A study using 24 years of emissions data confirms that California is leading the nation in reducing diesel particulate matter emissions — and the researchers are urging the U.S. Environmental Protection Agency to look west for a policy model.

The EPA is initiating a new round of rulemaking for the proposed Cleaner Trucks Initiative, which currently focuses only on emissions of nitrogen oxides. The authors of this study, published Thursday, believe diesel particulate matter, or DPM — which increases the risk of heart failure, asthma, lung disease and cancer — should be included in the new regulations, and that robustly regulating DPM at the federal level would save lives. 

Looking strictly at deaths from cardiopulmonary disease — which has the most easily quantifiable link to DPM — cross-referenced with county air quality levels, researchers concluded that 597 Californians died of those diseases in 2014 as a result of exposure to diesel particulates. Had the state been on the same trajectory as the rest of the country, 1,339 people — more than twice as many — would have died. 

"If EPA is looking for more effective rules than the ones they have, we have a blueprint right here, and we have a 30-year track record of demonstrating that it works," said Megan Schwarzman, associate director of the Berkeley Center for Green Chemistry and the lead author of the study. The authors plan to send a copy of the study, published March 24 in Science, to Michael Regan, who was sworn in earlier this month as the administrator of the EPA. 

DPM is emitted by many heavy-duty and medium-duty trucks, as well as buses, ships, heavy farm and construction equipment and other diesel-powered engines. It causes disease and illness, elevating the risk of cardiovascular, cardiopulmonary and respiratory disease and lung cancer; and it contributes to, among other things, high rates of asthma in children growing up in polluted neighborhoods near congested freeways and ports. 

In contrast with federal rules, California requires retrofit or replacement of old diesel engines in all heavy-duty diesel vehicles to comply with air quality rules; federal rules let old trucks run until they stop working, which the researchers say is worrisome because a typical truck lifespan is about 20 years or one million miles. California also has strict regulations on sulfur content in diesel fuels and particle filter requirements, among other rules.

The researchers found that from 1990 to 2014 — the last year before the EPA changed the way it reports the data — California reduced its DPM emissions by 78% while the rest of the country only reduced its emissions by 51%. If California had been doing business under the same regulations as the rest of the country, the state's emissions would have only fallen 58%, the researchers calculated. 

Overall, most of the emissions reductions were from heavy-duty diesel vehicles — trucks and buses. Thus, the "51% reduction" is an overstatement of the rest of the country's improvement, since the records do not capture the cross-country influence of California, which moves 60% of the nation's container cargo. The researchers also note that the regulations cleaned up the state's air and improved residents' health even as California's economy was growing at double the average national rate. 

State-level data as well as air quality studies in affected neighborhoods show that DPM pollution decreased significantly, and the declines were especially dramatic in the most affected neighborhoods. 

The most affected neighborhoods, Schwarzman said, are disproportionately low-income communities of color affected by redlining, the type of American segregation that systematically restricts where people of color, in particular Black people, can live and buy homes. 

"In cities across the country, redlined areas were where highways were built," she said, while higher-income white neighborhoods "got trees and parks. Just that practice of dividing cities created these profound disparities along racial lines that exist to this day." 

California has the two largest ports in the U.S., and neighborhoods near ports may be some of the most affected by pollution, Schwarzman explained. "As a truck gets older, it starts being assigned to shorter and shorter runs, until the oldest trucks are used as what's called drayage trucks, and those are the short-haul trucks that just go to the ports, receive a container off a ship, and take it to a nearby warehouse," she said. "And those are some of the highest-polluting trucks — the ones that are crisscrossing the neighborhood."

A study published last year in Lancet Planet Health — co-authored by one of the authors of this study, John Balmes — found that among 1,431 census tracts in eight California cities, historically redlined neighborhoods had 2.4 times the age-adjusted rates of emergency room visits for asthma than non-redlined neighborhoods. The mean DPM level for redlined neighborhoods was 1.8 times higher than non-redlined neighborhoods.

But California set itself apart from the rest of the nation by targeting the dirtiest trucks first, getting those engines off the road by requiring retrofit or replacement. 

The eventual aim should be for zero DPM emissions, which are inherently harmful to people who breathe them in, Schwarzman said. In the interim, stricter DPM emission restrictions, she said, would not only improve Americans' health, but also advance racial justice. 

The study, "Raising standards to lower diesel emissions," published in Science on March 25, was authored by Megan Schwarzman and Samantha Schildroth, University of California, Berkeley; May Bhetraratana and Álvaro Alvarado, California Air Resources Board; and John Balmes, University of California, Berkeley and University of California, San Francisco.

Correction: A previous version of this article misspelled Megan Schwarzman's name and misstated the asthma rates in the Lancet Planet Health study. The errors have been corrected.