Cleaning up dirty school buses

Every day, half a million school buses safely carry 24 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. 

* 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

Retiring a bus after 10 years is also a good rule-of-thumb. School buses that are not meeting 2007 federal engine standards should be retrofitted with diesel particulate filters (DPFs) and crankcase ventilation systems (CCVSs), cutting at least 85 percent of soot pollution.

School districts can apply for outside funding. To alleviate some of the economic burden for bus upgrades, school buses built after 2002 that have been retrofitted with DPFs and CCVSs could stay in service for the lifespan of the DPF even if this exceeds the 10-year retirement age. Typically, DPFs last about 10 years.