California’s Policies to Reduce Air Pollution Are Working

California officials have gone to bat for their ability to regulate pollution from vehicles. When the Trump administration weakened auto emissions standards under the Clean Air Act, for instance, the state used its authority to impose stricter ones, and even successfully facing off against former President Donald Trump when he tried to stop it. A new report published in Science on Thursday shows that compared with the rest of the nation, California’s pollution controls have dramatically reduced emissions of diesel particulate matter, and argues the state policies should serve as a model for the rest of the country.

Diesel particulate matter is the toxic black exhaust that cars, trucks, buses, trains, ships, and other diesel-powered vehicles produce. It’s been shown to increase risk of cancer as well as cardiovascular and respiratory disease. Some bits of it are so small that they can also enter the bloodstream when we breathe them in, rendering them even more dangerous. This pollution disproportionately affects low-income communities of color.

California is the largest emitter of diesel particulate matter in the U.S., which is unsurprising since it’s home to more cars than any other state as well as the country’s two largest ports. But over the past 30 years, the state has also imposed a number of policies to reduce that pollution, including continually tightening regulations on diesel engines, making efforts to switch to electric-powered ships, and switching over to using a lower-sulfur form of diesel fuel which emits less fine particulate matter when burned.

To analyze the effect of all of these state policies, the study authors looked at Environmental Protection Agency data on diesel particulate matter emissions from 1990 to 2014, comparing emissions in California with those from the rest of the U.S. The results suggest that the regulations are absolutely making a difference.

“California has also led the nation with the largest overall reduction in metric tons of diesel particulate matter emissions from mobile sources,” the study says.

The authors clocked a 78% reduction overall in California’s diesel particulate pollution. In that 24-year period, the country at large saw just a 51% decrease. The biggest portion of California’s reduction, the study found, came from tractor trailers, but emissions from the construction sector, marine vehicles, and passenger cars also fell.

The researchers also found that thanks to the improved air quality, there were far fewer cardiopulmonary deaths. Deaths from heart and lung disease linked to diesel pollution fell by 82% statewide.

Interestingly, California saw this major decrease even though its use of diesel fuel steadily rose over the period examined. That suggests that the state’s mandates to move to cleaner fuels and retrofits of existing marine and auto engines with pollution filters made a big difference. This could have big implications for the rest of the country.

“In the rest of the U.S., older, dirtier engines can operate without upgrades,” Megan Schwarzman, a physician and environmental health scientist at the University of California, Berkeley who was the lead author on the study, said in an email. “Since diesel engines last about 20 years, waiting for fleet turnover is just too slow. A federal requirement to update the oldest, dirtiest diesel engines would have an enormous impact, particularly for the lower income communities of color that are disproportionately exposed to emissions from truck traffic.”

None of this means that California doesn’t still have a tremendous amount of work to do. In particular, the study notes the state has struggled to lower emissions from tractors and other vehicles used in the agricultural sector, which accounts for 18% of the state’s total diesel pollution. Yet while other industry’s saw big drops over the study period, the agriculture sector was “less than 1% of the total emissions reductions in California between 1990 and 2014,” the study says.

The moves toward cleaner fuels and pollution filters are also imperfect solutions. Though they do effectively lower levels of toxic and planet-warming air pollution, they do not wipe it out altogether. California—and the country—could stand to make some more systemic changes to move toward electric vehicles and also boost its public transportation infrastructure. Luckily, there’s a new plan for that.