Who Breathes the Dirtiest Air from Vehicles in Colorado?

, Senior vehicles engineer | July 16, 2019, 8:29 pm EDT
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This post was written in collaboration with David Reichmuth

Most people know that cars, trucks, and buses from our highways and city streets are a significant source of air pollution. While this pollution impacts all communities in the state to some degree, Coloradans who face the greatest exposure to transportation pollution are those who live near highways, along major freight corridors, and in urban areas.

To help understand exactly which communities bear the greatest burden and breathe the highest concentrations of this dangerous air pollution, we used a computer model to estimate the amount of fine particulate matter air pollution (known as PM2.5) produced by on-road vehicles that burn gasoline and diesel. The findings, which are not likely to be a surprise to many residents, are quite troubling—they show that people of color are disproportionally exposed to vehicular PM2.5. For example:

  • African Americans are exposed to 64 percent higher PM2.5 concentrations from on-road transportation than the average PM2.5 exposure for all Coloradans. Asian Americans and Latinos experience concentrations 24 percent and 15 percent higher, respectively, than the average resident. At the same time, white residents have an average exposure that is 9 percent lower than the average for the state.
  • African American, Asian and Latino residents are exposed to vehicular PM2.5 pollution levels, on average, that are 81, 37, and 27 percent higher, respectively, than the exposure experienced by white residents.
  • A higher percentage of white residents than the state average live in the cleanest areas: white residents make up 76 percent of the people who live in census tracts where exposure is less than the state average, yet white residents make up just 69 percent of the state’s population.

What is PM2.5 and why is it so important?

The science is clear: no level of particulate matter is safe to breathe, says the American Lung Association. Although fine particulate matter—referred to as 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. Exposure to this dangerous pollutant is the largest environmental risk factor in the United States, responsible for 63 percent of deaths from environmental causes.

They include particles smaller than 2.5 millionths of a meter in diameter—at least 20 times smaller than the diameter of fine human hair—so they can penetrate deeply into the lungs. The ultrafine particles – smaller than 0.1 millionths of a meter – are particularly dangerous, as some can enter into the bloodstream.

Chronic exposure to PM2.5 causes increased death rates attributed to cardiovascular diseases, including heart attacks and strokes, and has been linked to other adverse impacts such as lung cancer, reproductive and developmental harm and even diabetes and dementia. Chronic exposure to PM2.5 in children has also been linked to slowed lung-function growth and the development of asthma.

PM2.5 is formed in many ways. A significant source of PM2.5 is fuel combustion. The combustion engines of cars burn gasoline and diesel. Power plants burn natural gas and other fuels to produce electricity. Burning wood for cooking and in residential fireplaces, as well as wildfires, are examples of biofuel combustion. To make things worse, not only does burning fossil fuels and biofuels produce PM2.5 directly, but the combustion reaction also emits gases such as nitrogen oxides, sulfur dioxide and volatile organic compounds that go on to form additional PM2.5 through complex chemical reactions in the atmosphere.

Because there are so many ways in which particulate matter is formed, you may ask yourself if some pose more health risks than others. Indeed particles can bind with bacteria, pollen, heavy metals, elemental carbon, dust and other building blocks, and so have a broad range of effects on human health. But size is one of the most important factors, and PM2.5 is responsible for a very heavy burden of disease, disability and death. That is why we focused our analysis on this pollutant.

Greater pollution for people of color

The results are clear: PM2.5 pollution burden from cars, trucks, and buses is inequitably distributed when looking at the exposure experienced by racial and ethnic groups in the state. People of color experience an undeniable “pollution disadvantage”.

We estimated exposure to PM2.5 pollution using a recently developed model from the University of Washington, and data from the EPA’s National Emissions Inventory and the US Census Bureau. This model allows us to 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.


 In an equitable world, one might expect that every area with the same pollution level would have an approximately equal representation of all racial groups. In other words, the burden would be shared equally. But only 11 percent of all white residents in the state live in the dirtiest census tracts, where pollution is more than twice the state average, while 38% of all African American residents live in these polluted areas.

We can look at this data in a different way. In the cleanest areas of Colorado—in census tracts with average annual PM2.5 concentrations less than half the state average—whites make up 76 percent of the population, while constituting only 69 percent of the state’s total population. In contrast, the most polluted census tracts have a higher proportion of people of color. Almost 12 percent of people in the highest burden areas, where concentrations are more than 2.5 times the state average, are African American, compared with a state population that is just 4 percent African American (Figure 2). The inequities are clear.


