Alternative Fuel School Bus Emissions: What the Data Says
Across key pollutants, propane and biofuel school buses show similar or higher emissions to diesel, while electric school buses eliminate all tailpipe pollution.
As school districts across the United States replace aging diesel-burning school buses, reducing emissions is top of mind. That’s because the pollution from diesel-burning school buses can harm kids and communities. Nearly 500,000 school buses operate across the U.S., transporting about 20 million students each day, and most still run on diesel or other fossil fuels.
Reducing tailpipe emissions is expected to reduce pollution exposure near bus routes and schools, which research has shown is associated with improved health outcomes for children.
Nitrogen oxides (NOx), particulate matter (PM), and volatile organic compounds (VOCs) are among the most harmful kinds of pollutants because they can cause serious health problems:
- NOx can cause asthma, respiratory conditions and heart conditions.
- PM can cause asthma, heart attacks, irregular heartbeat, decreased lung function, respiratory conditions, and premature death for those with underlying conditions.
- Long-term exposure to VOCs can damage the liver, kidneys, and central nervous systems, and cause cancer.
Fortunately, tools like AFLEET, developed by the U.S. Department of Energy’s Argonne National Laboratory, make it easier to compare emissions produced by different types of school buses.
There are two main types of pollution that school buses can cause:
Tailpipe emissions come directly from the bus when it’s running, and are breathed in by people riding or standing near the bus.
Well-to-wheel emissions include tailpipe emissions plus all the pollution that’s created when the fuel or feedstock—like diesel, electricity, propane, or vegetable oil for biodiesel—is generated and transported.
The AFLEET data also looks at greenhouse gas emissions as part of the well-to-wheel emissions. With greenhouse gas emissions, it doesn’t matter as much where the emissions happen. The real damage from greenhouse gas emissions comes from their accumulation in the atmosphere where they trap heat and contribute to the warming of the planet.
AFLEET includes data on several different kinds of buses. Some represent entirely new vehicle technologies, while others are fuels that can be used in existing diesel buses:
- Not currently available on the school bus market: compressed natural gas (CNG), liquified natural gas (LNG) and hydrogen fuel cell.
- Require new vehicle systems or engines: battery electric, propane and ethanol blends.
- Can be used in existing diesel-burning school buses: biodiesel blends and renewable diesel.
From zero tailpipe emission options like electric school buses to alternatives such as propane and biodiesel, the data help show how these fuels differ.
Right now, diesel-burning school buses are the most common type of school bus, so we think of that as the baseline.
Let’s take a closer look.
Tailpipe emissions
The AFLEET data show that electric school buses are the only school bus type on the market with zero tailpipe emissions.
Every other alternative fuel school bus on the market produces tailpipe pollution. In many cases, these emissions are similar to diesel-burning buses. In some cases, they are higher for certain pollutants.
For example:
- Biodiesel and renewable diesel buses produce nearly the same tailpipe emissions as conventional diesel-burning buses for most pollutants. The one difference is lower SOx emissions.
- Propane and E85 buses produce lower NOx and SOx emissions than diesel-burning buses. However, they produce higher emissions of VOCs and CO, and similar levels of particulate matter.
- CNG and LNG buses also produce lower NOx and SOx emissions than diesel-burning buses. However, they produce higher emissions of CO, and similar levels of VOCs and particulate matter.
Each of these types of alternative fuel school buses, aside from electric and hydrogen, has tailpipe emissions as bad as or worse than diesel for at least four of the six tailpipe pollutants in the data.
This means switching from diesel-burning buses to propane, CNG, or E85 buses could expose children to higher levels of some tailpipe pollutants. Using biodiesel or renewable diesel could leave children exposed to similar levels of most pollutants as conventional diesel buses.
Cutting these tailpipe emissions is crucial to giving kids a clean ride to school. And when we consider that Black, low-income, and disabled students are more likely to ride the bus to school than other students, it’s clear that reducing tailpipe emissions is also vital to a more equitable future for all kids.
Well-to-wheel emissions
To understand the full emissions impact of school buses, it is important to look beyond the tailpipe, especially as the emissions for some alternative fuel buses vary greatly from diesel on a fuel lifecycle basis.
