Photo: Eric Sonstroem/Flickr

Air Pollution from Vehicles in California: People of Color Bear the Biggest Burden

, Senior vehicles engineer | February 5, 2019, 12:00 pm EST
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Cars, trucks, and buses are a significant source of air pollution in California. But how much pollution is attributable to these vehicles and who is exposed to this pollution? To help answer these questions, I’ve used a computer model to estimate the amount of fine particulate matter air pollution (known as PM2.5) created by using on-road vehicles (cars, trucks, and buses). The findings are troubling, both because they show that people of color are exposed to higher levels of harmful air pollution and because this result is likely not to be a surprise to many Californians (full report available in English and Spanish). The study supports the claims many have been making for decades – that on average, African American, Latino, and Asian Californians are exposed to more PM2.5 pollution from cars, trucks, and buses than white Californians. In fact, these groups are exposed to PM2.5 pollution 43, 39, and 21 percent higher, respectively, than white Californians.

What is PM2.5 and why is it important?

Petroleum-powered cars, trucks, and buses produce emissions that lead to harmful air pollution.

Exposure to PM2.5 (particulate matter smaller than 2.5 micrometers in diameter) is linked to increased illness and death, primarily from heart and lung diseases. These particles are small —20 times smaller than the diameter of fine human hair— so they can penetrate deeply into the lungs, and the smallest particles can even enter into the bloodstream. While PM2.5 is not the only air pollutant that adversely affects health, it is estimated to be responsible for approximately 95 percent of the global public health impacts from air pollution. Long-term exposure to PM2.5 causes increased death rates attributed to cardiovascular diseases, including heart attacks, and has been linked to other adverse impacts such as lung cancer. Chronic exposure to PM2.5 in children has also been linked to slowed lung-function growth, development of asthma, and other negative health impacts.

On-road vehicles like cars, trucks, and buses are a significant source of harmful emissions in California. The burning of fossil fuels such as gasoline and diesel has multiple negative effects: it produces climate-changing emissions such as carbon dioxide and pollution that reduces air quality. PM2.5 pollution is of particular concern in California, as the state has seven of the 10 most polluted US cities in terms of PM2.5 pollution.

Greater PM2.5 pollution for Latinos and African Americans, low-income households

We estimated exposure to particulate matter air pollution using a recently developed model from the University of Washington and data from the US Census Bureau. This model lets us calculate how vehicle tailpipe and refueling emissions ultimately lead to ground-level pollution exposure so we can understand how exposure to PM2.5 varies among groups and locations.

The results are clear: PM2.5 pollution burden from cars, trucks, and buses is inequitable when looking at the exposure experienced by racial groups in California. Latinos are, on average, exposed to 15 percent higher PM2.5 concentrations than the average Californian, and African Americans in California experience concentrations 18 percent higher than average. White Californians have average exposure that is 17 percent lower than the average for the state. This means that, on average, African American and Latino Californians are exposed to PM2.5 pollution that is 43 and 39 percent higher, respectively, than white Californians.

African American and Latino Californians are exposed to higher than average levels of particulate matter pollution from cars, trucks, and buses

Unequal pollution burdens can also be seen at the community level. In census tracts with average annual PM2.5 concentrations less than half the state average, whites make up 48 percent of the population, while only constituting 38 percent of the state’s total population. In contrast, the most polluted census tracts have a higher proportion of people of color. More than 60 percent of people in these highest burden areas are Latino, compared with a state population that is just 39 percent Latino. The inequities and disparities are clear.

Communities with higher percentages of white population have less exposure to particulate matter from cars, trucks, and buses.

Our research also links inequitable disparities in household income to pollution exposure, with less affluent households having higher exposure to PM2.5 pollution from on-road transportation. On average, households with the lowest incomes (less than $20,000 per year) are exposed to more than 25 percent more particulate matter air pollution than the highest-income households (greater than $200,000 per year).

