Cleaning up dirty school buses

Solutions include replacing and retrofitting old buses

Every day, half a million school buses safely carry 25 million American children to school, field trips and athletic events.

Unfortunately, most buses are powered by diesel engines that actually pollute the air inside the bus. Studies show the pollution gets trapped inside the bus, where kids breathe it in.

The Science Behind School Bus Pollution

Yale University researchers demonstrated high pollution levels inside buses by attaching monitors to children’s backpacks and recording the pollution levels during their trips to school. Levels spiked when buses arrived to pick them up, remained elevated on board, spiked again as they exited the bus, and returned to low levels upon entering the school.

Several independent research teams using different monitoring methods have documented this effect for many buses in numerous locations. One noted that “While children may only spend a few hours per day on school buses, the high levels of exposure encountered on board school buses can add considerably to their daily and annual exposures to air pollutants such as [diesel particulate matter] and PM2.5.”

Another found that average exposures to fine particulates on school buses were 5-6 times greater than ambient (outdoor) levels and approximately three times higher during bus rides than during the average walking commute.

The California Air Resources Board found that school bus trips can increase children’s daily exposure to black carbon up to 34 percent, compared to regular passenger cars. Particle (PM) levels inside a school bus can be 5-10 times the levels outside the bus.

A University of California study (using data from the California Air Resources Board) notes “the total mass of bus pollution inhaled by bus riders likely exceeds the total bus pollution inhaled by the [entire] remaining public, despite bus riders being a relatively small group.”

What Are the Best Retrofit Options?

To clean soot from engines, crankcase filters can be installed in all school buses.

To clean up tailpipe emissions, the technologies used are:

  • Diesel particulate filters (DPFs) are the best solution. They remove the most particulate pollution (PM or soot).
  • Flow-through filters (FTFs) are the next best option, depending on the age and condition of the vehicle.
  • Diesel oxidation catalysts (DOCs) are the least effective but preferable to no filtering. 
Retrofit type for tailpipes Emission reductions
of fine PM mass
Estimated Cost installed
Diesel particulate filter (DPFs) 85% or greater $9,000 - 15,000
Flow-through filters (FTFs) 50 - 70% $8,000 - 12,000
Diesel oxidation catalysts (DOCs) 20 - 30% * $2,000 -  3,000

* At maximum efficiency.  Only removes soluable organic fractions from emissions.

Tapping Funding for Bus Retrofits

The U.S. Diesel Emissions Reduction Act authorized $50 million in national funds and $15 million for California projects. Washington, California and New Jersey have state-funded schoolbus clean-up programs.

Texas has $130 million in annual funding through the Texas Emissions Reduction Program (TERP).

A variety of government sources provide funding, including the following:

  • Environmental Protection Agency (EPA). Monies for retrofitting school bus engines are available through its Clean School Bus USA program. Clean School Bus USA and Blue Skyways Collaborative expect to award more than $3 million this year in grants for school bus diesel-emission reduction projects.
  • Congestion Mitigation and Air Quality Improvement Program (CMAQ). The CMAQ program provides funds to metropolitan areas failing to meet federal clean air health standards (National Ambient Air Quality Standards) or to implement appropriate air cleanup plans (State Implementation Plans) in line with Transportation Control Measures (TCMs). TCM funds are used to reduce pollution from cars and trucks by cutting traffic congestion.
  • Other sources. The Environmental Protection Agency has an extensive list of funding sources including state programs. School districts can also apply for funding from state air control boards and environmental agencies. 

Guidelines for Schools

School districts that contract with school bus companies

When school districts contract with outside bus providers, they should require diesel particulate filter (DPF) and crankcase ventilation system (CCVS) retrofits for all buses that don’t meet the 2007 federal engine standards. While putting limits on the age of the bus is helpful, contracting for the cleanest emissions is the real goal. 

A good contract should require pre-2007 school buses to retrofit the tailpipe and engine to remove at least 85 percent of soot pollution. Retiring a bus after 10 years is also a good rule-of-thumb.

