This paper presents the results of 810 pre-project baseline samples collected over four years, and 7,210 off-site samples and 14,314 perimeter samples collected over seven years during the Calaveras Dam Replacement Project (CDRP). The principal asbestos particles were chrysotile from serpentinite and glaucophane-winchite amphibole from blueschist. The baseline data showed that asbestos concentrations are station-dependent, and are influenced by the location on or near naturally occurring asbestos (NOA)–containing units, duration and intensity of disturbance, wind direction, and time of year. The increase of background over the time period shows that baseline sampling cannot be assumed as a measure of “background” or “ambient” conditions. Project perimeter and off-site data also revealed a strong influence by these variables, particularly the composition and location of NOA-bearing lithologic units. To differentiate local source impacts from the primary CDRP asbestos emissions, the data were analyzed independently for chrysotile and amphibole, and by composition of the amphiboles that comprise each unit. Of particular value was the comparison of the calcic-group amphibole to total amphibole ratio measured in air samples. This analysis delineated three asbestos sources: (1) alluvium in the Sunol Valley with high calcic ratios, (2) imported road surfacing aggregate with moderate calcic ratios, and (3) blueschist with low calcic ratios. When the data were sorted by wind direction, the analysis showed that the contribution of CDRP-generated asbestos to monitoring stations was significant near the point of disturbance only and did not significantly affect off-site stations that were located at or near sensitive receptors. The data also emphasize the shortfalls of applying a commercial definition of asbestos on large scale and long-duration construction projects that disturb NOA. Excluding amphibole particles that are compositionally deviant from those applied to building materials may result in a data set that is not representative of the minerals that actually are disturbed and may lead to a severe underestimation of exposure to workers and off-site receptors.