The EPA lowered the annual standard for particulate matter smaller than 2.5 microns (PM2.5) from 12 to 9 micrograms per cubic meter (µg/m³) of air. The move comes in the wake of dozens of studies that have shown how the tiny particles can travel deep inside the lungs and heart, increasing the risk of heart disease, lung cancer, and strokes.

However, based on current air quality monitoring data, much of California—and notably all of the most intensively farmed counties, including those that make up the San Joaquin Valley—will likely not meet the newly updated standard anytime soon.

The San Joaquin Valley Air District, which is home to massive tracts of almond, citrus, produce, and dairy operations, has yet to meet the 12 µg/m³ standard, which the EPA established in 2012. At a recent workshop held by the San Joaquin Valley Air District, a spokesperson for the district described how despite progress lowering PM2.5 levels, “initial modeling completed by CARB [the California Air Resources Board] suggests that attainment of the 2012 standard by 2025 is impracticable.” Instead, the spokesperson said that the district and CARB are revising a plan, and requesting a five-year delay, to reach the now outdated standard by 2030.

Community advocates expressed their continued frustration at the district’s failure to achieve clean air standards. “Back in 2018, we saw the aggregate commitments and weak rules come forward and warned that we would not meet the standard. We said, ‘We need to do more,’ and we were ignored. And here we are today,” said Genevieve Amsalem, research and policy director for the Central California Environmental Justice Network, at the workshop. She calls San Joaquin Valley’s failure to meet air quality standards a civil rights issue: “The people most impacted are more often low-income people of color.”

The San Joaquin Valley is the largest agricultural producing area in the nation; it produced  crops, livestock, and agricultural commodities worth $36.5 billion in 2022. The southern half of California’s 450-mile Central Valley is also home to some of the worst annual air pollution in the nation. Mountain ranges trap emissions from highway traffic, locomotives, municipal composting facilities, tractors, and burning. But agriculture’s full impact is difficult to assess using the data available to the public.

The PM2.5 levels in the valley have decreased in recent years, due largely to state-wide regulation of automobile and industrial emissions. When asked over email about the policy actions that have helped lower emissions, the San Joaquin Valley Air District pointed to “tougher regulations on various industrial sources such as boilers, industrial flares, glass melting furnaces, and engines.” But those changes have only gotten the region so far.

Since 1992, the air district has employed 670 rules to reduce air emissions in the valley, yet only a handful pertain to agriculture. “Ag is the sector that has gotten off the hook,” says Catherine Garupa, executive director of the Central Valley Air Quality Coalition.

As industrial and mobile sources of pollution decline, air quality advocates and members of the public are paying more attention to agricultural emissions, explains Mark Rose, the Sierra Nevada program manager for the National Parks Conservation Association, who has monitored the San Joaquin Valley’s air quality efforts for seven years.

Agriculture’s contribution to PM2.5 stems from burning, soil management, and gaseous emissions from both tractors and soil. While most of the focus has been on farm equipment, burning and soil management are also coming under increased scrutiny—especially as the San Joaquin Valley braces for a massive land transition.

An estimated 500,000 to 900,000 acres of irrigated farmland will likely be taken out of production to satisfy state-level groundwater laws by 2040. The worry is that fallowed farmland, especially in the eastern Central Valley where it is turning to desert, could generate more dust leading to more PM2.5 in the air.

“The most cost-effective way to prevent dust is to maintain [living plant] cover—which is difficult in areas that are desert,” says Andrew Ayres, an economist professor at the University of Nevada, Reno, and the co-author of a report on dust and air quality for the Public Policy Institute of California (PPIC).

According to the 2018 emission inventory, farm operations were estimated to generate 13 tons of direct PM2.5 emissions a day, or roughly one-third of the total direct emissions in the area.

But farming also contributes to PM2.5 in other ways. PM2.5 is also formed in the atmosphere when gases react to form tiny particles. Ammonium nitrate, one of the most widespread particle types formed in the atmosphere, contributes half of the daily PM2.5 in the San Joaquin Valley, according to the California Air Resources Board (CARB).

Ammonium nitrate forms when nitrogen oxides, a by-product of combustion from sources such as trucks and tractors, combines with atmospheric ammonia from fertilizers, which are abundant in the valley. But a growing number of studies, including international ones, suggest that soil microbes are also a significant source of nitrogen oxides, especially in intensively fertilized areas—which could hinder efforts to decrease PM2.5.

Farmers Struggle to Address Air Pollution

McIsaac and Roger Isom, president and CEO of the California Cotton Ginners and Growers Association and Western Agricultural Processors Association, anticipate that agriculture will be a target for additional regulations to achieve the stricter air quality standard.

“We have the most stringent air quality regulations of anybody in the country for ag,” says Isom, who has spent over a decade working with the state air board and local air district to find ways for growers to lower their contributions to PM2.5, both fugitive dust and nitrogen oxide emissions. While there may not be many rules aimed at agriculture, he says agricultural burn bans, conservation management, and dust mitigation plans, as well as incentives for cleaner truck and farm equipment replacement, have all had a significant impacts on farmers’ bottom lines.

Citing the difficulty of securing a burn permit in recent years, McIsaac’s operations have turned to chipping orchard residue, which is more expensive than burning. The practice reduces PM2.5 emissions and can build soil health when the resulting mulch is incorporated into soil.

A near-total burn ban goes into effect in 2025, and Mark Rose and Genevieve Amsalem consider it a significant step toward cleaner air. But, Rose says, if the air district hadn’t postponed adoption of the law for 20 years, the valley might have attained some of the previous air quality standards.

Incentives have been key for growers to find alternatives to burning, such as mulching the crop residue on or offsite, as well as replacing dirty vehicles. At least $760 million in state funds have supported growers who adopt new technology and approaches to farming that would result in less particulate matter in the air. Isom says the incentive approach is crucial to give growers flexibility and still achieve air quality gains.

