Workplace Exposure to Asbestos:
Review and Recommendations

ASBESTOS
SAMPLING AND ANALYSIS

Memorandum on Asbestos Update and Recommended Occupational Standard
I. Asbestos Nomenclature/Definitions
II. Asbestos Sampling and Analysis
III. Biologic Effects of Exposure to Asbestos in Animals
IV. Biologic Effects of Exposure to Asbestos in Humans
V. Smoking and Asbestos
VI. Exposure to Asbestiform Minerals other than Commerically Mined Asbestos
VII. Non-Occupational Exposure to Commerical Sources of Asbestos
VIII. Dose-Response Relationships
References

Update on New Methods

Since the NIOSH revised recommended asbestos standard was published in December, 1976, there have been several new developments in the area of sampling and laboratory analysis. Kim et al. (1979) have developed a quick screening test for chrysotile crocidolite, and amosite which can be used for bulk material samples. The test is based upon the formation of color complexes with Mg ⁺² and Fe ⁺² released from asbestos upon acid digestion. The Mg ⁺² from chrysotile is complexed with p-nitrobenzenazo-s-naphthol. The Fe ⁺² from crocidolite and amosite is complexed with 1,10-phenathroline. A positive test is indicated by formation of colored complex for Mg ⁺² and/or Fe ⁺² . The test by Kim et al. is not specific for asbestos; however, the detection limit is reasonable for bulk samples with 1-2 mg being detectable in any given sample. Of 70 samples tested, 52 were correctly classified as containing asbestos or not, 18 samples gave false positives, and there were no false negatives. The method has little promise for airborne samples.

Lange and Haartz (1979) have developed a method for chrysotile asbestos determinations by X-ray diffraction. The method for membrane filters involves ashing followed by redeposition on silver membrane filters. The 7.33A peak for chrysotile is primarily used in an integrated mode. Normalization using reflections from the silver membrane is employed along with X-ray absorption corrections. The lower limit of detection is reported to be 2 mcg on a filter with good linear response to over 200 mcg per filter. Minerals such as antigorite, lizardite, kaolinite, and possibly chlorite are potential interferences with chrysotile The method has not been adapted for amphibole determinations.

Lilienfeld and Elterman (1977) and Lilienfeld et al. (1979) have developed a portable monitor capable of real-time determinations of airborne fiber concentrations. The monitor is based upon rotation of elongated particles by means of a rotating electric field of large voltage gradient. Fibers of various lengths are then detected by synchronous detection of modulated light scattered from a continuous-wave helium-neon laser beam with modulation generated by the rotating particles. Concentrations between 0.001 and 30 fibers/cc are reported to be detectable. At a concentration of 1 fiber/cc, a relative standard deviation of 10% is reported. The minimum detectable fiber length and diameter are estimated to be 2 mcm and 0.2 mcm, respectively. The instrument is not specific for asbestos as other elongated particles align within the electric field, and the instrument cannot be easily used for obtaining "breathing zone" samples.

Gale and Timbrell (1979) have reported progress in development of an automated method for determining fiber density on membrane filters. The method involves first clearing the membrane filter by conventional methods followed by aligning fibers on the filter in a strong magnetic field. The sample is then placed in a specially designed microscope on a motor driven stage. Fiber density is determined by measuring light scattered from the rotating fibers. The method is not yet commercially available, but is predicted to have a lower limit of detection of about 0.1 fiber/cc based on a 4-hour sampling period.

Optical Microscopy

The phase contrast method recommended by NIOSH for compliance sampling in the occupational setting was reviewed in the 1976 NIOSH document. Since that review, Leidel et al. (1979) have reported studies to better define precision of the method at lower levels. Minor changes in fiber counting methods have also been recommended by NIOSH to correct a potential statistical bias.

Based on the most recent data available, Leidel et al. (1979) estimated the coefficient of variation for the membrane filter sampling-phase contrast counting method to be 0.11 to 0.15, given a total count of at least 100 fibers. With a reduced fiber count of 10 fibers in the analysis, the coefficient of variation is estimated to be 0.41. Statistical tests based upon these estimates of precision are recommended by NIOSH for determining compliance or noncompliance with regulatory standards (Leidel et al., 1979). Procedures are available for single full shift samples, multiple samples covering the workshift, or short "grab samples."

The phase contrast method is clearly capable of measuring airborne fiber levels down to 0.1 fibers/cc (fibers longer than 5 mcm) given that due consideration is given to inherently high variability at such levels. The method is highly sensitive for detection of fibers longer than 5 mcm; however, specificity of the method for identifying asbestos fibers may be a serious problem under certain circumstances. Fiber identification is based only upon fiber length and aspect ratio; therefore, the method is not specific in situations where a mixture of asbestos and non-asbestos fibers occur or where large numbers of other elongated particles are present. The lack of specificity becomes more serious at lower fiber concentrations, and alternate methods for identification are likely to be required. The most likely choice for fiber identification in airborne dust samples is electron microscopy where both electron diffraction and microchemical analyses may be used to identify fibers (NIOSH, 1976). The fraction of asbestos fibers determined by these methods could then be multiplied by phase contrast determinations to arrive at asbestos fiber levels. It seems reasonable that such determinations only need be made for a statistically determined sample and not for each airborne dust sample, with subsequent determinations made only upon process or product modifications. The statistical confidence of the airborne asbestos fraction determinations should be taken into account in determinations of compliance or noncompliance.

In addition to the problem of lack of specificity for fiber identification, only a fraction of all airborne asbestos fibers are actually accounted for by the phase contrast method, which considers only fibers longer than 5 mcm. The phase contrast method, therefore, can only be considered an "index" measure of fiber exposure. In fact, the fraction of airborne fibers longer than 5 mcm is extremely variable, ranging from 1 to 50%, depending on fiber type and industrial operation (Dement et al., 1976). In addition to determinations of fiber identification by electron microscopy, it may also be desirable to determine airborne fiber size and specifically the fraction of airborne fibers longer than 5 mcm.

Memorandum on Asbestos Update and Recommended Occupational Standard
I. Asbestos Nomenclature/Definitions
II. Asbestos Sampling and Analysis
III. Biologic Effects of Exposure to Asbestos in Animals
IV. Biologic Effects of Exposure to Asbestos in Humans
V. Smoking and Asbestos
VI. Exposure to Asbestiform Minerals other than Commerically Mined Asbestos
VII. Non-Occupational Exposure to Commerical Sources of Asbestos
VIII. Dose-Response Relationships
References