Development of custom calibration factors for respirable silica using standard methods compared to photometric monitoring data

The goal of this study was to investigate the relationship between particulate matter (PM) concentrations recorded by real-time monitors and respirable crystalline silica, in the form of quartz, (RCS) concentrations measured by the standard NIOSH 7500 method using an aluminum cyclone attached to a filter-cassette. Development of correction factors for real-time monitoring instruments would provide instant estimates of RCS concentrations in order to more immediately assess worker exposures and efficacy of controls implemented for worker health and safety. Sampling occurred in two environments: (a) during iron casting at a foundry in the Western United States and (b) in a controlled testing environment, using Arizona Test Dust (ATD) in a low-speed wind tunnel. Two DustTrak DRX 853 monitors were used to collect area samples of PM1, PM2.5, PM4, PM10 and PMtotal, and two SidePak AM510 monitors were used to collect personal samples of PM2.5. Calculated r-squared (R-2) values analyzed how well paired data from real-time monitors and filter-based methods fit a line of regression, and how well real-time monitor measurements predicted RCS concentrations. Among the different PM size fractions recorded by the DRX monitors, the highest coefficient of determination was between RCS quantified by XRD and PM10 for DRX-1 and DRX-2 (both R-2 = 0.80), however, their respective regression equations differed. The coefficients of determination calculated for each SidePak were 0.19 and 0.016 respectively, demonstrating a poor ability for these monitors to predict RCS concentrations. There was variability among the identical real-time monitors, especially among the regression equations. Results of this study indicate that it is reasonable to determine a calibration factor for DRX direct-read monitors, enabling them to estimate concentrations of RCS in real-time. Dust containing quartz percentages that are significantly different would require calculation of separate calibration factors. Successful development of silica-dust calibration factors from this study is not intended to replace either the OSHA or NIOSH standard silica sampling and analytical methods. Future work in this area should include using test material with a known, homogenous concentration of silica in a controlled environment to more accurately determine device-specific calibration factors.