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 c. Analyte Choice Because diesel exhaust is a highly complex mixture, a surrogate of exposure must be selected. In the early nineteen-nineties, NIOSH researchers considering potential surrogates sought an overall measure of the particulate fraction because animal studies linked lung tumor induction to unfiltered diesel exhaust [1,3], and most (90% in one study) [30] of the exhaust’s mutagenic potency was associated with the particulate fraction. At the time, gravimetric methods for respirable combustible and submicrometer dusts were being used in mines, but gravimetric methods lack selectivity and are not suitable for low-level (e.g., < 200 µg/m3 ) measurements. Methods for characterization of the soluble organic fraction of diesel particulate matter also were available; others have since been developed. Although measurement of specific organic compounds, particularly genotoxins, may be relevant in characterizing the potential toxicity of diesel exhaust, a single compound or compound class would not reflect exposure to the particulate fraction—even if unique markers are found—because the composition of the exhaust is highly variable. Carbon is a logical exposure surrogate to monitor because diesel particulate matter is predominantly (typically more than 80%) carbon [31-33], but carbon in the organic fraction (i.e., organic carbon, or OC) of diesel particulate matter is not a selective measure because other sources of OC (e.g. cigarette smoke and other combustion aerosols, asphalt fumes) are present in many workplaces. Elemental carbon (i.e., carbon in the soot particle core, or EC) is a better surrogate [34] to monitor because it is a more selective measure of particulate diesel exhaust and still constitutes a sizable fraction (30%–90%) (see reference [28], Part I, Section 1) of the particulate mass. Fine EC particles are derived primarily from the combustion of fossil fuels, and diesel engines are major sources of these particles. Carbonaceous aerosols such as cigarette and wood smokes contain little, if any, EC [34, 35]. Gasoline engines emit far less EC than diesels, so the contribution of this source is relatively small. Other sources such as coal combustion, incinerators, and tire debris can contribute to the background (environmental) levels of EC, but diesel engines are the primary emitters (see reference [2]: Sawyer and Johnson; Cass and Gray). In occupational settings, where diesel equipment is used in relatively close proximity to workers, the contribution of these remote sources is negligible, especially when EC levels are well above background. Environmental EC concentrations typically range from about 1–3 :g/m3 [34]; workplace levels are generally much higher [36–43]. Although higher background levels (near 5 :g/m3 ) have been reported for more polluted U.S. cities (e.g., Los Angeles), the higher levels were attributed to diesel vehicles [44, 45] (see also reference [2], Cass and Gray). At one monitoring site (Glendora, CA), examination of the data collected at 1-minute intervals revealed that emission plumes from diesel vehicles located 50 meters from the site contributed up to 5 :g/m3 above the background level [46]. Materials such as coal dust contain EC (content depends on coal rank), but mechanically generated particles are larger (generally > 1 :m diameter) than combustion-based particles. Therefore, coal dust, and other mechanically generated dusts, can be effectively excluded

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