In an earlier investigation, computer-controlled scanning electron microscopy (CCSEM) and X-ray diffraction (XRD) were used to investigate respirable quartz in several bulk coal fly-ash samples. It was shown that the XRD result for quartz in bulk fly-ash was much larger than the amount of respirable quartz determined by CCSEM because the XRD signal was derived primarily from quartz particles too large to be classified as respirable. In the current paper, CCSEM and XRD were used to investigate coal fly-ash collected using cyclone-filtered personal and area samplers at two different power plants during maintenance activities. The amounts of respirable quartz determined by the two methods were found to be in good agreement, confirming the need for cyclone-filtered samples. The amounts of respirable quartz determined by CCSEM ranged from 1.5 to 3.5 vol.% for the plant burning a Southwest sub-bituminous coal and from 0.6 to 1.9 vol.% for the plant burning a Mid-West bituminous coal. Very small amounts (average similar to 0.2 vol.% for both plants) of Si-rich glass or amorphous silica were also detected by CCSEM. This paper is important because of the health implications associated with inhaling quartz particles. It demonstrates that the amounts of respirable quartz to which power plant workers are exposed can be accurately measured by two very different measurements on samples collected in cyclone-filtered samplers designed to emulate the inhalation process of human airways. (C) 2011 Elsevier Ltd. All rights reserved.