Document rBMg2kGpy8VeZbMga8qKaYaNe

DEPARTMENT Of HEALTH. EDUCATION. AND WELFARE rusuc HKAUTH acnvics NATIONAL CtMTCA UAOAN AND IMOU|tAI NCAlTM 19I4AA040V4T QMONNATt, OMO AIMS March 27, 1968 fttm TO: Hr. L. V. Thatcher Safety Ihgineer &igineerlng Department Chambers Works Construction E. I. du Font de Nemours it Company Deepwater, New Jersey 08023 Dear Mr. Thatcher: We here been concerned with the problem of measuring asbestos dust ex posure since the beginning of our epidmaiological study in 1964 and Z am happy to give you our thoughts on the subject. the present Threshold Limit Value of 5 mppcf is applicable to counts obtained by the standard light field impinger counting method. Briefly, this method uses a 1QX objective to count dust collected in either a Qreehburg-Smith or Midget impinger. Although the ATI procedure prefers ths Grssnburg-Aaith, 1 believe that there is little differeace between tbs. This method is the only one which may be used with reference to standards end eodee based on the TLV. However, the impinger method cells' for the counting of ell dust, both grains and fibers, end thus gives an Index of overall dustiness. The Impinger is an Inefficient fiber collector and ths optical system used does not reveal the smaller fiber*. In ml vd dust environments, such as insulation, the counts will include many particles which are not asbestos. TO provide a more direct index of asbestos exposure, we have developed the membrane filter counting method described in the attached reprint. This method dess collect and reveal fibers end provides our best method at present of estimating fiber concentration. Some other methods which have been used are magnesias fnalysls for chrysotlls asbestos, Z-ray diffraction end light scattering dust counters. All of these have their disadvantages end we still prefer the membrane filter method la our* research despite the difficulties of dust counting. Based on simultaneous samples taken in asbestos textile mills, ws astlmate that - 30 fibers longer then 5 H per ee by mnabrane filter ere about equlvalot to the TLV of 5 mppcf by impinger. There is some question, however, about the safety of this TUT* The British, who use a similar fiber counting method, recommend e limit of below 2 fibers 1.2 G * DOTNDANTS - EXHIBIT ijm 120646 DUP 0900960 SC-DP-02240