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<2 -oc a ec: fi. D. Drummond - Wilmington Hovmmber 25, 196? CHARLES D. CROOK smii itbers OLD BZCKORT, TEW. METHOD OR KSPOBTTHJ ASBEST38 DOST LEVELS Tbit it in reply to your letter of October 2, 1969, Inviting consent on the propotition: "The bett method to rl>ort asbestos dutt level it fibert per milliliters longer than 5 microns in length." I have reviewed your letter and the diteuttlon which it contain* and Z have alto reviewed a letter written by D. D. Drummond to P. J. Tracey, Auguat 21, 1969, copies of which you and I received. I have teen the reference* cited in Drummond's letter and, in addition, have eeen the Threshold Limit Value* of airborne contaminant* adopted by AOQH for 1969 and the intended change*. The current Threthold Limit Value for aebeatoa dutt remain* at five million particlet per cable foot of air bated on lmpinger tamplea counted by light-field technique*. It it now generally recognised that thia level and thia method have outlived their uaefulneaa. Thia la reflected in tbt notice of intended change* (ACQXB, 1969) In which an option la preaented of uting two million partlelea per cubic foot of air baeed on lmpinger tamplea counted by light-field technique*, or limit of 12 fibert greeter than five micron* in length per milliliter of air at determined by tbe membrane filter atthod at A50 timet phase contrast magnification. The latter method of collectin' and counting Is meetlnc with increasing favor among those who are faced with the task of evaluating asbestos dust exposures because it gives a more direct measure of the agent which needs to be controlled in order to eliminate asbestos!*. DUP 1142253 C. D. Crook Pag* 2 It can be argu*d that a aound rational* vould edit for adopting 0.5 ppcf baaed on lmpinger samples counted by light-field techniques b*eaus* thia vould represent the application of a safety factor of four to one of the optional ACQZB levels. Vo go beyond this rationale, as the Construction Division has done, and utilising a level of 0.5 mppef as detazsined by the membrane filter method at fcJO tines phase contrast magnification is to introduce an Innovation vhich is In consistent vith the efforts at standardisation. I ooncur in the proposition that our best bet at the present time vould be to count asbestos particles greater than five microns in length at times phase contrast magnification on samples collected by membrane filter. DIGGED BY JAMES &. MORGAN JAMES J. MDBGAH JIM: Urn DUP 1142254 /-><? 3 fi.g-i.~- ~ rv-jfigiwHMdwIl- 4 ?*!stoa CfsJl DUP 1 H2255