Much of the pollution produced from electricity production occurs at power plants, where emissions are released through smokestacks that are generally located away from population centers and dispersed higher in the atmosphere. As a result, on a pound-for-pound basis these emissions can have a much lower health impact than emissions from tailpipes in areas of higher populations.
Electric school buses produce the lowest well-to-wheel emissions of NOx, CO, and VOC, while PM levels are similar to diesel buses.
Here’s how other fuels perform in several categories for well-to-wheel emissions:
- School buses using an ethanol blend produce the highest well-to-wheel emissions across all pollutants analyzed.
- Propane buses produce less well-to-wheel NOx emissions than conventional diesel buses, but more CO, and VOC emissions.
- Renewable diesel and biodiesel fuels also produce higher well-to-wheel emissions of NOx, CO, and VOCs, and similar levels of PM compared with conventional diesel.
- CNG and LNG buses produce higher well-to-wheel VOC and CO emissions than diesel school buses.
All alternative fuel bus types have estimated SOx emissions that are elevated compared to diesel. Coal's role in electricity generation, the primary source of SOx, is expected to continue to decline sharply by 2040, even when factoring in recent policy decisions. This means future grid SOx emissions will likely be lower than current assumptions used in AFLEET models.
Greenhouse gas emissions
School buses also produce greenhouse gas emissions like carbon dioxide (CO2) and methane (MH4), which contribute to climate change. These emissions come from burning fuel and producing energy. The United States has nearly half a million school buses, so their fuel choices can have a large impact on climate pollution.
AFLEET data shows that electric school buses produce the lowest greenhouse gas emissions across the full fuel lifecycle. This is true even after accounting for emissions from generating the electricity that powers the buses.
Reducing greenhouse gas emissions is important because climate change affects extreme weather, air quality, and public health. These impacts affect the safety and well-being of students and communities.
Among other fuels analyzed:
- Propane buses produce the highest greenhouse gas emissions of any school bus currently on the market.
- Renewable diesel, biodiesel and E85 fuels can lower greenhouse gas emissions compared with conventional diesel. However, when land use emissions are included—something the AFLEET model does not fully account for—biodiesel made from vegetable oils can have higher greenhouse gas emissions than fossil diesel. These fuels also continue to produce harmful air pollutants during operation.
Like tailpipe emissions, greenhouse gases hurt some communities more than others. Low-income people and communities of color have the fewest resources to prepare for, and recover from, extreme weather fueled by climate change. That means these emissions will hurt them even more, and leave them more at risk.
Market influence on school bus choice
Hydrogen, CNG, and LNG school buses are not commercially available. So even if a school district wanted to buy one of these buses today, they would not be able to find a manufacturer selling it.
Other alternative fuel buses have very limited market options. For example, only one manufacturer currently sells propane-burning school buses. This means districts have little competition or choice when purchasing propane buses.
Biodiesel and renewable diesel remain limited. Outside of the Pacific Coast states with Clean and/or Low-Carbon Fuel Standards in place—California, Oregon, and Washington—access to renewable diesel is essentially non-existent. There are only five publicly accessible fueling stations supplying renewable diesel across the remainder of the country.
In addition, biodiesel and renewable diesel can typically be used in existing diesel-burning school buses without modifying the vehicle. While this may change the fuel used, it does not replace the underlying combustion engine or remove diesel-burning buses—and the harmful pollution they produce—from the road. The same is true for ethanol blends. While they require engine modification, ethanol buses can still run on regular gasoline and do not remove gasoline-burning buses from the road. This raises concerns about waste, fraud, and abuse as funding programs would have difficulty verifying that biodiesel or ethanol is being used instead of fossil diesel and gasoline.
Electric school buses, however, are available from 13 manufacturers in 26 models. After more than a decade of operation, electric school buses are now on the road in 49 states, DC, Puerto Rico, American Samoa and more than 20 Tribal nations. And in 2025, electric school buses were an estimated 7.5% of new school bus sales nationwide, showing growing adoption and confidence in the market.