Document 2J6QZq5GVpXGEMnoqE05Z1Ojb

THE METHOD USED BY THE U.S. PUBLIC HEALTH SERVICE FOR ENUMERATION OF ASBESTOS DUST ON MEMBRANE FILTERS C. H. Edwaxds and J. R. Lynch CS. Department of Health, Education, and Welfare, Public Health Service, Bureau of Disease frrvenuoe and Environmental Control, National Center for Urban and Industrial Health, Occupa tional Health Program, 1014 Broadway, Cincinnati, Ohio (Rectivtd 30 May 1947) Abstract--Samples of air-borne mineral dusts are collected by drawing air through cellulose ester membrane filters. Particles smaller than nominal pore size are retained. Varying sampling rate and filter holder controls the distribution of dust across the filter face. Samples are mounted on standard microscope slides using a high viscosity solution of membrane filter material in a mixture of diethyl oxalate and dimethyl phthalate to render the filter transparent. The mount is ruble for 28-30 days, but after this period migration of the dust particles causes a loss of areal concentration. The grain and fiber dust particles, which lie on the surface of the filter, are counted with a phase contrast microscope, using 10X eye-picas and a 4 mm "high-dry" objective. The field area is delineated with a Porton graticule. Count data are recorded in a form allowing data (eduction and statistical analysis by electronic computer. The method, although not suitable to determine compliance with the threshold limit value for asbestos that was developed from a different enumeration method, is highly suited for developing basic dau for a more precise recommendation of the limit for asbestos. INTRODUCTION Is the early epidemiologic studies of the asbestos industry by Fulton (1935) and Drilsen (1938), the impinger was settled upon as the standard method for appraising dustiness. Since few libers were seen it was necessary to count both grains and fibers t obtain a statistically useful count despite the assumption that asbestosis was caused by fibers. This inconsistency notwithstanding, dust control based on impinger results has considerably reduced asbestosis where it has been conscientiously applied in the asbestos textile industry. However, a number of investigators (Lynch and Smith, 1955; Doll, 1955) have suggested other health hazards associated with asbestos, and a more relevant and sensitive method is required for estimation of the hazard, especially in industries using only part asbestos in their product. A further weakness in the impinger method is that it is not amenable to long personal samples. It is believed thiu personal samples best approximate the worker's actual exposure, and Roach (1966) has recommended a sample duration of 10 work shifts for mineral dusts. New methods were therefore needed, and among those studied were grain and 1 CHEVBB 0003051 2 C. H. Edwards and J. R. Lynch fiber counts at 430X phase contrast on membrane filters that had been made trans parent. This paper deals with the development and standardization of this method. The principle variable which needed to be considered was the mounting procedure in relation to maximum transparency and the effect of time on the dimensional stability of the mount Methods using immersion oil, airplane glue or acetone were found to be either too cumbersome or resulted in an imperfectly cleared or mottled filter, particularly as viewed with phase contrast optics. In methods which involved the application of liquid over the dust deposit the movement of the meniscus across the deposit sometimes caused redistribution of the dust. The use of a 50-50 mixture of diethyl oxalate-dimethyl phthaiate was attempted and found to result in satis factory clearance. However, since this medium dissolved the filter, the migration of particles resulted in changing counts with time and needed to be examined to evaluate the method. SAMPLING PROCEDURE Samples for count are collected on Millipore type AA filters* by personal samplers operated by battery powered pumps worn by the employees, and as simul taneous samples collected in conjunction with impingers. The filters are contained in plastic filter holders and are supported on thick pads which also aid in controlling the distribution of air through the filter. Several methods of admitting air to the filter have been attempted. With the face cap removed and the filter completely exposed, the air may enter the filter evenly, but large high velocity particles and foreign objects may also hit the filter and possibly cause damage. As an alternative, the face cap may be left in place and the small plug