Comments on the Diesel Exhaust in Miners Study.

We are writing in response to four recent articles describing the historical reconstruction of diesel exhaust exposures in underground mines (Diesel Exhaust in Miners Study or DEMS) for use in epidemiological analyses of exposure–response relationships between mining-associated diesel exposure and health effects, e.g. lung cancer (Coble et al., 2010; Stewart et al., 2010; Vermeulen et al., 2010a,b). We have particular concerns about the estimation of historical respirable elemental carbon (REC) levels for underground miners. Those estimates were back extrapolated using historical carbon monoxide (CO) area measurements and estimated levels of historical ‘adjusted horsepower (HP)’ of diesel fleets to estimate levels of CO as surrogates of diesel exhaust (DE). As discussed below, those measures are at best imprecise and may not be valid measures of DE. Accordingly, we have concerns about the validity of the articles’ conclusions, which we believe are insufficiently justified. The following communication briefly describes those concerns. 1. Measurements of CO by colorimetric tubes are imprecise and may be unreliable. The historical reconstruction relied on CO data obtained in the DEMS survey, a 1994 Feasibility study, and a compilation of MSHA data from 1976 to 2001 (MIDAS). In the DEMS and Feasibility studies, CO measurements were obtained using long-term (i.e. 8 h) colorimetric tubes. The MIDAS surveys used short-term colorimetric tubes (i.e. 5–15 min) or gas chromatography, but the numbers determined by each of those methods were not described. The precision of CO colorimetric tubes is limited, especially at low exposure levels. Prior to the NIOSH certification program, colorimetric and length-ofstain detector tubes were considered ‘inaccurate’ (Perkins, 1997). Early studies reported that over the range from 25 to 100 p.p.m. , all commercially available CO detector tubes were worse than –25% of the true value, while only some brands yielded results within –50% of the true value (Morgenstern et al., 1970). CO tubes are currently required to yield results –35% the true value at 12.5 p.p.m. , but precision and accuracy decline as concentrations approach the limit of their recommend use ( 5 p.p.m.). Because of imprecision, inaccuracy, and observer variability, historical and current authorities agree that CO colorimetric tubes should only be used to detect the presence of CO and for range finding purposes, not for quantitative measurements (WHO, 1976; Stern and Mansdorf, 1999; Todd, 2003) and that after CO detection ‘a more accurate . method’ should be used (Lodge, 1988). In 1976, the year MIDAS surveys began, WHO recommended that detector tubes only be used ‘for estimating the concentration of CO at concentrations above 5 mg/ m, (i.e. 4.35 p.p.m.) (WHO, 1976). Accordingly, it is notable that the great majority of CO measurements in the DEMS reports were below 4.35 p.p.m. We are also concerned by a table footnote indicating that some CO values were ‘corrected for measurement technique (detector tube versus bistable)’, but the reports described neither the method for and effects of such ‘correction’, nor the number of samples so ‘corrected’. Thus, the CO measurements are subject to still further uncertainty. 2. The majority of CO measurements were below the recommended range for use of colorimetric tubes. The reports do not describe the actual CO measurements across the seven mines, but summary statistics were provided for samples obtained at the underground production face. Across the seven mines in the DEMS survey, the geometric means of CO samples ranged from 0.8 to 4.5 p.p.m. Among historical production face CO measurements, the geometric means across all mines grouped by decade ‘typically’ ranged from 1 to 3 p.p.m.