Critical analysis of exposure assessment in the NIOSH-NCI Diesel Exhaust in Miners Study

In a recent series of published papers, researchers at NIOSH and NCI described a complex reconstruction of historical diesel exhaust (DE) exposures of miners employed at 8 underground mines.  A principal aim was to develop “retrospective estimates of exposure to Respirable Elemental Carbon (REC), a constituent of DE”, from the first year of mine dieselization (e.g., 1947) until 1998.  However, measurements of REC were not available prior to 1994.  Thus, it was necessary to utilize surrogate exposures as the primary basis for estimating historical REC levels. The reconstruction was conducted as follows:  First, the relationship between REC and carbon monoxide (CO) levels was determined in side-by-side area samples obtained during 1998-2001.  Then, historically measured CO levels were compiled for each mine.  Those historical CO levels were converted to REC levels, based on the REC:CO relationship determined for 1998-2001.  However, CO measurements were “almost exclusively” available beginning only in 1976.  Accordingly, earlier CO levels were “predicted” for each year and each mine on the basis of estimated annual diesel fleet horsepower (HP), estimated hours of diesel fleet operation, and estimated exhaust air flow. Mine-specific inventories of fleet diesel equipment and their corresponding HP ratings were available for only “a few years in the 1970s”; inventories were not available for earlier years.  Thus it was necessary to estimate historical fleet HP levels for most years.  Annual average total airflow rates were available for each mine overall, but not for specific areas of underground mines.  In addition, mine processes (e.g., quantities of fuel used, mining methods) and engineering controls (e.g., use of catalytic converters, preventive engine maintenance) were considered “if available”.  The components of this complex reconstruction are each associated with known or predictable uncertainty.  This presentation will detail the reconstruction methods, identifying their relative strengths and weaknesses.