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California has long had more cars on the road than any other state. As its population exploded in the first half of the 20th century, so did the number of drivers, particularly in Los Angeles. By the 1940s, exhaust from millions of cars, fumes from power plants and a booming oil industry shrouded the famously sunny city in a noxious brown haze that left Angelenos wearing gas masks on days they couldn’t see more than three blocks.

A chemist identified automobile exhaust as the major source of the smog that regularly darkened city skies, laying the groundwork for California to pass the nation’s first tailpipe emissions standards in 1966. 

The state has continued to implement the most aggressive air pollution policies in the country. But even as they cut exposure to one of the deadliest components in vehicle exhaust by nearly three-fold statewide over two decades, exposure disparities persisted or increased for people of color and residents of overburdened communities, a new study reports.

California environmental and climate policy has long focused on reducing air pollution for everybody because that clearly has big health benefits, said Joshua Apte, an air quality engineering expert at the University of California, Berkeley, who led the study, published in the peer-reviewed journal Science Advances. Apte and his colleagues wanted to know if state policies designed to address climate change and improve public health in California also reduced air pollution exposure disparities. 

The team focused on pollution from vehicles, the largest source of greenhouse gases in California and the primary source of fine particulate matter, or PM2.5, which kills an average of 5,400 residents a year, according to the California Air Resources Board, or CARB. Vehicles release PM2.5 directly from tailpipes and indirectly when byproducts of gasoline combustion form particles through chemical reactions in the atmosphere.

To track the disparate exposures to PM2.5 across a state with nearly 36 million registered vehicles, Apte forged a unique partnership with two agencies under California’s Environmental Protection Agency, CARB and the Office of Environmental Health Hazard Assessment, or OEHHA. CARB provided estimates of mobile emissions by year and vehicle type from 2000 to 2019 at a fine geographic scale. Three scientists from OEHHA, which funded the work, collaborated on the study design and data analysis.

The team used CARB models to track both direct particle emissions and the gases that form atmospheric particle pollution. To understand how cars, light trucks and heavy-duty vehicles contribute to PM2.5 exposures across the landscape, they created a user-friendly tool called ECHO-AIR.

California’s aggressive policies to control vehicle emissions reaped across-the-board benefits, the team found, reducing PM2.5 emissions by 65 percent. Groups that have historically lived near the worst PM2.5 pollution saw the biggest declines in absolute terms, Apte said. But as exposures continued to drop for white residents, they held steady or increased for Hispanic, Black and Asian Californians and for residents of “overburdened communities,” where people are disproportionately affected by hazardous pollutants.

The 65 percent decrease from the transportation sector is a “big public health win,” said Alvaro Alvarado, chief of OEHHA’s Community and Environmental Epidemiology Research Branch and a study coauthor. “But the challenge remains that the most polluted are still the most polluted.”

A Silent Killer

Exposure to PM2.5 increases the risk of heart attacks, asthma and other respiratory problems and mounting evidence shows that even short-term exposure can kill. Health experts call air pollution a silent killer because particles like PM2.5 are invisible to the naked eye. Recent research found that Black Americans accounted for the highest proportion of deaths from exposure to PM2.5, due to multiple factors that make them more susceptible, including poverty and lack of access to healthcare.

Vehicle emissions tend to be disproportionately concentrated in overburdened communities, and communities that are predominantly Black, Hispanic and Asian, Apte said. These groups are more likely to live near major roads and traffic arteries around the state, sandwiched between freeways in cities like Oakland and Los Angeles or next to long-haul trucking routes that serve agriculture and the oil industry in the Central Valley.

“This new research project is an important contribution to understanding the disparate and disproportionate impacts that low-income people, people of color, disadvantaged communities or environmental justice communities are confronting in their everyday lives,” said Michael Méndez, an expert on environmental justice and policy at UC Irvine who was not involved in the study.

Although California has consistently been a global leader in air quality and climate policy, many of those policies are geographically neutral, said Méndez, author of “Climate Change from the Streets.” What’s needed, he said, “is a more contextual understanding of the historical impacts that those particular pollutants and the sources of that pollution have had on local communities.”

A history of discriminatory policies often pushed people of color into marginal living conditions. The federal interstate highway system bulldozed homes and buildings in Black and Brown neighborhoods through the ’60s and ’70s while racially discriminatory housing and lending policies confined many to communities hemmed in by polluting freeways and industries.

CARB spokesperson Amy MacPherson said the new study highlights the need for approaches that focus on communities most impacted by exposure to air pollution. It’s a challenge the state is already addressing with equity-driven solutions that focus on these communities, she said, pointing to the A.B. 617 Community Air Protection Program.

The A.B. 617 program started just two years before the end of the study, which found that exposure disparities for the nearly 3 million people living in these overburdened communities were more than three times as large as those endured by Hispanics, the most exposed racial-ethnic group. Communities designated as “disadvantaged,” based on multiple environmental, socioeconomic and public health indicators, fared even worse.

Marginalized communities disproportionately host other sources of PM2.5, including power plants, chemical manufacturers and refineries, said Rachel Morello-Frosch, an environmental health expert at UC Berkeley who collaborates with Apte but did not contribute to the study. “Those kinds of stationary sources also emit PM2.5 and in California tend to be disproportionately located in communities of color and poor communities.”

These communities typically experience multiple stressors that compound environmental harms, including higher rates of poverty, food insecurity and less of the green space that reduces pollution burdens, Morello-Frosch said. “Such neighborhood and place-based factors create stressors that can enhance their vulnerability to the adverse outcomes associated with PM2.5,” she said.

The new study can help policymakers find ways to improve people’s health by showing which vehicles are causing the highest pollution burdens in which places, Morello-Frosch said. One way to address persistent disparities would be to reroute truck traffic or accelerate the adoption of zero-emission vehicles in the most polluted communities.

For Méndez, the study underscores the need to address systemic inequalities that have left communities of color overburdened with environmental pollution for decades. Regulators need to identify specific pollution hotspots where they can increase enforcement and target more resources to mitigate some of these impacts, he said.

California is moving toward a zero-emissions future that will help reduce disparities in air pollution exposure, said CARB’s MacPherson. She pointed to incentives to make carbon-free vehicles more accessible to low-income communities and to help residents replace polluting vehicles with zero-emission models.

“When it comes to exposure and especially disparity, place matters,” Apte said. His team’s ECHO-AIR tool should allow community members to simulate how different emissions policy scenarios might play out in their neighborhoods. He hopes the tool gets into “as many hands as possible” to help drive emissions reductions in the places that need it most. 

Ultimately, Apte said, environmental policy in California and across the country must target persistent exposure disparities to ensure that everyone benefits from clean air.