To alleviate the cost burden for bus upgrades, bus providers can apply for outside funding for retrofits. Fleet operators can negotiate that buses built after 2002 that have been retrofitted with DPFs and CCVS can stay in service for the useful lifespan of the DPF even if this exceeds the 10-year retirement age. Typically, DPFs last about 10 years.   

School districts that own their own buses

Today’s buses are much cleaner than buses built a decade ago. Because of this, retiring a bus after 10 years is a good rule-of-thumb.

If school districts own older buses, then the buses should be retrofitted with diesel particulate filters (DPFs) and crankcase ventilation systems (CCVSs), cutting at least 85 percent of soot pollution. If school districts contract with outside bus providers, they should also require DPF and CCVS retrofits for all buses that don’t meet the current federal engine standards. To alleviate the cost burden for bus upgrades, incentive funding for both diesel retrofits and replacements is often available.

Four steps to cleaner buses

School buses are still the safest and smartest way to travel to school. But with lower diesel emissions, they could be even safer. New buses and retrofits are good solutions. Here are four ways to make a difference:

#1: Replace - Add new buses

New buses are much cleaner than older ones. Thanks to strict new Environmental Protection Agency standards, buses meeting the cleanest standards are 90 percent cleaner than the buses they replace.

New buses are expensive, however. (A new bus costs $75,000-80,000.) One solution for cash-strapped schools is to outfit buses older than 1995 with newer engines and retrofits.

#2: Retrofit - Clean up older buses

Affordable pollution-cutting filters are available. For just $9,000-15,000 each, older buses can be fitted with tailpipe and crankcase filters that reduce soot emissions by up to 90 percent, making them as clean as new buses for a fraction of the cost.

Every dollar spent on retrofitting a diesel school bus is worth at least $12 in health benefits (such as avoided emergency room visits) — a very smart investment.

State-of the-art technology and ultra-low sulfur diesel, now required for diesel trucks and buses, will result in cleaner-burning buses.

#3: Reduce idling - Turn off engines

Eliminate idling. Bus engines should be turned off when waiting, especially within 500 feet of a school. During hot summer or cold winter months, drivers can make arrangements to wait inside schools between jobs, or auxiliary power can be used to warm or cool the bus.

When a bus remains running for more than three minutes, emissions from the bus increase more than 65 percent. Buses also waste countless gallons of gas through idling. By reducing idling a 25-strong bus fleet can save thousands of dollars a year in fuel costs.

Other ways to minimize the effects of idling:

  • Line up buses diagonally rather than single file. When buses park in single file, the exhaust flows easily filters into other buses through open doors.
  • Move the exhaust pipe to the opposite side of the bus to minimize pollution from entering the doors of adjacent waiting buses.
#4: Routing - Smart fleet use
  • Routes and travel times should be scheduled to achieve the most efficiency. Bus companies and school districts will save money, too.
  • The cleanest buses should travel the longest distances and the most days.

American Lung Association, 2007. State of the Air: 2007.

California Air Resources Board, Characterizing the Range of Children’s Pollutant Exposure During School Bus Commutes [PDF], ARB Staff Interpretive Summary of Study Results, 2003.

Clean Air Task Force, Hill, Bruce et al, 2005. A Multi-City Investigation of the Effectiveness of Retrofit Emissions Controls in Reducing Exposures to Particulate Matter in School Buses [PDF].

Health Canada, "Exposure of School Children to Diesel Exhaust from School Buses," 2006. Marshall, J and E. Behrentz, 2005. Vehicle Self-Pollution Intake Fraction: Children’s Exposure to School Bus Emissions. Environ. Sci. Technol., 39 (8), 2559 -2563.

Ontario Public Health Association, School Buses, Air Pollution & Children’s Health: Improving Children’s Health and Local Air Quality by Reducing School Bus Emissions [PDF], 2005.

State and Territorial Air Pollution Program Administrators & the Association of Local Air Pollution Control Officials, 2000. Cancer Risk from Diesel Particulate: National and Metropolitan Area Estimates for the United States [PDF].