Document 3N52JOnNVqyx1wKxyqdg6qzgD

the INDUSTRIAL ENVIRONMENT - Its EVALUATION & CONTROL LAM021568 U.S. DEPARTMENT OF HEALTH, EDUCATION, AND WELFARE Public Health Service Center for Disease Control National Institute for Occupational Safety and Health 1973 6113 18264 BGG231-8 For sale by the Superintendent of Documents, U.S. Government Printing Office > having a low concentration of dust are di luted as little as possible. Dense samples are diluted so that no more than about 2,000 particles/mm' will appear in the counting area of the cell. Not less than 5 ml of original or diluted sample should be taken for further dilution, and dilu tions should be made in steps not exceed ing 10 parts of dilution liquid to 1 part of original or diluted sample. The dust suspension must be shaken vigorously by hand for at least 30 sec. before a portion is removed for dilution. Preparation for Counting A. The sample to be counted is shaken to ensure a uniform suspension and a por tion is transferred immediately to a clean cell by means of a clean pipette, taking care to prevent the inclusion of air bubbles. B. Two cells are filled from each sample and from a "blank" flask. C. Sample counting should start at the end of the settling time and should be completed in 10 min. The settling time should be 30 min/mm of cell depth. Counting A. Before counting, the ocular grid should be cleaned to remove dust particles. B. The counting plane is the bottom liquidglass interface of the cell. The microscope is focused up and down slightly with the fine focus adjustment in order to bring in dividual particles in and out of focus for more positive detection and counting. C. Fields selected for counting should be uni formly distributed over the counting plane of the cell. Observation should not be made through the microscope while fields are being selected. D. At least five fields of equal area should be counted in each of two cells. For a dust sample, when the first five fields of the first cell counted yield a total count of less than 100 particles, additional fields of known area should be counted; the total area counted is recorded and used in cal culation of concentration. For each cell from the "blank" flask only five fields need be counted. E. The same total area should be counted in the second cell as is counted in the first. F. Total counts from the two cells of the same sample should be compared; and when the ratio of the greater to the lesser count is larger than 1.2, additional pairs of cells should be counted until a pair yields counts which satisfy this criterion. The count of this pair should be used for calculating the concentration of the sample. G. Five fields of the same area as that used for dust sample counting should be counted in each of two ceils from a "blank" flask. The average blank count should be used in calculation of net count. If the blank count exceeds 30 particles/mm2 of counted area, all the samples should be rejected. H. Observers are cautioned that their ability to see particles probably improves during the first few minutes of counting as their eyes become accustomed to the task. A brief period of counting is suggested prior to recording data. Fatigue can cause a deterioration in counting efficacy; conser vative judgment should be exercised on when to discontinue counting because of fatigue. Membrane Filters The membrane filter method is commonly used for assessing asbestos dust exposure. In order to view the dust under the micro scope it is necessary to use immersion oil to render the filter transparent. The refractive index of membrane filters is about 1.5, close to that of chrvsotiie asbestos (ND = 1.55), so that under ordinary illumination the chrvsotiie may itself be very nearly invisible when using an immersion oil closely matched to the filter. A phase contrast microscope is therefore used to increase the visi bility of the chrysotile. The method given below is based on the standardized techniques recom mended by the National Institute for Occupational Safety and Health. Sampling Procedure Samples for evaluation of asbestos exposure are collected on Millipore AA Membrane filters (37 mm diameter, 0.8 ^m pore size) by personal samplers operated by battery-powered pumps, worn by the employees. 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. The face cap of the filter holder is removed and filter used open face during sampling. The sampling rate is about 2 liters/min. A minimum sampling period of 15 minutes (for evaluation of excursion limit) and several samples of up to 4 hours for evaluation of 8-hour average are normally required. Samples with a visible deposit may be too heavy to count; compare the appearance of the collected samples with a clean filter. Heavy concentrations of visi ble dust in the air (100 to 500 fibers/ml) may re quire short sampling periods of only 5 minutes, or less. The following specifications should be con sidered minimum for the microscope used for counting of asbestos fibers. 1. Microscope body with a binocular head and a fine focus accuracy of 0.005 mm. 2. Binocular with 10X Huygenian eyepieces 3. Porton reticle 4. Mechanical stage 5. Kohler illumination (preferably built in and having provisions for adjusting light intensity) 6. Abbe condenser with an adjustable iris 7. 40-45X (0.65-0.75 N.A.) Positive (bright field) phase-contrast objective 8. Annular ring condenser diaphragm (cor responding to the objective) 9. Phase ring centering telescope 10. Green filter 6113 11. Stage micrometer 182 65 148 BOG-319 LAM021569