What You Should Know About Reducing 

Diesel Exhaust from School Buses 

S

chool buses provide our nation’s children with 

safe, convenient transportation. Diesel exhaust 

from school buses, however, poses a health 

risk, particularly to children. Diesel exhaust contains 

small particles as well as smog-forming and toxic air 

pollutants. Exposure to diesel exhaust can cause 

lung damage and respiratory problems and can 

exacerbate asthma and existing allergies. Buses that 

idle outside schools can pollute the air inside the 

school building as well as outdoors. Fortunately, 

there are several steps that schools can take to 

reduce diesel exhaust from school buses. 

Recommended Actions 

to Reduce Diesel Pollution 

Clean School Bus USA encourages school districts 

to reduce the health risks associated with exposure 

to diesel exhaust by: 

Reducing school bus idling time and reinforcing 

smart driving practices, such as following at least 

three car lengths behind any vehicle with visible 

exhaust or a noticeable odor. 

Retrofitting current school bus fleets with new


technologies and introducing cleaner fuels.


Replacing the oldest buses with new ones that 

meet stringent pollution control standards. 

How Are Children Affected? 

Air pollution from diesel vehicles has health 

implications for everyone, but children are 

more susceptible to this pollution because 

they breathe at a faster rate than adults. 

Children’s lungs are still developing, and 

children are more likely to play actively 

outdoors. More than 24 million children in 

the United States ride a bus to and from 

school every day. 

Help Clear the Air with 

Clean School Bus USA 

Clean School Bus USA is an initiative sponsored 

by the U.S. Environmental Protection Agency 

(EPA) to help communities reduce pollution from 

school buses. It’s a partnership of educators, 

industry, businesses, school officials, school bus 

fleet and transportation managers, state and local 

governments, public health officials, and other 

community leaders who are committed to 

protecting children’s health and modernizing 

America’s school bus fleet. 
C

Portland, Maine 

i

Clean School Bus USA 

Case Studies 

ommunities across the country are 

developing effective local 

partnerships to reduce air pollution from 

diesel school buses. School children in 

these areas reap immediate air quality 

benefits from effective anti-idling policies, 

buses that run on cleaner fuels, and 

buses retrofitted with state-of-the-art 

emission reduction technologies. 

In the last f ve years, the Portland school 

district has replaced 90 percent of its 

school bus fleet with newer, cleaner 

buses. This pioneering effort is part of a 

multi-faceted statewide campaign to 

reduce emissions from diesel school 

buses. The campaign—a partnership 

between the Portland School 

Transportation Department, the Maine 

Departments of Education and 

Environmental Protection, the Asthma 

Regional Council, and the U.S. 

Environmental Protection Agency— 

focuses on: anti-idling and fuel 

conservation; aggressive investments in 

newer, cleaner buses; and route 

management to assign the cleanest buses 

to the longest routes. 

State of Washington 

With support from Washington’s 

legislature, schools throughout the state 

will retrofit bus fleets, reducing toxic 

continued on next page 

Here are some specific actions school districts can take 

to achieve these goals: 

Implement Anti-idling and 

Smart Driving Practices 

• Train school bus drivers to turn off their buses as 

soon as they arrive at loading or unloading areas and 

to refrain from restarting their buses until they are 

ready to depart. 

• Establish a program to recognize drivers who


successfully reduce idling. 


• Consider changing circuit configurations if necessary 

to power flashing lights with the battery. 

• Limit idling time during early morning warm-up to 

what is recommended by the manufacturer (generally 

3 to 5 minutes). In colder climates, block heaters can 

help warm the engine of older vehicles to avoid 

starting difficulties and shorten warm-up time. Newer 

buses are designed to start easily at all temperatures 

without idling. 

• In the winter, provide a space inside the school where 

bus drivers who arrive early can wait. 

• Follow the anti-idling laws and guidelines that many

states have in place. Post signs as reminders. 

• Revise bus schedules and operational logistics to

minimize school bus caravanning. 

• Assign cleanest buses to the longest trips. Inform

drivers that following other diesel vehicles too closely 

can contribute to higher concentrations of diesel 

exhaust inside and outside the bus. 

I

Typical school buses burn 

T

dling wastes fuel and money. 

approximately one-half gallon of 

diesel fuel for each hour of idling. 

he less school buses idle, the more 

money school districts can save. 
Work Closely with Fleet Managers 

and Bus Drivers to Reinforce 

New Practices 

•	 Make sure both fleet managers and bus drivers 

understand the potential health risks from 

breathing diesel exhaust and the benefits of not 

idling or caravanning. 

•	 Highlight the economic benefit of reduced fuel 

consumption as a result of less idling. For example, if a fleet operates 50 buses and each bus 

reduces its idling time by 30 minutes per day, at 

$1 per gallon of diesel fuel, the fleet would save 

$2,250 per school year in fuel costs. 

