Document M92NJn5KK5vdzvQYanQJ1mx

miLLIPORE Monitoring Airborne Asbestos with the Millipore Membrane Filter Prolonged exposure to high levels of air borne asbestos dust has been linked to a number of occupational diseases, in cluding asbestosis (pulmonary fibrosis), mesotheliomas and certain lung cancers. Recognition of this hazard has prompted many countries to impose strict limits on exposure, based on the number of fibers per milliliter of air. The United States, for instance, enacted regulatory legisla tion under the broadly worded Occupa tional Safety and Health Act of 1970. The standard for exposure to asbestos dust, adopted under this act, prescribes the use of the membrane filter method for the determination of asbestos fibers. The current standard states that the 8hour time-weighted average (TWA) air borne concentration of asbestos dust to which employees are exposed shall not exceed 5 fibers longer than 5 micrometers (/tm) per milliliter (Federal Register, Vol. 37, No. 467). However, the standard was amended (Federal Register, Vol. 37, No. 110) June 7, 1972, to read: "As of July 1,1976, TWA concentrations of asbestos fibers longer than 5 micrometers will not be allowed to exceed two fibers/ml, with a ceiling value of 10 fibers/ml." THE MEMBRANE FILTER METHOD The standards differ from country to country, but the most generally accepted method of monitoring airborne asbestos is based on use of the membrane filter for sample collection. A vacuum pump is used to draw a measured,,volume of air through the filter.-Because of the uni formity of its pore openings, a Millipore* membrane filter retains on its surface I##*/. f all fibers or particles larger than its rated pore size (e.g.,, #.5 jtm). These fibers are retained in a single plane, so they may be examined readily under the microscope.. The filter is then mounted and examined under a micro scope to determine the size distribution and number of fibers in the sample. Fig. 1. For personal sampling, the Millipore Aerosol Monitor is usually clipped to the worker's clothing. Vacuum is provided by a battery powered pump worn in a special belt. (Photo courtesy of Raybestos-Manhattan, Inc., Manheim Division, Manheim, Pa.) Application Procedure AP501 50^4'0Z7 3 1. Check the flow rate of the pump (see manufacturer's recommendation regard ing frequency of calibration) and be sure the battery has been fully recharged. 2. Attach a 3' length of flexible rubber hose. 3. Remove the red (outlet) plastic plug from the Aerosol Monitor and insert the Aerosol Adapter (Fig. 3). Fig. 2. Photomicrograph of asbestos dust trapped on a Millipore membrane filter showing particles and fibers (400x). SAMPLING For detailed instructions on sampling, the reader is urged to study the most recent accredited procedures (see refer ences 1, 2 and 3). In general, the proce dure calls for drawing air through the filter at a sampling rate of 2 liters/min. for a period of 15 minutes to 8 hours, de pending on the expected fiber concen tration. Personal sampling is recom mended. This is usually accomplished by attaching the sampling device to the worker's shirt pocket with a clip. The sampler is then connected to a batteryoperated vacuum pump worn in a spe cial belt (Fig. 1). (Table I contains order ing information for those items produced by Millipore.) Air can also be sampled from a fixed location in the room by connecting the Aerosol Monitor to a vacuum pressure pump plugged into an electrical outlet. When using a Millipore Vacuum-Pressure Pump, be sure to install the appropriate flow-limiting orifice in the Aerosol adapter. The flow-limiting orifice is not necessary with the battery powered pump. The following steps will serve as a gen eral guide in sampling: Fig. 3. The Aerosol Adapter is inserted into Aerosol Monitor. Fig. 4. The Aerosol Adapter allows a hose to be attached to the Monitor. Install a flowlimiting orifice in the adapter when sampling with the Vacuum-Pressure Pump. 4. Remove the top section of the Mon itor as shown in Figure 5. 5. Attach the other end of the hose, to the Adapter and clip the Monitor/hose assembly to the vest pocket area of The wearer. (The inlet should point down ward or to the side and no clothing should obstruct it.) Fig. 5. The top of the Monitor is removed for "open" aerosol sampling. 6. After sampling, replace the top por tion of the Monitor and remove it from the Adapter and hose. Label the Monitor appropriately. MOUNTING THE SAMPLE Normally, the Monitors are accumulated fn a laboratory facility for analysis.'To count fibers on a filter, it must first be mounted and rendered transparent. TWo mounting media have been used. The following technique (described in Ref erence 1) is most commonly used in the U.S.: 1. Dissolve clean membrane filter ma terial in a 1:1 solution of dimethylphalate and diethyl oxalate (0.1 gm filter material per ml of solution). 2. To mount the sample, place a drop of this medium on a freshly cleaned 25mm x 75mm glass microscope slide (see Fig. 6). 3. Next, pry apart the Monitor and re move both the top and the retaining ring. Leave the filter in the Monitor. 4. Use a scalpel to cut from the filter a wedge-shaped piece with an arc length of about 1 cm. 509$0Z7T Millipore is a Registered Trademark. rK]n I AM SOIO/P Printnrl in 11 ^ A Cnnt'riciht'5*' IQ'i'i Miliinrtrn rft.n BrXfn,(i MA HI p;rCt Fig. 6. Here, a glass slide is being washed with ultra-clean Freon from a Millipore Solvent Filtering Dispenser. Fig. 7. Mounting medium can be ultracleaned as it is dispensed, using a Micro Syringe Filter Holder containing a solvent resistant FluoroporeTM (PTFE) filter. 