Effective Collection Area: A Potential Error in Asbestos Air Monitoring

Abstract

The current National Institute for Occupational Safety and Health method to measure airborne fiber concentration has one of the highest variability levels of the analytical techniques used by industrial hygienists. It is common practice to assume that the effective collection area (ECA) of a 25-mm filter is 385 mm². However, measurements of the inner diameter of filter cowls cast doubt on this assumption. Thus, a laboratory study was conducted to characterize the ECA of nine cassette types. The effective filter areas were blackened by sampling a carbon black aerosol. Computer image files were created from photocopies of the filters by means of a scanner at a resolution of 600 dots/inch, and the numbers of black pixels in the image files were determined. Areas were calculated by comparing the numbers of black pixels in filter images with the numbers in the images of circular area standards. Three cassette brands were found to be significantly larger than 385 mm². Assuming that the number of fibers per counting field is inversely proportional to the ECA, the use of 385 mm² in calculations can lead to the underestimation of airborne fiber levels by an average of 4, 5, and 9 percent for these three brands. The coefficient of variation of ECA ranged from 0.09 to 3.49 percent for the cassette types tested and thus probably contributes only slightly to method variability. Also, it was concluded that ECA could not be accurately predicted from the cross-sectional area of the cowl.