in Fresno?
Through the year, the valley sees ozone on hot, sunny days, wildfire smoke in fire season and fine particles in winter, when inversions, fog and wood smoke trap pollution. In our hourly data for mid-October 2026, Fresno's AQI was highest around 1 pm and from 10 pm to midnight, and lowest around 6 pm.
When does Check Before You Burn start in Fresno?
The Valley Air District's Check Before You Burn programme runs from 1 November through the end of February, with a daily wood-burning status for each county. As of 22 October 2026 we could not confirm the district's 2026–27 announcement, so check its website, the Valley Air app or 1-800-SMOG-INFO before lighting a fire.
Is it safe to exercise outside in Fresno?
On most autumn days in 2026, Fresno's air was Moderate, which AirNow says is acceptable for most people, though unusually sensitive people may be at some risk. Check the live AQI and forecast first. In mid-October 2026 the late afternoon was the cleanest time, and people with asthma or heart or lung disease should follow their doctor's advice.
