NIOSH Manual of Analytical Methods, Fourth edition. Third supplement. Cincinnati, OH: National Institute for Occupational Safety and Health, 2003 Mar; :229-259
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 components can be significant contributors to the measured EC. For this reason, an analytical protocol that maximizes removal of refractory OC components (and carbonates) and corrects for the char formed through thermal decomposition is important. In the NMAM 5040 analysis, the transmittance of some samples continues to decrease as the temperature is stepped to 870 °C in helium, which indicates charring is not always complete at the lower temperatures used in some methods. Partial char loss is sometimes observed (at 870 °C), but loss of original EC is uncommon. Good interlaboratory agreement between OC-EC results has been obtained by laboratories using NMAM 5040. Because a certified reference material is not currently available, confirmation of results in a second laboratory using NMAM 5040 is advisable, particularly when samples char during the analysis because interlaboratory results for these are typically more variable. Organic materials that char are useful as quality control samples. Accurate pyrolysis correction was obtained in the NMAM 5040 analysis of an alginic acid solution spiked onto blank filters and diesel soot samples. Sucrose has traditionally been used as a check on the method's char correction, but alginic acid is more representative of complex, plant-derived components present in some air samples. To ensure data quality, participation in proficiency testing among laboratories involved in major studies also is advised. At present, seven commercial laboratories (six in the United States and one in Canada) offer the NMAM 5040 analysis, and over 60 instruments are available globally for environmental and occupational monitoring. Method standardization is critical if results obtained by different laboratories are to be compared. Interlaboratory studies are useful in exposing differences among methods, but results of such comparisons must be interpreted with a clear understanding of each method's limitations (e.g., in dealing with interferences). Additional research is necessary to address the potential health effects of diesel exhaust and other types of fine particle air pollution. Although the organic compounds associated with diesel particulate matter have potential health effects, those traditionally measured are not unique to diesel exhaust. Nevertheless, characterization of this fraction may be useful in assessment of exposure risks, particularly if compounds enriched in diesel relative to other particulate emissions can be used as indicators of mutagenic potency (e.g., specific nitro- PAH). This information also may reveal engine types or operating conditions that produce higher emission rates of genotoxic compounds. Undoubtedly, much will be learned through additional research. As reasoned previously , regardless of whether the potential adverse health effects of diesel particles are due to the carbonaceous cores, adsorbed compounds, or a combination of both, monitoring and control of the particulate component are necessary if effects exist that are particle related. In most workplaces, diesel engines are the primary source of fine-particle EC. Other combustion sources may contribute to environmental levels of EC. These sources may be relevant from an emission control perspective, but if the potential toxicity of these particles is similar, their origin is not relevant from a health perspective.