PM2.5 exposure from cars and trucks varies greatly within California

Click to view interactive map.

Los Angeles County has the highest average PM2.5 pollution exposure from cars and trucks in the state: on average, 60 percent higher than the mean value for the state. One quarter of the population in Los Angeles County experiences pollution levels that are more than double the state average. And because Los Angeles County is the most populous in the state, this higher level of pollution affects millions of people. Only six counties have an average exposure from on-road transportation that is greater than the state average, but four of them (Los Angeles, Orange, San Bernardino, and San Diego) are in the top five most populous counties in California, with a combined population of almost 19 million people.

Other areas, such as the San Francisco Bay Area, have zones of higher pollution but have much lower average exposure to vehicle-related particulate pollution compared with the state average. The worst regions of the Bay Area (such as downtown Oakland and San Jose) have annual average PM2.5 concentrations equal to the average across Los Angeles County.

Opportunities to reduce harmful impacts of vehicle use

Particulate matter air pollution from on-road transportation places significant health burdens on Californians, and those burdens are inequitably distributed. However, there are opportunities to greatly reduce the exposure to PM2.5 by reducing tailpipe and refueling emissions, making much of this burden avoidable.

Electrification of vehicles, both passenger and freight, could greatly reduce emissions. Battery-electric and hydrogen fuel cell vehicles in particular have no tailpipe emissions (however, there are minor amounts of PM2.5 emissions from tire and brake wear that all vehicles produce) and completely avoid the need for, and emissions associated with, gasoline refueling. Electricity generation and hydrogen production can produce emissions; however, California has renewable content standards for both hydrogen for transportation and electricity that will limit additional emissions.

While Californians can make a difference by buying cleaner vehicles, much of the pollution comes from sources outside an individual’s direct control, like heavy-duty trucks and buses. The state needs to continue to move forward on regulations, incentives, and other policies to reduce vehicle emissions. Equity and meaningful involvement of disadvantaged communities should be key considerations in designing policies and strategies to reduce pollution from vehicles. The state will need to continue to make progress on reducing emissions and should prioritize actions that reduce the inequitably distributed burden of air pollution in California. Programs like the Enhanced Fleet Modernization Program (incentives to help retire older, polluting cars) and low-income clean vehicle rebates are examples of ways the state can help, but clearly more can and should be done to address the problem of harmful air pollution in California.

Photo: Eric Sonstroem/Flickr
Photo: Jimmy O'Dea

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  • MarkRocchio

    There is a reason this so-called study was only self-published without peer review. A cursory look at the data, methodology and analysis shows a severely flawed study. Air and therefore air pollution is not stagnant and atmospheric PM vary depending upon climatic conditions, wind and season. When was the data collected? Over what time period? So many other unanswered questions… I can speak about the Greater Los Angeles area with its half-dozen micro climates let alone its dense and diverse population. Temperatures can vary 20c on any given 24 hour period. Did you taken into consideration the Koppen Climate Classifications? I understand your need for funding… so you put out a racially charged news release hoping to get some press and donations. Shameful.

    • Perhaps you should take more than ‘a cursory look’ at our work before making these claims.
      As clearly discussed in our methods ( , we used a peer-reviewed model ( that represents the chemical and physical mechanisms that lead to PM2.5 formation and transport in the atmosphere.
      The model also includes detailed geographic and weather conditions, including the unique variables that contribute to high pollution levels in Los Angeles. We did not consider the Koppen Climate Classifications as they are much too broad a measure of climatic variables for this type of detailed modeling.

  • I think you are not acknowledging the full impact of non-tailpipe vehicle PM emissions, which include PM 10 as well as fine particulates, in your treatment of electric vehicles:
    “Electrification of vehicles, both passenger and freight, could greatly reduce emissions. Battery-electric and hydrogen fuel cell vehicles in particular have no tailpipe emissions (however, there are minor amounts of PM2.5 emissions from tire and brake wear that all vehicles produce) and completely avoid the need for, and emissions associated with, gasoline refueling.”

