Governmental vehicle emission standards began in 1959 in California. The federal government became involved eight years later as Congress passed the Air Quality Act of 1967, which designated air quality regions throughout the country and gave states the responsibility for adopting and enforcing pollution control standards in those regions. In 1970, President Richard Nixon brought those responsibilities under one umbrella with the establishment of the Environmental Protection Agency. Since then, the EPA has regulated diesel fuel emissions on an almost annual basis.

Currently, engine manufacturers are operating under a 1998 consent decree that, among other things, mandated meeting the 2004 emission standards by October 2002.

But engine manufacturers already are looking forward to 2007 when the EPA's most stringent diesel emission standards ever will take effect. Diesel's most common pollutants -- Nitrogen Oxides (NOx), Particulate Matter (PM) and Hydrocarbons (HC) -- must be reduced to 0.20, 0.01 and .14 grams/brake-horsepower-hour, respectively.

See DieselNet and the EPA for more in-depth information regarding diesel emission standards.

The following table compares emission standards applicable for school buses from 1984-2007.

Heavy-Duty Emission Standards for Diesel Engines (g/bhp-hr)

*This is a combined Non-methane HC + NOx standard for heavy-duty diesel engines.