Sources of interference in field studies of diesel exhaust emissions

This article describes interferences encountered in a variety of occupational settings during industrial hygiene surveys of diesel particulate material (DPM) using the NIOSH 5040 Method. The method yields time-weighted-average measurements of elemental carbon (EC), organic carbon (OC), and total carbon (TC = EC + OC). NIOSH recommends EC as proxy for DPM, but other agencies (e.g., MSHA) regulate exposure as TC. Surveys were conducted in an engine factory and a wood treatment plant where diesel equipment was used, and in a foundry where its use was being considered. Full shift samples were collected using open-faced cassettes and cyclones fitted with 37-mm quartz fiber filters analyzed by the NIOSH 5040 Method. Non-DPM-related interferences were noted for both the OC and EC. In the engine factory and wood treatment facility, OC measurements were very high (range of 10.0-1600 μg/m³), while EC levels were mostly below the LOD. These findings almost certainly reflect interferences by cutting oil mists and airborne creosote respectively. In the foundry, EC levels were high and comprised mainly of larger (>4 μm) particles (open face samples: arithmetic mean = 136 μg/m³, geometric mean = 74.0 μg/m³; cyclone samples: arithmetic mean = 30.2 μg/m³, geometric mean = 14.7 μg/m³). These findings suggest that OC interferences should be suspected if the EC:TC ratio is <0.35 and, if DPM surveys are performed with open-faced samplers, at least a small number of size-selective samplers should be employed to assure that results do not reflect EC interference by larger (i.e., >1-4 μm) particles. They also support the ACGIH decision to modify its proposed DPM TLV® to specifically consider elemental carbon, rather than total carbon.