Most of Los Angeles' school districts are banning diesel fuel in new or replacement school buses. The districts plan to use a cleaner-burning natural gas to decrease the amount of smog produced by school buses. California's South Coast Air Quality Management District recently mandated that the State improve air quality in the Los Angeles area. The agency based its directive on a 1999 study that identified diesel fuel as the primary toxic substance contributing to poor air quality. Diesel fuel is responsible for 71% of airborne toxics in the Los Angeles Basin.

Initially, about 200 diesel school buses will be replaced with buses running on cleaner-burning natural gas. Within 20 years, all of the area's 8,500 diesel school buses will be replaced.

The smog-control agency is focusing on school buses due to rising health concerns over diesel exposure. Recent studies have found a correlation between diesel pollution and impaired lung growth in children.

Some of the Los Angeles Basin's smaller school districts oppose the agency's mandate. According to critics, the cost of phasing out diesel school buses will be significant. However, the program exempts school districts that can demonstrate that the cost of the replacement program is prohibitive.