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Reducing Diesel Emissions

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There are over 1,800 publicly owned and contracted school buses, transporting over 75,000 school age children over 12 and million miles annually in Vermont. The majority of these buses are diesel-fueled, with high amounts of exhaust emissions. Although school buses are one of the safest means of transporting children to and from school, emissions from diesel-powered school buses constitute a serious health risk to children. While it is true that breathing diesel exhaust has health implications for everyone, children are much more susceptible to this pollution than healthy adults because they breathe more air relative to their body weight and their respiratory systems are still developing.

A recent study conducted by researchers at Yale University has shown that children are exposed to unhealthy levels of diesel exhaust and that the increased levels are directly related to idling school buses. Diesel exhaust from idling school buses can accumulate at ground level and enter both the passenger compartments of the buses and school classrooms through ventilation systems. Therefore, limiting engine idling time whenever practical can make a significant impact on protecting the health of students as well as bus drivers, teachers and other school staff. In addition to reducing harmful diesel exhaust fumes, turning engines off when buses are not in use can save money on fuel and reduced engine wear.

What is diesel exhaust?

Exhaust from diesel-fueled trucks and buses contains small particles known as fine particulate matter, or PM (aerodynamic diameter < 2.5 microns). Diesel powered trucks and buses make up about 2% of the motor vehicle fleet in the Northeast states, but emit roughly 75% of vehicle-related fine PM. Diesel PM can penetrate into the lungs and therefore pose a serious health risk. Diesel-fueled engines also emit carbon dioxide, nitrogen oxides, volatile organic compounds and carbon monoxide.

What are health and environmental impacts of diesel exhaust?

  • Fine particulate matter increases both mortality and morbidity in young children, the elderly, and people with heart and lung disease. Exposure to fine particles can aggravate existing heart and lung disease (i.e. chronic obstructive pulmonary disease and asthma), change the body's defenses against inhaled materials, and damage lung tissue. Fine particles also contribute to haze which reduces visibility and can damages painted surfaces, soils, clothing and furniture.

  • Carbon dioxide is a greenhouse gas that contributes to global climate change by trapping the earth's outgoing energy as heat in the atmosphere.

  • Nitrogen oxides and volatile organic compounds are the primary ingredients in the formation of ground-level ozone. High ozone levels can cause respiratory tract problems such as difficult breathing and reduced lung function. Ozone can also cause asthma, eye irritation, nasal congestion, reduced resistance to infection, and premature aging of lung tissue. Nitrogen dioxide can cause bronchitis and pneumonia, irritate the lungs, and lower resistance to respiratory infections.

  • Carbon monoxide is a colorless, odorless and poisonous gas that weakens the heart's contractions and lowers the amount of oxygen carried by the blood. It reduces the body's ability to exercise and is dangerous for people with chronic heart disease. It can cause nausea, dizziness, headaches and, at high enough concentrations, even death.

Who is most at risk?

Children are especially vulnerable to these emissions.This special vulnerability has several aspects to it. First, school-age children are growing and developing physiologically. Exposure to toxic and hazardous substances at this period in human development can have compound effects. Breathing rates of children are enhanced over adults so their exposure to the same concentrations of air pollution will deliver a greater dose of toxic substances to their bodies. The stature of children is such that they are closer to the point of engine exhaust and must walk through a concentration of vehicle exhaust daily as they load and unload from buses. Lastly, it has been documented that buses idling in the vicinity of school buildings allows for engine exhaust to be taken in by building air handling systems, resulting in the distribution of engine exhaust contaminants through out the school building.

The Centers for Disease Control (CDC) estimates that 4.8 million children in the U.S. have asthma, symptoms of which are exacerbated with exposure to diesel exhaust. Exposure to diesel exhaust can also adversely affect children with other respiratory illnesses such as bronchitis and lung infections and can enhance the effects of allergens.

What the Federal Government is doing:
  • The U.S. Environmental Protection Agency (EPA) has adopted new standards for diesel fuel and engines resulting in the use of advanced emissions control technologies and ultra-low sulfur fuel. 2007 & newer school bus engines are up to 95 percent cleaner than earlier models.

  • The EPA is also promoting voluntary anti-idling efforts as well as the use of cleaner fuels and technologies to reduce emissions in existing diesel-powered school buses through the Voluntary Diesel Retrofit Program.
What more the Federal Government could be doing:
  • Adopt air quality standards within school bus passenger compartments that adequately protect children from the harmful effects of fine PM in diesel exhaust.

  • Require states to conduct routine inspection & maintenance of emissions control equipment for school buses.

