There were no outliers in this round of silica analysis according to the statistics used for the PAT program. Therefore, the arithmetic and geometric means were calculated from the results of all 62 laboratories submitting data. The wide scatter of x-ray intensities from the same membrane filter confirms the conclusion from image observation that the distribution of dust particles over the filter is not uniform. The excellent agreement of the amount of silica found demonstrates that the multiple-exposure approach is effective and realistic.
of other laboratories participating in the PAT program. This in situ x-ray diffraction method achieves substantial timesaving, delivers reliable data, and may be used to substitute the more demanding NIOSH method for fast screening analysis of silica in airborne dust.
Conclusions For a maximum of 5 mg total dust collected on a membrane filter (overloaded if higher than 5 mg), the effect of particle distribution overrides the matrix effect in x-ray diffraction analysis for silica by an external standard method. Through an image development technique, it has been shown that the dust distribution on the filter is not uniform. To cope with this nonuniform distribution and to preserve the simplicity of analysis, a multiple-exposure x-ray diffraction method for silica analysis has been developed for routine and PAT samples. The results obtained are in good agreement with those
Literature Cited
Acknowledgment The author acknowledges the review and discussion of Richard W. Scott. (1) Anderson, P. L., A m . Ind. Hyg. Assoc. J., 36 (lo), 767-78
(1975). (2) U.S. Dept. of Health, Education and Welfare, “NIOSH Manual of Analytical Methods”, GPO, 1974. (3) Chung, F. H., J . Appl. Crystallogr., 8,17-19 (1975). (4) Chung, F. H., Lentz, A. J., Scott, R. W., X - R a y Spectrom., 3, 172-5 (1974). ( 5 ) Chung, F. H., Adu. X - R a y Anal., 19,181-90 (1976). A m . Ind. Hyg. (6) Allen, G. C., Samimi, B., Ziskind, M., Weill, H., ASSOC. J., 35 ( l l ) , 711-7 (1974). Received for review August 2,1977. Accepted April 24,1978.
Correction In the article, “Composite Hazard Index for Assessing Limiting Exposures to Environmental Pollutants: Application Through a Case Study” [Enuiron. Sci. Technol., 12, 802-7 (1978)],by Elizabeth M. Rupp, Dennis C. Parzyck, Phillip J. Walsh*, Ray S.Booth, Richard J. Raridon, and Bradford I,. Whitfield, on page 804, column 2, the second line of the caption to Figure 2 should read Helena smelter (ng/m3), not (mg/m3) as printed.
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