removed. The passage of air through this 4 mm opening at a flow rate of 2 1. per min produces a velocity of 265 cm/sec, which yields an impaction parameter of 0*081t for 1 micron (}i) unit density spheres. Although this impaction parameter would predict a very low collection efficiency of lp particles by impaction, some larger particles would tend to be impacted on the center of the filter. This was found not to affect count distribu tion, which is dominated by small particles; however, it does result in obscuring a small portion of the filter, particularly if many large particles are present, as in friction product plants. The third alternative was to drill 6 additional 4 mm holes in some face caps and to fit these perforated caps to the filter holder during sampling. This reduced the velocity to 38 cm/sec thus eliminating the impaction problem while still protecting the filter from damage. MOUNTING THE SAMPLE The mounting medium used in this method is prepared by dissolving 0*1 g of membrane filter per ml of a 1:1 solution of dimethyl phthaiate and diethyl oxalate. The exact proportions of the 3 components are not critical, but the medium must have as high a viscosity as possible without being difficult to handle. The index of refraction of the medium thus prepared is Nd " 1*47. */ Millipore filters are named in this paper since the method was developed using this brand of filters. Similar techniques may be developed for other filter brands. tRanz. W. E. and Wong, J. B. (195?.) Ardu. md. Hyg. 5,462. CHEVBB 0003052 Fig. 1. The wedge-shaped piece of membrane filter is placed dust side-up on a drop of mounting medium which has been applied to a standard microscope slide. The mount will be covered with a #It covenlip. UMmgp.1 3HEV BB >003053 tut u*- -* -- * i.i*' iiwi mm un membrane iutert 3 To ,.epare a sample for microscopic examinaw.4, a drop of the mounting medium is placed on a freshly cleaned standard (25 mm x 75 mm) microscope slide, using a dropper or applicator. The volume of the drop is approximately 0*05 ml. A wedge-shaped piece about 1 cm x 2 cm is excised from the filter using a scalpel and forceps, and placed dust side up on the drop of mounting solution (Fig. I). A #11 coverslip carefully cleaned with lens tissue is placed over the filter wedge. Slight pressure on the coverslip achieves contact between it and the mounting medium. Gearing of the filter with this method is slow, requiring about 15 min. The sample may be examined as soon as the mount is transparent. The transition of the dust-supporting substrate from translucent solid membrane filter cellulose ester material to a transparent, optically homogeneous gel is very gentle, and at no time does a meniscus sweep across the dust deposit. Rather, the substrate slowly softens beneath the dust, and there is no opportunity for washing away or redistribution of the dust The optical homogenity of the resulting mount is nearly perfect with only a slight background granularity under phase contrast which disappears within 1 or 2 days. STABILITY Observers noted a reduction with time in apparent concentration for samples mounted with only dimethyl phthalate and diethyl oxalate. By observation, this loss was due to the migration of the dust particles outward from the center through the gelatinous material of the fixed mount, with a decrease in areal concentration. The cause of this migration was not determined but may be related to a disordering or distortion of the membrane filter polymer caused by partial solution. To determine the magnitude of this loss, a group of samples was mounted using 1:1 dimethyl phthalate and diethyl oxalate only, and counted at 1-3 day intervals for periods ranging up to 76 days. Counts were displayed as fractions in per cent of the original, or first day counts (Fig. 2). At the end of 1 week, samples mounted by this method yielded only 75 per cent of their original values. By 30 days after mounting. F. 2. Decrease in percentage of original particle count with time for the original lzldimethyl phtfialate-dirthyl oxalate mixture and for the high vinaity mixture awming q.1 g/ml of membrane filter matcriaL CHEV BB 0003054 4 C. H. Edwamb aad J. R. Lynch the counts usins this method had declined to 40 per cent of their first day value, and continued at this level with no further decline until counting was terminated 76 days after mounting. . A group of samples mounted in the method described in the section Mounting the Sample wing high-viscosity mounting medium were counted at 1-day intervals for 11 days. No loss