“If it [was] mandatory, there would have been more farms going out of business,” he says  Since 2016, for example, over 12,000 tractors have been replaced with lower-emissions models via a voluntary cost-share program.

But Ansalem would like to see tractors face the same regulation as other vehicles in the state. “Agricultural tractors are a huge source of NOx in the San Joaquin Valley,” she says. “They remain one of the only mobile sources in the state not regulated by the California Air Resources Board.”

Land manager McIsaac also points to the fact that growers are required to have a dust management plan that details the steps they take to minimize dust. Many farmers avoid field work on windy days to the extent they can or water roads to tamp down dust. Their plans are described as mandatory, and subject to annual inspections every year if out of compliance, otherwise inspected every five years.

Rose and Amsalem, however, don’t think the current rules go far enough. “If you look at all the regulations that exist for agriculture, there’s basically a de facto policy in the state of California not to regulate agriculture as an industry,” Rose tells Civil Eats.

And as farmers determine which parcels of land must be taken out of production to satisfy groundwater laws, there are no guidelines for how to manage the dust on fallowed land. “There is a little bit more urgency in terms of evaluating what needs to be done with those lands,” says University of Nevada’s Ayres.

The San Joaquin Valley Air District is in the midst of a process to evaluate potential additions to the region’s Conservation Management Practices rule. First adopted in 2004, the rule promotes reduced tillage and other management practices on cultivated lands to reduce dust and improve soil health. The air district continues to evaluate the impact of those practices, but they currently estimate up to 5.5 tons of PM2.5 have been reduced per day.

Ayres anticipates the new EPA standard could light a fire under the effort to bring more conservation practices onto fallowed land. “If you are taking this ag land out of production, and don’t have a management requirement, that could be very negative moving forward,” he says. The goal should be to avoid the worst possible outcomes—such as patchwork fallowing with no dust management practices—and to figure out how to do that most cost effectively, he adds.

Isom says farmers are at the table to find further ways to cut emissions, but it’s going to be tough because the easiest cuts have already been made. “The low-hanging fruit is gone; now, we’re going after the hard stuff,” he says. “What ultimately comes out of [efforts to meet the new standard] in terms of new regulations or tighter regulations, we are certainly very scared,” he adds.

Farms’ Contribution

Complicating matters, pollution sources are shifting. “Air pollution in the 21st century is not what it was in 20th century,” says Ian Faloona, an atmospheric researcher at University of California at Davis.

For example, PM2.5 is increasingly formed in the atmosphere through what is called a secondary process when abundant gases such as nitrogen oxides and ammonia react to form tiny particles, notably ammonium nitrate. Reducing secondary PM2.5 requires a robust accounting of gas sources to determine how best to prevent this process.

Nitrogen oxides—nitric oxide (NO) and nitrogen dioxide (NO2)—are collectively known as NOx. Since NOx is in shorter supply than ammonia in the atmosphere, the air district has focused their efforts on reducing it as the most cost-effective way to minimize ammonium nitrate, which makes up half of the daily PM2.5 in the San Joaquin Valley.

In San Joaquin Valley, farm equipment and food processing contribute roughly 22 percent of NOx emissions, according to the air district’s 2018 emissions inventory. The bulk of NOx emissions are estimated to come from vehicles.

But NOx emissions from soil may also be higher than previously understood. In 2018, Faloona co-authored a study that found that agriculture’s contribution increased the NOx budget by 20 percent to 51 percent above what state agencies estimate. The researchers found that the existing models for soil NOx emissions underestimate emissions, “leading to a poor assessment of the relative roles of mobile and agriculture sources of NOx in the region.”

“Essentially, our study was a process model based on fertilizer applications, soil temperature, and moisture, while the air district based their numbers on extrapolations from a few soil measurements,” explains Faloona.

When the state conducted a follow-up study in 2020, their numbers agreed with the San Joaquin Valley Air District. Other studies, however, have also found that higher levels of soil NOx in the San Joaquin Valley and in intensively farmed areas in China and the European Union contribute to PM2.5.

If there is a larger pool of unaccounted-for NOx emissions in the San Joaquin Valley, it could stymie progress on efforts to curb PM2.5. Faloona’s 2023 paper, for example, found that the amount of NOx in the air has not declined in California rural areas in recent years.

Since 2018, Rose and Amsalem have been asking the air district to investigate whether nitrogen emissions from soil microbes—a potentially large source of agricultural emissions—is being overlooked.

CARB is in the process of establishing an expert review panel on nitrogenous emissions from soils. “In 2018, we asked them to look into soil NOx,” says Amsalem. “That didn’t happen until 2023, and we still don’t know who is on the expert panel.” CARB expects to finalize and announce the panel within the next month.

“Ag is a big piece of the [pollution] puzzle, but the state won’t even tell us how much agriculture contributes to the overall problem,” adds Rose.

While growers are wary of its implications, the new federal PM2.5 standard still isn’t as stringent as that of the World Health Organization, which lowered its air quality guidelines to 5 µg/m³ in 2021. And it’s not just people’s health that’s on the line—air pollution’s cost to taxpayers is also staggering. A 2021 study found that strengthening PM2.5 standards to 8-10 µg/m³ in California would provide public health benefits valued at $42–$149 billion.

And yet Isom questions whether the San Joaquin Valley can even achieve the 9 µg/m³ standard. “I don’t see us getting into attainment in my lifetime, but hopefully my kids will,” he says.

“We realize [farms are] part of the issue, and we’ve got to do our part,” adds Isom. “We just have to find ways to improve that without putting us out of business.”

Reporting for this piece was supported by the Nova Institute for Health.