5. Remove this wedge-shaped piece from the Monitor using smooth-tipped MF Forceps, and replace the top of the Monitor. This will prevent damage or contamination of the remaining portion, should it become necessary to cut a sec ond wedge. 6. Place this wedge, sample side up, on ~ the mounting solution and cover it with a No. IVi coverslip. Slight pressure on the coverslip will bring it into uniform contact with the mounting medium. The filter will become "water-white" trans parent in about 15 minutes. Reference 2 describes a similar proce dure developed in the U.K., using triacetin (glycerol triacetate). In low-fiber-count work, perform this procedure in a clean environment such as a clean bench. In this case, the clear ing medium should also be filtered as it is dispensed (Fig. 7). COUNTING PROCEDURE Fibers retained on the filter surface must be counted using a research-quality bin ocular microscope equipped with phasecontrast accessories. Use flat field objec tives. Fiber counting requires a substage condenser with-cenferable annular dia phragms and at least 20 watts of tungsten illumination. Counting should be carried out at 400 to 450x magnification (typi cally, a 40x phase contrast objective and lOx Huygens eyepieces). One eyepiece must contain a Porton or Paterson Globe and Circle reticle or equivalent size ref erence. Use a hand activated counter. Note: Good microscope techniques are critically important in resolving asbestos fibers. Consult the manufacturer of the microscope if you'are not current on phase contrast microscopy. The fibers to be counted have the char acteristic shape shown in Figure 2. For purposes of counting, however, a fiber is defined as having an aspect ratio of at least 3:1 (i.e., it is at least three times longer than it is wide). All other particles are considered as background and should not be counted. Most standards require counting of all fibers 5 /am or longer as determined by comparison with the ret icle size reference. The following steps will serve as a gen eral guide to counting: 1. Using 400x phase contrast illumina tion, adjust the microscope for optimum illumination and focus. 2. Scan several fields of view to assure that fiber distribution is random (you may also do this at lower magnification, say lOOx). 3. Count the number of fibers 5 /am and longer in 20 randomly selected fields of view. 4. Record results and calculate the num ber of fibers/ml. 5. Label the mounted slides. HANDLING DATA To calculate the numbers of fibers/ml, use the following formulas: NrxAf EFA Where: F = Fraction of the filter analyzed Nf = Number of fields of view counted (typically 20) Af = Area of any field of view in mm* (as determined with a stage micrometer) - EFA = Effective Filtration Area fn mm* (this value is 855 mm* for the Aerosol Monitor described). Note: Once calculated, F will become a constant for a fixed number of fields (N,). The total number of fibers is: F x V x 1,000 Where: Nt = Total number of fibers/ml. Nc = Number of fibers counted in N* fields. V = Volume of air in liters. You will find it advisable to prepare a standard data sheet to simplify calcula tions and the recording of data. REFERENCES 1. Edwards, G.H. and Lynch, J.R., "The Method Used by the U.S. Public Health Service for Enumeration of Asbestos Dust on Membrane Filters," Annals of Occu pational Hygiene, Vol. 11, pp. 1-6, Pergamon Press, Great Britain (1968). Re prints are available from the U.S. Dept, of Health, Education, and Welfare, Public Health Service, 1014 Broadway, Cincin nati, Ohio 45202. 2. "Technical Note 1: The Measurement of Airborne Asbestos Dust by the Mem brane Filter Method," (Sept. 1971); avail able from the Asbestosis Research Coun cil, P.O. Box 40, Rochdale, Lancashire, England. 3. "Criteria for a Recommended Stand ard - Occupational Exposure to Asbes tos," U.S. Department of Health, Educa tion, and Welfare, Public Health Service, National Institute for Occupational Safety and Health, Washington, D.C. (1972). 4. Bayer, S.G., Zumwalde, R.D., and Brown, T.A., "Equipment and Procedures for Mounting Millipore Filters and Count ing Asbestos Fibers by Phase Contrast Microscopy," (July 1969). Available from U.S. Dept, of Health, Education, and Welfare, National Institute for Occupa tional Safety and Health, 1014 Broadway, Cincinnati, Ohio 45202. Table I: Apparatus described in the procedure available from Millipore Corporation Product Description . Millipore CaL No. Price Aerosol Monitor Disposable filter holder containing 0.8 /xm filter MAWP 037 AO (50/box) $ 33.00 Aerosol Adapter MF Forceps Solvent Filtering Dispenser Fits into Monitor and accepts rubber tubing Smooth tipped forceps for handling filters For cleaning glass micro scope slides XX62 000 04 XX62 000 06 XX66 025 00 9.50 2.75 42.00 MF-Millipore Filters Filters for solvent filtering dispenser HAWP 025 00 (100/pkg.) 12.50 Glass Microscope 25 mm x 75 mm slides for Slides mounting filters XX10 07615 (pkg. of 72) 5.00 Counter 2-gang hand activated counter XX75 000 02 45.00 Micro-Syringe'"" Filter Holder Attaches to a syringe to dispense clean mounting medium XX30 025 00 22.00 Fluoropore Filters Solvent resistant 1.0 /xm pore size filters for Micro Syringe FALP 025 00 (100/pkg.) 40.00 Vacuum-PressurePump For sampling of room air from a fixed location (not for personal sampling) XX60110 50,115V/50 Hz or 110.00 XX60 220 50. 230V/50 Hz 129.00 Gum Rubber Hose Flow-Limiting Orifice 4' length 12' length 2 liters/min. XX71 000 04 XX71 000 12 XX50 000 20 4.00 9.00 3.50 Flow-Limiting Orifices Set of five: 1.0, 3.0,4.9,10.0, XX50 000 00 14.0 liters/min. 13.50 k \\ 50010277