  • Require states to expand air quality monitoring networks to better assess actual exposures to hazardous air contaminants.
What State Governments are doing:
  • A number of state governments have adopted regulations limiting school bus idling time with varying degrees of enforcement. Voluntary initiatives to reduce school bus emissions have also been implemented in several states. Below is a summary of idling regulations and initiatives for the Northeast states.

Summary of School Bus Idling Regulations and Initiatives in the Northeast
Idling Time*
Enforced by
Not more than 3 minutes. CT DEP;
State and local enforcement officers
Program to encourage schools to reduce school bus idling time and retrofit buses with pollution control devices.
Not more than 5 minutes.
Program to encourage schools to reduce school bus idling time and retrofit buses with pollution control devices.
Not more than 5 minutes. MD DOT  
Not more than 5 minutes. Police and Fire Departments;
Board of Health Officials;
Building inspectors and designees within respective jurisdictions.
Program to encourage schools to reduce school bus idling time and mandatory retrofit of buses with pollution control devices.
Not more than 5 minutes.
State Police
Program to encourage schools to reduce school bus idling time and retrofit buses with pollution control devices.
Not more than 3 minutes.
State, county and local police
Program to encourage schools to reduce school bus idling time and mandatory retrofit of buses with pollution control devices.

NYC: Not more than 3 minutes.
State: Not more than 5 minutes (may be extended by federal, state or local agency regulations).

NY DEC Enforcement Officers;
State Police
Program to encourage schools to reduce school bus idling time and mandatory retrofit of buses with pollution control devices.
Not more than 5 minutes.
 Program to encourage schools to reduce school bus idling time and retrofit buses with pollution control devices.


(only for buses on school property VT School Bus Idiling Regulation)

Not more than 5 minutes.
VT Department of Education
Programs to encourage schools to reduce school bus idling time, retrofit buses with pollution control devices, and accelerate bus replacements.
*Most state idling regulations have various exemptions to address special circumstances such as traffic conditions, emergency vehicles, mechanical difficulties, vehicle service, vehicle inspections, engine warm-up, and heating/ cooling equipment, often with ambient temperature thresholds which range from < 20°F to < 40°F.

What more Vermont could be doing:
  • Enhance the enforcement of limits on school bus idling

  • Expand programs to retrofit existing buses with pollution control technologies.

  • Expand programs to replace older school buses with new low-emission and alternative-fuel buses.

  • Require routine inspection & maintenance of emissions control equipment for school buses.

  • Expand ambient air quality monitoring networks to characterize local variability of air pollution levels in order to better assess public exposure.

What Local Governments and School Districts are doing:
  • Various local governments and school districts are working with state agencies and/or other organizations to implement programs to reduce children's exposure to diesel emissions.

What more Local Governments and School Districts could be doing:
  • Adopt policies to minimize school bus idling in order to improve air quality both on the buses and in the vicinity of the schools

  • Limit the duration of time that children spend on buses by reducing the length of bus routes and thereby reduce exposure to diesel exhaust.

  • Require routine maintenance of emissions control equipment for all buses.

  • Relocate school bus parking and loading areas and/or school intake air systems that currently contribute to unhealthy levels of air pollution in and around school buildings.

Vermont schools that are making a difference.

  • Barre Town, Brighton, Cambridge, Chittenden East , Chittenden South, Rutland Northeast , Stockbridge- In 2007, AQCD received an EPA Clean School Bus USA grant to retrofit over fifty school buses state wide with a combination of diesel oxidation catalysts (DOCs), closed-crankcase ventilation (CCV) systems, and auxiliary fuel operated heaters (FOHs).

  • Windham Northeast- In 2005, Windham Northeast supervisory union replaced five of it's oldest diesel school buses in serveral communites with new, cleaner diesel buses.

  • Burlington School District - In 2004, the Burlington School District was recognized by EPA as a national leader in improving indoor air quality in schools. The district's efforts include the implementation of an anti-idling policy and an agreement with delivery vendors that helped solve indoor air quality problems created by air intake systems located near school loading docks. The effects have been remarkable, with one school reporting a drop in the rate of absenteeism among asthmatic students from 31 days to 2 days in the first year.

  • North Country Union High School (Newport) - North Country Union High School has implemented a policy to prohibit idling of school buses and delivery trucks on school property. NCUHS subsequently received national recognition with an award from EPA for excellence in its indoor air quality program.

  • Warren Elementary School - In Spring 2005, Warren Elementary School, formed a partnership with the Quebec-Labrador Foundation (QLF) to demonstrate an innovative approach to reducing school bus idling. A fuel-fired auxiliary heater was installed on a school bus and operated by an automated timer to preheat the bus's engine on cold mornings in lieu of extended periods of engine idling. The resulting benefits - significant reductions in fuel consumption, engine wear, maintenance and air pollution - were so impressive that the school is planning to purchase auxiliary heaters for all of its buses in the future.


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