The potential for fiber exposure during historical use of chrysotile-containing joint compounds (JCC) has been documented, but the published data are of limited use for reconstructing exposures and assessing worker risk. Consequently, fiber concentration distributions for workers sanding JCC were independently derived by applying a recently developed model based on published dust measurements from sanding modern-day (asbestos-free) joint compound and compared to fiber concentration distributions based on limited historical measurements. This new procedure relies on factors that account for (i) differences in emission rates between modern-day and JCC and (ii) the number of fibers (quantified by phase contrast microscopy [PCM]) per mass of dust generated by sanding JCC, as determined in a bench-scale chamber study using a recreated JCC, that convert respirable dust concentrations to fiber concentrations. Airborne respirable PCM-fiber concentration medians (and 95% confidence intervals) derived for output variables using the new procedure were 0.26 (0.039, 1.7) f/cm3 and 0.078 (0.013, 0.47) f/cm3, and corresponding total fiber concentrations were 1.2 (0.17, 9.2) f/cm3 and 0.37 (0.056, 2.5) f/cm3, in enclosed and nonenclosed environments, respectively. Corresponding estimates of respirable and total PCM fiber concentrations measured historically during sanding of asbestos-containing joint compound—adjusted for differences between peak and time-weighted average (TWA) concentrations and documented analytical preparation and sampling artifacts—were 0.15 (0.019, 0.95) f/cm3 and 0.86 (0.11, 5.4) f/cm3, respectively. The PCM-fiber concentration distributions estimated using the new procedure bound the distribution estimated from adjusted TWA historical fiber measurements, suggesting reasonable consistency of these estimates taking into account uncertainties addressed in this study.