The vast majority of America's school buses run on diesel fuel, and the exhaust from their engines contributes significantly to air pollution as well as presenting serious health risks to children. Many newer school buses (those manufactured after 2006) have improved engine efficiency and greatly reduced emissions when operated using low-sulphur diesel fuels.

For those older buses still on the road (a typical diesel engine can last twenty years or more), there are several types of pollution-reducing retrofits available.

Most of the particulate matter (PM) pollution inside school bus cabins appears to come from the engine crankcase. Closed Crankcase Ventilators (CCVs) reduce particulate matter almost to ambient air levels.

Diesel Oxidation Catalysts (or DOCs) are similar to catalytic converters used on cars. These can reduce diesel particulate matter from tailpipes by about 20% and hydrocarbons by about 50%.

The other retrofit is a diesel particulate filter, sometimes referred to by its trade name "CRT" for "Continually Regenerating Technology." This passive-design filter requires the use of ultra-low sulphur fuel, and can reduce tailpipe particulate matter by up to 95%.


Resources:

INFORM, a non-profit whose mission is educating the public about the effects of human activity on the environment and public health, has produced an excellent fact sheet called Diesel School Buses: Health Effects and Opportunities for Change.

The Clean Air Task Force (CATF) is a nonprofit organization dedicated to restoring clean air and healthy environments through scientific research, public education, and legal advocacy. Their Fact Sheet on School Bus Engine Crankcase Emissions and Controls provides excellent information.

The U. S. Environmental Protection Agency has a section of its web site devoted to diesel retrofit technology. It lists all of the currently available products, along with their test results.

The EPA also has a list of federal funding available for retrofit programs as part of its Clean School Bus USA program.