A subset of air samples from a 1988 EPA study was reanalyzed for asbestos by TEM using an indirect transfer technique. The samples were originally analyzed using a direct transfer technique. The document presents the results of the reanalysis and extends the discussion to include data from six other studies. The development of the two techniques and their respective advantages and disadvantages are described. The data support the general opinion that TEM analysis of air samples using indirect transfer methods tends to provide estimates of total airborne asbestos structure concentration that are higher than those obtained using direct transfer methods. There is no single factor that can be used to convert measurements made by one method to a value that is comparable with measurements made by the other because the quantitative relationship is expected to depend on details of the sampling and analytical protocols and the nature of the asbestos in the air. The ratio of indirect measurements to direct measurements ranges from 3.8 to 1,700 for the studies considered. Additional research is needed to determine which transfer technique more accurately reflects biologically meaningful airborne asbestos concentrations. Breakdown of larger structures into smaller ones during indirect preparation does not appear to be sufficient to explain the difference in measured concentrations.

Includes bibliographical references. Spine title: Transmission electron microscopy using direct and indirect techniques. "EPA 560/5-89-004." "March, 1990." "EPA Contract No. 68-02-4294." Prepared for the Exposure Evaluation Division, Office of Toxic Substances, Office of Pesticides and Toxic Substances, U.S. Environmental Protection Agency.