Asbestos Counting

Standard deviations from two to five or more. This means that the upper seventeenth percentile may be as much as from two to five times the mean. This variabihty is compounded by the problem of estimating the exposure of a group of workers having differing exposures to find the most exposed workers. Compared to this environmental variabihty, the variabihty introduced by the sampling and analytical error is smah, even for those methods such as asbestos counting, which are relatively imprecise. 

The sampiing medium for air sampies shaii be mixed ceiiuiose ester fiiter membranes. These shaii be designated by the manufacturer as suitabie for asbestos counting. See beiow for rejection of bianks. 

Purchase filters which have been selected by the manufacturer for asbestos counting or analyze representative filters for fiber background before use. Discard the filter lot if more than 4 fibers/ 100 fields are found. 

Since only about one-fourth of a 25-mm sample membrane is required for an asbestos count, any PAT sample can serve as a standard for replicate counting. 

Each laboratory engaged in asbestos counting shall set up a slide trading arrangement with at least two other laboratories in order to compare performance and eliminate inbreeding of error. The slide exchange occurs at least semiannually. The round robin results shall be posted where all analysts can view individual analyst s results. 

Each laboratory engaged in asbestos counting shall participate in the Proficiency Analytical Testing Program, the / bestos Analyst Registry or equivalent. 

When ordering the Graticule for asbestos counting, specify the exact disc diameter needed to fit the ocular of the microscope and the diameter (mm) of the circular counting area. Instructions for measuring the dimensions necessary are listed ... 

Differentiai Counting The term applied to the practice of excluding certain kinds of fibers from a phase contrast asbestos count because they are not asbestos. 

Fiber counting A microscopic technique which is of particular relevance to asbestos, where the fibers are counted on a filter paper. 

RM 8411 consists of a section of collapsed mixed-cellulose-ester filters with a high concentration (138 fibers/o.oi mm) of chrysotile and a medium concentration (43 fibers/o.oi mm) of amosite. It is intended for use in evaluating the technique used to identify and count asbestos fibers by TEM. 

RICE (Regular Interlaboratory Health and Safety Asbestos fibre counting in the... 

Churg, A. and M. L. Wamock (1977). Correlation of quantitative asbestos body counts and occupation in urban patients. Arch. Patrol. Lab Med. 101 624-634. 

Air drawn through a 0.8 to 1.2 mm cellulose ester membrane filter, asbestos fibers counted by positive phase contrast microscopy technique sample prepared by acetone/triacetin method (NIOSH Method 7400, 1985). 

Alternatively, asbestos fibers collected on the filter counted by TEM, following the modified Jaffe wick technique for sample preparation (NIOSH Method 7402, 1987). 

Guidance on the minerals and fibers to be counted under the OSHA Asbestos Standard may be obtained by reference to the Federal Register. 

An identification based solely on morphology in situations where the analyst is inexperienced or interference is severe may have positive errors if fiberglass, plant fibers or other fibers are counted as asbestos or negative error if an attempt is made to exclude such fibers without other optical tests. 

The NIOSH method for the analysis of asbestos (9) states that "The method is intended to give an index of employee exposure to airborne asbestos fibers of specified dimensional characteristics. It is not meant to count all asbestos fibers in all size ranges or to differentiate asbestos from other fibrous particulates". 

The NIOSH method (101 also states, however, "In an atmosphere known to contain asbestos, all particulates with a length to diameter ratio of 3 to 1 or greater, and a length greater than 5 micro meters should in the absence of other information, be considered to be asbestos fibers and counted as such1 (emphasis added). 

When asbestos is in a mixture with other fibers, it is possible to bracket the asbestos concentration by determining the percent of asbestos fibers in the mixture removed from the membrane and applying this percentage to the total fiber count on the membrane. 

The percent by number of asbestos in talc samples is determined by particle counting. If a scan of two slides shows no fibers present, there is no need for a particle count. 

This paper will not discuss counting rules for asbestos. The phase contrast method is fully described by NIOSH (38). It is... 

Bayer, S.G., Zumwalde, R.D., Brown, T.A., "Equipment and Procedures for Mounting Mi 11ipore Filters and Counting Asbestos Fibers by Phase Contrast Microscopy", 1-15 (July, 1969), U.S. Department of Health, Education and Welfare, Public Health Service, Bureau of Occupational Safety and Health,... 

Units of Exposure. Consideration and comparison of quantitative data on asbestos inhalation studies are complicated by the fact that a number of different methods have been used to measure asbestos levels in air. Currently, the standard method for measuring asbestos concentrations in workplace air employs phase contrast microscopy (PCM). A particle visible under PCM is counted as a fiber if it is 5 micrometers (pm) long and has a length/thickness ratio of 3 1. However, the method cannot detect fibers thinner than about 0.3 pm and caimot distinguish between asbestos fibers and other fibers (NIOSH 1987). 

Analyses of bronchoalveolar lavage fluid samples or sputum samples can directly reflect alveolar concentrations of retained fibers and, although they do not reflect the proportion of deposited fibers that may move to the interstitium (Case 1994 Pinkerton et al. 1984), can provide information regarding past exposure to asbestos, especially to amphibole fibers. Obtaining sputum samples is much less invasive than obtaining bronchoalveolar lavage samples. In Libby, Montana vermiculite miners and millers exposed to fibrous tremolite, counts of asbestos bodies in sputum samples closely reflected intensity and duration of past exposure (Sebastien et al. 1988b), but asbestos body counts in sputum samples from volunteers from other cohorts of workers exposed to asbestos (predominately chrysotile or lower levels of amphibole fibers than in Libby) did not reliably reflect past levels of exposure (McDonald et al. 1988, 1992). 

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