The eight counties in the San Joaquin Valley experience some of the highest levels of air pollution in California, with levels of several pollutants far above the statewide average.

All eight counties rank within the top 10 among California’s 58 counties with the highest percentile scores for average pollution burden, according to the state’s Office of Environmental Health Hazard Assessment.

Coincidentally, the region also has a much lower percentage of battery-electric, plug-in hybrid and other zero-emission vehicles (ZEVs) on the road than the state as a whole. More than 97% of all on-road vehicles registered to owners in the Valley – and more than 99.9% of heavy-duty vehicles like trucks, buses and other large commercial vehicles – are powered by either gasoline, diesel or other fossil fuels that contribute to air pollution.

The correlation between air quality and zero-emission vehicle adoption, however, is simply coincidental, said Jaime Holt, the chief communication officer for the San Joaquin Valley Air Pollution Control District. The district oversees air-quality efforts for the eight-county region in collaboration with the California Air Resources Board and the U.S. Environmental Protection Agency. 

“There are a lot of different sources (of air pollution), but mobile sources make up the majority of it,” said Jon Klassen, director of air quality science for the Valley air district. 

And while the cars, SUVs, pickups and other light-duty passenger vehicles represent more than 95% of all of the vehicles registered in the Valley, they represent a relatively small part of the region’s air pollution concerns.

“It’s not really passenger vehicles that’s the biggest problem,” Klassen said. “It’s the heavy-duty vehicles that really is most of the pollution that we still have here in the region, and that can be heavy-duty trucks to heavy-duty ag equipment, construction equipment, locomotives, types of vehicles burning a ton of fuel.”

The types of air pollution most prevalent in the Valley, and their impact on the health of residents and workers, varies across the region’s counties.

  • Kern, Tulare, Fresno, Kings and Madera counties all rank within the top 10 counties for the highest burden from ozone, a component of smog, while Merced, Stanislaus and San Joaquin counties fall within the top half of the state’s counties.
  • Each of the Valley’s counties also rank high in measurements of fine particulate matter, often formed as a result of chemical reaction from pollutants emitted by automobiles, power plants and industries. Only Merced County falls outside the top 10 counties for particulates, but just barely, at No. 11 of 58 counties.
  • Not surprisingly seven of the San Joaquin Valley’s eight counties have higher prevalence of asthma, a chronic lung disease that is often triggered or exaggerated by air pollution, than all but 44 California counties.

Trucks and other heavy-duty vehicles account for about 70% of emissions of nitrogen oxides, or NOx, that contribute to ozone formation, Klassen said. That includes not only almost 150,000 trucks and other big vehicles registered within the Valley, but also thousands more big rigs that roll along Highway 99 and Interstate 5 through the Valley on a daily basis.

By comparison, the more than 3 million passenger vehicles in the Valley represent about 10% of the region’s NOx emissions. The remaining 20% comes from stationary sources like industrial plants or agricultural equipment.

The air district regulates pollution that comes from stationary sources, but has no authority over mobile sources including passenger vehicles, big rigs, buses and other heavy-duty vehicles. What the agency can do, however, is offer incentives for owners and drivers to switch to cleaner vehicles, whether those are ZEVs or newer vehicles that produce less air-pollution emissions than older models of cars or trucks.

Air pollution and passenger vehicles

ZEVs represent the cleanest option when it comes to air pollution, but because passenger vehicles don’t account for the bulk of emissions in the San Joaquin Valley, they’re not a top priority for the Valley air district, Holt said. And there are various reasons for that.

“Our focus has not necessarily been on getting people into zero-emission vehicles. It’s been getting people into cleaner vehicles, and part of that has to do with the age of vehicles in the Valley,” Holt said. 

The average ZEV costs significantly more than an equivalent vehicle powered by a traditional internal combustion engine, and paying the extra money can be a heavy lift for many Valley residents.

“The biggest issue when it comes to cleaning up the vehicle fleet in the Valley and getting people not only into EVs [electric vehicles] but into newer models is the socioeconomic issue,” Holt said.

The air district considers it a success when people get rid of older, less efficient cars into newer, cleaner-burning cars, whether powered by conventional gas- or diesel-burning engines or gasoline-hybrid engines. “And then we are starting to see folks pick up adoption of EVs,” Holt said.

Indeed, sales of new ZEVs have climbed across the region as a percentage of all new passenger vehicles sold and registered, from 3.5% in 2020 to almost 15% in 2024. Through September, ZEVs accounted for 15.3% of vehicles sold in 2025.