•	 Maintain engines properly. 

Work Closely with Fleet Managers 

to Retrofit Buses with 

Pollution Controls 

Oxidation catalysts and particulate matter filters are 

two retrofit technologies that can help reduce diesel 

particulate matter. Both devices are housed in the 

exhaust system where they break down the pollutants 

in the exhaust. 

Oxidation Catalysts 

Diesel oxidation catalysts are widely available and 

commonly used retrofit technologies. They are relatively simple, low cost devices that can be installed in 

almost all buses and require very little maintenance. 

Diesel oxidation catalysts can be used with regular 

diesel fuel. Diesel oxidation catalysts typically cost 

between $1,000 and $2,000 and reduce particulate 

matter emissions by 20 to 30 percent. Reductions 

may be even higher if used together with ultra-low 

sulfur diesel fuel. 

Particulate Matter Filters 

Diesel particulate matter filters provide even greater 

particulate matter reductions. Filters are a more complex technology than catalysts and generally are most 

appropriate for 1995 and newer buses. Filters typically 

cost between $5,000 and $10,000 and require the 

use of ultra-low sulfur diesel fuel. The combination of 

ultra-low sulfur diesel and particulate matter filter technology reduces particulate matter emissions by 60 to 

90 percent. 

Case Studies 

(continued) 

nearly 5 million miles. 

diesel emissions by 50 to 90 percent. This 

program, named the Washington State 

Clean School Bus Program, will affect 

approximately 5,000 of more than 9,000 

school buses throughout the state by 

2008, making it the largest state-funded 

voluntary school bus retrofit program in 

the country. The Puget Sound Clean Air 

Agency, in partnership with several state 

and local agencies, conducted pilot 

programs with U.S. EPA grants during the 

past two years to pave the way for rapid 

deployment of retrofitted buses. Retrofits 

for buses will involve either installation of 

particulate matter filters or oxidation 

catalysts on school bus exhaust systems, 

depending on the age of the bus and the 

regional availability of ultra-low sulfur 

diesel fuel. To maximize program benefits, 

older school buses that pollute the most 

will be the first to receive retrofits. 

Ardmore, Pennsylvania 

Nearly 70 percent of the Lower Merion 

School District’s fleet of 107 buses 

operate on compressed natural gas 

(CNG), providing noise relief and clean 

air benefits to the residential 

neighborhoods where these buses 

operate. To build local fast-fill CNG 

refueling stations and offset the upfront 

costs of the new buses, the district 

received several grants from community, 

state, and federal partners. Lower 

Merion’s CNG fleet has now logged 
Work Closely with 

Fleet Managers to 

Purchase Cleaner Fuels 

There are a number of alternatives to conventional 

diesel fuel that can help reduce particulate matter 

emissions from today’s school bus fleet. Most of these 

fuels can be used with little or no modification to the 

bus or its engine: 

•	 Alternative fuels such as compressed natural 

gas offer outstanding environmental benefits for 

buses designed to run on such fuels. 

•	 Biodiesel is a fuel that contains some 

domestically produced, renewable components. 

Blends of biodiesel (B20) can be used in 

unmodified diesel engines, but pure biodiesel 

(B100) may require certain engine modifications. 

Pure biodiesel may not be suitable for cold 

climates. 

•	 Emulsified diesel fuel is a blended mixture of 

diesel fuel, water, and other additives. 

Emulsified diesel can be used in any diesel 

engine to reduce emissions of particulate matter 

as well as nitrogen oxides. 

•	 Ultra-low sulfur diesel fuel is diesel fuel with an 

extremely low sulfur content (15 or less parts 

per million). The low sulfur levels enable 

particulate matter filters to perform most 

efficiently. Ultra-low sulfur diesel will be required 

nationwide for use in all highway diesel vehicles 

beginning in 2006 and is currently available in 

some parts of the country. 

Work Closely with 

Fleet Managers to 

Replace the Oldest Buses 

with New Ones 

•	 About one third of U.S. school buses were 

manufactured before 1990 and are not 

equipped with modern pollution controls and 

the latest safety features. These buses are 

excellent candidates for accelerated 

replacement. 

•	 By 2007, EPA will require that new buses rolling 

off the assembly lines be 95 percent cleaner 

than today’s models. Because some buses 

might meet EPA standards ahead of schedule, 

ask the manufacturer before purchasing a new 

bus to see if you can acquire one that meets 

these standards. 

•	 Both diesel and compressed natural gas technologies offer very clean replacement options. 

For more information about Clean School Bus USA: 

Visit www.epa.gov/cleanschoolbus 

Leave a voice message at (734) 214-4780 

E-mail cleanschoolbususa@epa.gov 

For more information on diesel retrofit options, visit www.epa.gov/otaq/schoolbus/retrofit.htm 

EPA420-F-03-038 	 November 2003