Document J3N3b02oVkYxYGD52wDw8aNQe

* Emventions, Inc. The following are com m ents pertaining to the FDA publication in the F ederal R egister, Vol. 33, #188, Septem ber 28, 1973, (Section 121.2006, paragraph c) of a method using polarizing light optics and media of selected indices of refraction to determ ine asbestos content in talc. We believe there are some serious technical lim itations of the method described and that an alternative approach of surveying with the scanning electron m icroscope should be used. The optical m icroscope has a practical resolution lim itation of 0.2 f \. At the m agnification of 400X described in paragraph c, the resolution is approxim ately 1 .0 ^ . These are seriously lim iting factors in the detection of asbestos fibers in talc. We know from electron m icroscopic studies that large num bers of asbestos fibers with diam eters between 0.025 )\ and 1.0 exist naturally and have possible biological significance. F ibers in this size range cannot be detected by the method described in paragraph c because of the lim ited resolving power of optical m icroscopes. In addition, talc is a m ineral which is crushed and processed to give it certain size characteristics. We know also by electron m icro scopic studies that naturally occuring asbestos fibers, particularly larger ones, can be broken down into sm aller fibrils by the sam e treatm ents and that these fibrils may then be undetectable by the optical m icroscope Emventions, Inc. requirem ent of 5.0 /q. The SEM survey method has been used by the Environmental Protection Agency and the Food and Drug A dm inistration as a survey tool for asbestos and should sim ilarly be used in surveying talc for asbestos content. The method is direct, economical, and m ore accurate than the method described in paragraph c. T ransm ission electron m icroscopy (TEM) of talc is generally not acceptable as a direct survey method because of the frequency of fibers lying on platelets; such fibers are not observed by TEM . This is also a lim itation of the m ethod described in paragraph c. In the SEM, however, all fibers lying between and on talc platelets are easily observed. The SEM survey method does not positively identify a fiber as being asbestos. We know, however, that with a suitable set of standards, the asbestos content in a particular sam ple could be definitely and accurately established below a certain level by m orphological characters alone. M oreover, the ability to do X -ray m icroanalysis on fibers enhances the accuracy of such a m ethod. T alc contamSincaantendinwg iEthle1c%troansbMe sitcorsogfirbaeprhss (c h ry so tile ). 0* 4 Figure 3. 5 ^ (7200X)|------------------------------------- j Figure 4. I---------------- 1 1 H (18, OOOX) T alc contamSciannanteindgwEitlhec1tr%o na sMb eisctroosg rfaibpehrss (c h ry so tile ).