Since 2020, San Joaquin County has not only the most ZEVs in the Valley, at more than 23,600, but also the highest percentages each year, including topping 20% in 2023, 2024 and so far in 2025.

Kings County, by contrast, has had the fewest new ZEVs purchased and registered, fewer than 1,450. But the percentage that ZEVs represent among new vehicles sold has climbed faster than almost all of the Valley counties, from just 2% in 2020 to 10.3% this year. 

But federal tax credits that helped people offset some of the increased cost of electric vehicles expired at the end of September, said Todd DeYoung, director of grants and incentives for the Valley air district.

The district also had one of the last remaining rebate programs in the state aimed at promoting adoption of zero-emission vehicles, but high demand forced the agency to stop accepting applications in October, DeYoung said.

And while it’s not aimed solely at zero-emission vehicles, “we also operate a program that allows folks in certain instances, and based on income qualification and things like that, to get into cleaner vehicles,” DeYoung added. “Those vehicles that are just not worth repairing, …and you’ve got to just get those cars off the road.”

While the number of ZEVs operating continues to climb, albeit slowly, DeYoung said there are also issues of having enough places for people to be able to charge up their batteries.

Finding a place to plug in

For residents who own their own home, for example, it’s not uncommon for owners of battery-electric or plug-in hybrid vehicles to have a charging system installed in their garages. There are also a growing number of commercial companies with charging stations at shopping centers, convenience stores, and office complexes – if you know where to look.

“But one of the challenges we’ve had in the larger charging infrastructure is, let’s say you have a large apartment complex and you want to put in 20 electric vehicle chargers, getting your utility to have the juice that can charge all of those has been an issue in the Valley,” Holt said.

That’s a particular problem in more rural areas, DeYoung added. “The electrical grid in some areas is just not able to accommodate the infrastructure needs that we have,” he said, whether for passenger vehicles, heavy-duty trucks, or even agricultural equipment.

In some areas, farmers and ranchers desire to switch to electric vehicles, “but they’re being told by the utilities that it could take 10 years, if ever, to get electrical infrastructure out to some of these rural sites,” DeYoung said. “It’s an uphill battle.”

Much of the commercial truck traffic on Interstate 5 and Highway 99 are trucks passing through the Valley, rather than local, and the air district is working on developing larger-scale charging sites along those two freeway corridors. But the same concerns about grid capacity also affect those efforts, DeYoung said.

How about those big-rig trucks?

Big rigs and the emissions they generate have been a point of contention in some cities, including Fresno, when large-scale distribution centers have been proposed in areas near low-income neighborhoods concerned over the increased volume of truck traffic and air pollution. In some instances, such proposals have resulted in lawsuits for which settlements include measures to minimize the effects on those surrounding neighborhoods.

There are about 150,000 heavy-duty vehicles registered to owners in the Valley, and all but a few hundred are powered by diesel, gasoline or some other fossil fuel.

But although those big trucks, buses and others are outnumbered by more than 20-to-1 by the Valley’s more than 3 million passenger vehicles, they combine to generate about seven times more NOx emissions than their smaller light-duty cousins.

Because big-rig trucks and other heavy-duty vehicles cumulatively represent the largest share of air-pollution emissions in the Valley, adoption of new technology there can help make a difference, even if such progress seems painstakingly slow. And that includes electric big-rig trucks for commercial hauling of goods.

“Our region, I believe, has seen probably one of the largest adoptions of zero-emission trucks,” DeYoung said. “We’ve done a couple of different demonstration projects, and now they’re into deployment of Tesla trucks and other technology.”

“Up in the Modesto area, we did the Frito-Lay plant; they’re operating 50 or 60 zero-emission trucks up and down the Valley with tremendous success,” he added. “Down here in the Fresno area, the Pepsi bottling company is just in the deployment phase of a significant number of trucks. We’ve got a couple of other really large-scale projects, and so they’re taking hold.”

But two limiting factors are the electrical infrastructure for charging and vehicle range.

“With these trucks if you don’t have the infrastructure to support long-haul trucking, it’s just not going to work,” DeYoung said. “But what we’ve seen is that these regional routes, these return-to-base or 300- to 500-mile routes, are perfect for zero-emission trucks, and they’re really starting to take a foothold here in the Valley.”

Tim Sheehan is a senior reporter and Health Reporting Fellow with the nonprofit Central Valley Journalism Collaborative. The CVJC fellowship is supported by a grant from the Fresno State Institute for Media and Public Trust. Contact Sheehan at tim@cvlocaljournalism.org. CVJC research assistant Marisol Herrera contributed to this story.