of areal concentration was apparent over this short period and a second group was similarly mounted and counted for a period of 46 days. The composite of these two series is shown in Fig. 2. No loss of concentration was detectable during the first 30 days, but the count declined to 85 per cent of its original value by the 46th day, when the series was terminated. A 6 mm square of membrane filter with a vacuum-deposited film of carbon was mounted in the manner described to provide a macroscopic observation of the distortion of the filter areas. After a few days, the square began to disintegrate at the edges, but the major portion showed no discontinuity during the initial 30 day period. By the end of 62 days, most of the square had disintegrated and the fragments migrated over an area 16 nun x 17 mm. EVALUATION The filter samples mounted in the manner previously described are evaluated in terms of grain and fiber concentration, and fiber size. Binocular research microscopes equipped with fixed mechanical stages have been used. Phase contrast optics fined to these instruments include an Abbe condenser with rotating phase turret and a 4 mm "high-dry" achromatic objective wed for all membrane filter determinatiow. In all cases 10X Huygens eyepieces, one of which contained a Porton reticle at the level of the field-limiting diaphragm, were used. The left half of the Porton reticle field served to define the counting area or field. A ribbon filament illuminator was critically adjusted to provide Kohler illumina tion through a clear blue filter. Twenty fields located at random on the sample were counted and total grains, total fibers, fibers greater then SfL, and fibers greater than I0p were recorded. Any particle having an aspect ratio of 3 or greater was considered a fiber. RECORDING OF DATA Since it was recognized early in the study of the asbestos products industry that the large number of samples to be collected would present a burdensome data pro cessing problem, the data were collected on forms designed for transcription to punch cards (Fig. 3). Computer programs were then written to calculate dwt concentrations and to produce statistical summaries consisting of mean, standard deviation, median, etc., for any group of samples by plant, operation, job, type, etc. These programs, written in Automath (Fortran II) are applicable to any problem wing the sample and count forms as source documents. Additional possible are correlation and regression of simultaneow samples and Poisson index of dispersion of field counts for various counters. CHEVBB 0003055 Th* method u*ed for enumeration eS RsbesJc* dust on membrane fiJim natt res t/*4 MO. BAT VtAR FLANT NO. Jiunt NO. fill! I' l CCD QD 10 II a <4 OBJ TVBI mature count count. rower n.no. E3ESZ3E3 17^ W IR T W *22 U2 m n trmmH.UMMNAnOM MAINS FMERS > S 10 NQCTS. 2> MT BLANK MM 37 BAINS M43 4* 4B SS TOTAL FIBERS >i LA >10 sr ..... . blank NO. 30 73 74 7B Fio. 3. Count tabulstion form used for the evaluation of dust samples taken on membrane filters. Membrane filter, U-- Elutriated membrane filter, it-- Breathing zone sample, G" General ample, i*-- Personal ample, 5 -- Sequential sample, C -- Count (1st), Jt Recount. COUNT provides for the tecfaniciaa's initials, OBJ TOWER indicates the magnification, M. NO provides for the microscope number for calibration purpose!, and IF and FC are light-field and phase contrast. SUMMARY A method for sampling and counting asbestos dust using membrane filters has been developed and evaluated. The mounting medium developed clears the filter gently so as to not redistribute the dust deposit, and produces a mount with adequate dimensional stability for a period of 30 days. This method has been used in developing environmental data in connection with the Public Health Service study of the asbestos products industry, and is recommended for research studies in other dusty trades although not for application to Threshold Limit Values. A system of data handling involving electronic computer processing, which is also applicable to other studies, was discussed. CHEVBB 0003056  C. H. Edwamh and J. R. LmcH " ~ 'REFERENCES Oou. X_ (1933) Br. /. Imt. Mot 12. Damn, W. G, 0*1X4 Value, J. ML, Eowaom, T. L. Main, J. M. and Savuu, X. K (193H) Pubi. HUM. Bull. So. 241, US. Gwcmment Printing Office, Wub, D.C Fuuon, W. B- Doouy. A, Matthcws, J: L. and Hours, R. L. (It3S) Spatial Butt. 42, Non. Dept, of Labor and Industry. Lynch, K. M. and Stone. W. A. (193:5) ^jh. J. CmKtr 24, 5C-64. Roach. S. A. (1966) Am. M. /for. A*uJ. 27,1. :hev bb )0030S7