    You didn’t acknowledge road dust (though you do acknowledge elsewhere in your excellent report).
    There have been several studies that conclude that “Electric vehicle PM emissions are comparable to those of conventional vehicles.”

    • Thanks for the comment.
      For gasoline cars in particular, primary PM2.5 emissions (both tailpipe and wear/dust) are only a fraction of the total PM attributable to vehicle use. Much of the total PM2.5 from vehicles is secondary PM2.5, formed in the atmosphere by reactions involving volatile organic compounds, nitrogen oxides, and other pollutants.

      Electric vehicles eliminate gasoline fuel vapor emissions and combustion emissions that lead to secondary PM2.5 formation, in addition to the elimination of primary PM2.5 exhaust emission. It appears that the source cited does not consider the full air quality impact of switching from gasoline & diesel combustion to electric motors, as it only addresses primary emissions.

      • CCM591

        Thank you for the report and the work you put into producing it.

        Regarding PM2.5 contribution from EV and ICEV, a recent study (Beltran et al., “When the Background Matters: Using Scenarios from Integrated Assessment Models in Prospective Life Cycle Assessment.” Journal of Industrial Ecology, November 2018 (open access)) shows that, in Europe at least, “Particulate matter formation” is actually higher for EV than ICEV (Figure 4 on page 10). The PM formation is expressed in “kg PM10 equivalent / vkm.” I don’t see where “Particulate matter formation” is explicitly defined, but presumably it includes both primary and secondary particle formation. The ICEV in this case is a small Euro5 diesel car (“Case Study” on page 7), presumably Euro5b which would effectively require a particulate filter (DPF).

        Based on the graphic in Figure 4, PM formation from EV will remain higher than diesel ICEV until at least 2035, and even then only if the electricity grid mix corresponds to the IPCC RCP scenario 2.6, which is the most aggressive carbon reduction scenario of all of the IPCC scenarios (IIRC).

        In this study, only the electricity generation mix was changed in the prospective LCA model runs – vehicle technologies were not changed, if I understand the paper correctly.

        I agree that much of the ambient PM2.5 is now secondary PM, but even then, the Beltran study does not show a benefit for EV (with respect to PM10 at least).

      • I don’t believe that the calculations from the Beltran et al study include secondary PM formation. They used the ecoinvent database for lifecycle emissions and the IMAGE model for scenario generation and it appears that both tools calculate the rates of primary PM emissions. Because secondary PM is a large fraction of total PM from vehicles in California, the results of the scenarios in the Beltran study are likely not directly comparable to our results.
        Also, our results are for PM2.5 and not PM10. While both classes of particles have negative health impacts, PM2.5 is generally considered worse.

        However, the most important difference in the studies is that we are looking at the spatial distribution of both PM pollution and the population. The exposure to PM2.5 pollution is the key metric we investigated and this requires modeling both pollutant formation and the transport of PM in the atmosphere. The Beltran study does not measure exposure, it looks at total emissions rates.

      • CCM591

        Thank you for your response. Just to be clear, I was not criticizing your study, just commenting on your reply to a previous comment.

        I don’t see how it’s possible to have significantly higher primary PM emissions from EV than diesel ICEV, which is what is shown in the Beltran paper. I’ve seen several studies that conclude that primary PM emissions from EVs are nearly as high as primary PM emissions from ICEVs, including the Timmers study, but I haven’t seen any that conclude that primary PM emissions are higher, in spite of the generally heavier weight (mass) of EVs relative to comparable ICEVs.

        Furthermore, ~0.0005 kg PM10 eq / vkm is ~0.5 g/km, which is two orders of magnitude higher than the Euro5 “type approval” for diesel ICEV PM mass emissions (0.005 g/km). Is there any other explanation besides the “particulate matter formation” includes secondary PM?