This chart shows the PM2.5 exposure in groups of census tracts, defined relative to state average. In areas where PM2.5 exposure is low, the fraction of white residents is high. As the analysis looks at more polluted areas, this fraction decreases. In the highest pollution areas, which correspond to urban centers with heavy traffic, the fraction of white residents is higher. Notes: Each column refers to census tracts in areas with similar PM2.5 pollution concentrations. The columns show the fraction of people belonging to each of eight racial groups living in those areas. The least polluted areas are on the left and the most polluted on the right. The 0–50% area refers to census tracts where PM2.5 pollution is less than half the state average, the 50–100% area refers to tracts where pollution is from half the state average to the state average, etc. The column at the far right shows the state’s racial composition.

Furthermore, PM2.5 exposure varies greatly within Colorado. People in the urban areas of the state, like Denver County, are exposed to vehicle pollution at levels similar to Los Angeles County in California. Denver County, the second most populous county in the state, and the most polluted, has average PM2.5 exposure from vehicles 237 percent higher than the state average.

Finally, the analysis also shows that exposure inequities are more pronounced between racial and ethnic groups than between income groups.

What is to be done?

Clearly air pollution from on-road transportation such as diesel and gasoline vehicles places significant, inequitable and unacceptable health burdens on Coloradans. This inequity reflects decades of local, state, regional, and national decisions about transportation, housing, and land use. Decisions concerning where to construct highways, where to invest in public transportation, and where to build housing have all contributed to a transportation system that concentrates emissions in communities of color. In many cases, transportation policies have left those communities with inadequate access to public transportation.

We have the tools and the technologies to transform our transportation system away from diesel and gasoline and toward clean, modern, and equitable solutions. Electrification of vehicles, both passenger and freight, could greatly reduce emissions. Battery-electric and fuel cell vehicles have no tailpipe emissions, with the exception of minor amounts of PM2.5 emissions from tire and brake wear. Not just that, but these vehicles eliminate vapor emissions associated with gasoline refueling.

Electric vehicles  can result in some additional climate emissions (carbon dioxide) from electricity generation, but these emissions are lower for an electric vehicle than for an average gasoline car, and vary depending on the location where the vehicle is charged. Seventy-five percent of people in the US live in places where driving on electricity is cleaner than a 50 mile per gallon car. It is very good news for Coloradans that Governor Polis has pledged for 100 percent renewable energy in the state’s electric grid by 2040.  By the way, Colorado ranks fourth in the US for solar potential, and eleventh for wind potential.

While residents can make a difference for local air pollution (as well as for climate emissions) by choosing cleaner vehicles and driving less, much of today’s air pollution comes from sources outside the direct control of individuals. Colorado needs regulations, incentives, and other policies to reduce vehicle emissions, with equity and the meaningful involvement of affected communities as key considerations in designing policies and strategies to reduce pollution from vehicles.

Last year, the state approved a low emission vehicle standard (LEV) for passenger cars and light trucks, which means Colorado will continue to sell vehicles that are progressively cleaner and more fuel efficient. Colorado is also considering adopting the California Zero Emission Vehicle (ZEV) standard, which would require automakers to increase the percentage of ZEV vehicles sold in the state. This state leadership is especially critical at a time when the federal government has proposed rolling back federal fuel economy and greenhouse gas standards. Furthermore, the Colorado legislature recently approved the extension of the income tax credit for purchase or lease of electric vehicles until 2025.

Other specific investments that could reduce inequities in air pollution include:

  • Investments in electric transit buses and school buses, with a priority on serving communities exposed to the highest levels of gasoline and diesel emissions
  • Expansion of electric vehicle rebate programs to provide financing assistance and larger rebates to low- and moderate-income residents
  • Utility investments in electric vehicle charging infrastructure, with a priority on serving communities exposed to the highest levels of gasoline and diesel emissions

Colorado has made much progress in reducing air pollution from vehicles, but it needs to continue this effort, placing a high priority on actions that reduce the inequitably distributed burden of air pollution in the state. This analysis provides important quantitative evidence of the need for and importance of such programs, and it can help inform and shape future actions to reduce on-road transportation pollution exposure and inequities in the state.

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