Noncancer health effects have also been associated with diesel exhaust exposure, including immunologic, respiratory, and cardiovascular effects. Diesel exhaust particles can act as nonspecific airway irritants at relatively high exposures. At lower levels, they can trigger release of mediators (cytokines, chemokines, immunoglobulins, and oxidants) of allergic and inflammatory responses [8]. Diesel particles may promote expression of the immunologic response phenotype (Th2) associated with asthma and allergic disease and appear to have greater immunologic effects in the presence of environmental allergens. Internationally, the prevalence of asthma (and related hospitalizations and mortality) continues to rise in adults and children. Children may be more vulnerable than adults [9-12]. Studies indicate children living along major trucking thoroughfares are at increased risk for asthmatic and allergic symptoms. In the United States, the number of individuals with self-reported asthma increased by 75% from 1980 to 1994 [13]. The immunologic evidence is consistent with results of epidemiologic studies that associate traffic-related air pollution, especially diesel exhaust particles, to an increase in respiratory diseases. Studies have consistently found positive associations between particulate air pollution and daily mortality [14–23]. The traditional U.S. air quality standard for particulate matter is based on particles having diameters # 10 µm (PM10 ) [24]. In 1997, EPA proposed a new standard (see http://www.epa.gov/airlinks/airlinks4.html) based on particles having diameters # 2.5 µm (PM2.5 ). These smaller particles originate mainly from combustion sources. The new standard was proposed because recent studies show higher correlation between fine particle pollution and adverse health effects. In an analysis [19] of data from six U.S. cities, fine particles were consistently associated with increased risk of death from chronic obstructive pulmonary disease, pneumonia, and ischemic heart disease. Positive associations between fine particle pollution and hospital admissions due to respiratory and cardiovascular illness also have been found [25–27]. Particles produced by combustion sources have been implicated in these findings. In addition to asthma, chronic inhalation of diesel exhaust particles may play a role in these adverse health outcomes. Comprehensive reviews on the health effects of diesel exhaust can be found elsewhere [1-7, 28, 29].
b. Composition Diesel engine exhaust is a highly complex and variable mixture of gases, vapors and fine particles. The amount and composition of the exhaust vary greatly, depending on factors such as fuel and engine type, maintenance schedule, tuning, workload, and exhaust gas treatment. The gaseous constituents include hydrocarbons and oxides of carbon, sulphur, and nitrogen. Particulate components consist of liquid droplets and soot particles bearing organic compounds, sulfates, metals and other trace elements. The organic fraction (droplets and particle adsorbed) is mainly unburned fuel and oil, but also contains thousands of compounds (e.g., aldehydes; polycyclic aromatic hydrocarbons [PAH])—some of which are genotoxic. Detailed information on the composition of diesel exhaust has been published previously [1-4, 28].
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NIOSH Manuual of Analytical Methods
