There is evidence that diesel exhaust causes cancer and premature death, and also exacerbates asthma and other respiratory illness. Noting that the vast majority of the nation's school buses run on diesel fuel, this report details a study examining the level of diesel exhaust to which children are typically exposed as they travel to and from school. Further, the report is intended to inform parents, administrators, and policy makers of the cleaner alternatives that are readily available. The air was continuously sampled inside four elementary school buses that drove an actual bus route of about 45 minutes for 4 to 6 repetitions over 5 hours. Nearly 20 hours of sampling results were obtained. Findings indicated that children riding inside a diesel school bus, even buses not emitting significant amounts of black smoke, may be exposed to as much as four times the level of toxic diesel exhaust as someone standing or riding beside the bus, translating to from 23 to 46 times the cancer risk level considered significant under federal law. The report details the harmful health impact of diesel exhaust, describes cleaner alternatives being used by an increasing number of school districts, and notes that short-term measures such as particulate traps have not been proven to decrease risks to children and are available in only a limited number of areas. Funding sources are identified to help public and private schools purchase cleaner alternatives. The report concludes with recommendations for districts, government agencies, and parents. The report's eight appendices include a study protocol, an estimation of potential cancer risk to children in school buses, emissions comparisons for various school bus models, and sample letters for advocacy purposes. Contains 146 end notes. (KB)