Despite their small numbers compared to the total number of vehicles in California, diesel engines account for 40 percent of the nitrogen oxide and about 60 percent of the total particulate matter from mobile sources. The extensive scientific literature demonstrates that exposure to diesel exhaust increases the risk of developing lung cancer and other non-cancer health problems. In listing diesel as a toxic air contaminant, CARB determined that the increased cancer risk from diesel particulates could cause premature deaths in more than 14,000 Californians exposed to diesel pollution over a lifetime. Diesel exhaust contains hundreds of constituent chemicals, including many that are human toxicants, carcinogens, or present reproductive hazards. Forty chemicals in diesel exhaust are on California's list of toxic air contaminants, and California residents face high diesel-related health risks based on the heavy concentration of diesel truck traffic in urbanized areas and recent reports demonstrating that diesel cancer risks far outweigh cancer risks from other toxic air contaminants.
Diesel exhaust is a major source of particle pollution in California.
Ninety-four percent of diesel emissions are estimated to be fine particles, less than 2.5 microns in diameter, which can bypass respiratory defense mechanisms and lodge deep in the lungs. Numerous studies have found that fine particles impair lung function, aggravate respiratory illnesses such as asthma, bronchitis and emphysema, and are associated with premature deaths. Dozens of studies link airborne fine particle concentrations to increased hospital admissions for respiratory diseases, chronic obstructive lung disease, pneumonia and heart disease. Diesel is also a major contributor to ozone pollution in California. Ozone air pollution, generated by nitrogen oxide and hydrocarbons from fuel combustion, is a powerful respiratory irritant that may lead to shortness of breath, chest pain, wheezing, coughing, and exacerbation of respiratory illnesses such as asthma.