Introduction
The yellow school bus remains an American icon, looking much the same since 1939. That year, to improve safety, attendees at a national education conference agreed on a standard bright color: orangish yellow.
School buses have gained other safety features since then, and with relatively few road accidents, have a strong safety record. Yet almost all of them miss a critical opportunity to protect not only the 26 million children riding them in the United States but also everyone living near their routes.
A 1939 Dorothea Lange photograph of children boarding a school bus in Malheur County, Oregon. Dorothea Lange / Library of Congress
That opportunity is electrification. About 95% of school buses run on diesel fuel, which emits pollution that harms people’s health and warms the planet. Children using these buses and parents waiting at bus stops breathe dirty air, as do all their neighbors. The health impacts are worse in dense urban areas—home to many low-income residents and communities of color.
Switching from diesel to all-electric or battery-powered buses improves health and reduces greenhouse gas emissions. And new federal funds make it easier to cover the costs. The 2021 infrastructure bill includes $2.5 billion for electric school buses.
Quantifying the benefits of such a conversion, though, requires a detailed analysis of how many people live near schools and along bus routes. The benefits can vary widely based on a neighborhood’s density.
This project shows how switching from diesel to electric school buses will improve health for children and families in Prince George’s County, a Maryland suburb of Washington, DC.
Diesel exhaust and health
Diesel school buses impact the health of riders, drivers, and people living along routes. Pollution from diesel fuel use can hurt the lungs, heart, brain, and other major body systems, leading to increased risk of asthma and breathing problems, heart issues, stroke, reproductive health complications, cancer, and more.
Graphic by Oregon Physicians for Social Responsibility
Kids exposed to diesel exhaust are at particular risk, because their lungs and brains are still developing.
Photo by Adam Cohn, used under CC BY-NC-ND 2.0
Diesel buses also contribute to climate change through carbon dioxide (CO 2 ) emissions.
Changing the pollution game
The air pollution impacts from an electric bus depend on the electricity grid that powers it. As climate goals become more ambitious, the grid will get cleaner over time.
In the short term, transitioning to electric vehicles (EVs) reduces the overall amount of carbon dioxide and nitrogen oxides, among other pollutants. But some pollutants can increase overall in the short term, depending on where and how the new power for the electric buses is produced.
Photo by Jeswin Thomas on Unsplash
Geography matters
Total emissions matter, but to fully understand how going electric impacts pollution, we have to look at how where those emissions are concentrated and how many people work and live near the source of pollution.
Electrification changes the amount, location, and concentration of a school bus’s emissions.
Swapping diesel buses for EVs in Prince George's County
In Prince George's County, MD, the school district is exploring how to transition their bus fleet from diesel to electric.
The district serves more than 136,500 students across 208 schools (shown in green) with a fleet of 1,282 buses; the county's bus lots are marked as red squares.
How do exposure and health risk compare between a diesel fleet like the one in Prince George's County and an EV fleet making the same number of trips each school year?
When a bus is converted to an EV, the power to drive it comes from the electric grid instead of from burning diesel. In general, this change shifts some emissions from diesel tailpipes in neighborhoods to power plant smokestacks.
Health risk is related to the toxicity of and total exposure to pollutants. The health harms that result from exposure to pollution depend on how many people are exposed to the pollution and how much they breathe in (how close they are to the source).
To understand and quantify total health risk and likely health impacts from diesel and power plant emissions, we must look at not just total emissions, but also the human geography of nearby areas.
Pollution from new power generation is distributed across the electric grid
Because power isn't generated right where it is used, pollution stemming from new electricity demand in one location is dispersed across power plants throughout a region's power grid.
This map shows one year of estimated new NOx emissions likely from increased power demand from a PG County School District bus fleet electrification, with darker dots showing a greater volume of new emissions.
To put this in context, here's the same map showing this volume by size instead of color...
....with the volume of NOx emissions estimated from one year of Prince George's County Public School's diesel fleet operation shown for scale in red.
The population density is very low in the areas where power plant emissions are likely to increase if EV buses are adopted. County population density is shown here in blue —darker areas are denser.
Let's see what this looks like up close, at the census-block level.
In two out of three of these plants (circled here with a half- mile radius in red), the population density is below 50 people per square mile.
Diesel school bus emissions are concentrated in dense residential zones
Because buses have to pick up more students, more miles of bus travel will occur in more densely populated areas. Most schools will also have many bus routes terminating at them. Buses also have their tailpipes ground level meaning that exposure to their pollution is focused in the areas they travel in, as opposed to power plants with tall smokestacks which can spread their pollution across a much greater area.
Prince George's County has a population density of about 1,900 people per square mile—about 10 times higher than the areas around power plant smokestacks. Here we see the county's bus lots again in red, and all elementary schools in orange.
When we look closely at how that density is distributed, we see that most schools are clustered near denser areas. The shaded areas shown here are elementary school service territories; all but the lightest shade of blue are as dense or denser than the county average.
When we look at density at the level of census blocks again, we can see even more clearly that schools (and bus route termini) are concentrated near densely populated areas.
But emissions aren't confined to where buses park or drop off students—they occur along the entire bus route.
Here’s a closer look into one of these denser areas in Prince George's County.
A number of schools are shown again in orange, along with a bus lot in red. To get students to school, buses may visit densely populated residential areas where most students live and move through similar areas en route to the school itself.
Some Prince George’s County census blocks are over 170 times denser than the county average (318,000 per sq. mi). If diesel emissions converge in these places, then total risk to the population is likely much greater than if the same amount of emissions were spread evenly over the whole county.
The most concentrated exposure, however, happens on the buses themselves.
Exposure on the bus
Bus riders breathe in their own bus's tailpipe emissions. Emissions concentrations in school buses have been found to be four times higher than ambient levels outside the bus.
This exposure is estimated to cause up to 46 more cases of cancer per million students than would otherwise be expected.
Not only do EVs avoid this “self-pollution” problem, the outlook for total EV emissions is also getting brighter every year as Maryland move toward a cleaner power grid.
Maryland has set ambitious targets of 50% renewables by 2030 and 100% clean electricity by 2040.
Estimated near-term versus future PM 2.5 emissions from plants generating power for buses
Diesel school buses increase health risk more than the volume of their tailpipe emissions alone would suggest, by concentrating it among populated corridors.
The grid is getting cleaner. EVs open the door to a lower-emissions future that protects both student health and the climate.
Photo from PGCPS
Conclusion
Electrifying school buses yields safety and environmental benefits, which will continue to increase as the power grid gets cleaner. New federal funds could ease this win-win shift to EVs.
There is no time to waste. Transitioning to electric school buses opens the door to a lower-emissions future that protects people—especially children—and the climate.
