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 TLV would be ten times higher than the OEHHA REL. The ACGIH withdrew diesel exhaust particulate, as EC, from the 2002 Notice of Intended Changes (NIC) and placed it under study [86]. An EC (or TC) standard, rather than submicrometer particulate mass, would simplify interpretation of the analytical results because the target analyte and exposure standard would be the same. This would eliminate the problem of extrapolation (to submicrometer mass), which is not straightforward and introduces unacceptable analytical error. As discussed previously, OC interferences are an issue when making TC measurements; variable EC content is a problem when measuring EC. If EC concentrations are high and samples are relatively free of OC contaminates, the TC concentration is a reasonable measure of the diesel particulate mass, and the EC:TC ratio is representative of the EC fraction of the mass. Unfortunately, this situation is uncommon outside of mines. In general industry, EC and OC levels are normally much lower. When EC levels are low, the EC:TC ratios are not reliable estimates of the EC fraction of diesel particulate matter because OC interferences can skew the ratios low and increase variability in the apparent ratio. In such cases, TC is an inaccurate measure of the diesel particulate concentration. Exposures in the mining industry are of particular concern because diesel particulate concentrations in mines [62, 83, 84] sometimes exceed 2 mg/m3, which is one thousand times higher than a typical environmental level. In its final rule [62] on metal and nonmetal mines, MSHA proposed an interim exposure standard of 400 :g TC per cubic meter of air. Five years after publication of the rule, a final standard of 150 µg TC per cubic meter was to apply. In response to a legal challenge, MSHA recently initiated limited new rulemaking [85] to revise certain provisions of the final rule. Among other amendments, MSHA agreed to propose a change of exposure surrogate from TC to EC. Comments on appropriate interim and final limits for EC were requested in the new rulemaking. MSHA exposure standards consider health risks and technical and economic feasibility, based on available engineering controls and their cost.

5.

SUMMARY NMAM 5040 is based on a thermal-optical analysis technique for particulate carbon. Its intended application is assessment of workplace exposure to particulate diesel exhaust, but thermal-optical analysis has been routinely applied to particulate carbon measurement in general. Both OC and EC (and TC as OC + EC) are determined by the method. For reasons discussed in this chapter, EC is a better exposure surrogate than TC. Over 30 epidemiological studies are consistent in finding a positive association between exposure to diesel exhaust and lung cancer, but quantitative exposure data are lacking. Accurate monitoring methods are necessary in quantifying the risks of exposure. The thermal-optical instrument incorporates an optical feature that corrects for the char formed during the analysis of some materials. EC results of purely thermal methods show positive bias when samples contain materials that char. Although minimal charring has been observed with most diesel soot samples from mines, workplace (and environmental) air often contains components that char. Depending on the workplace and thermal protocol, these

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