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users per 100 users for the currently accepted APF value of 10 used for professional practice. However, in two of these three studies^[1] the results were biased to an unknown degree. Thus the actual control-failure rates for these two studies could be higher than 14 users per 100 users.

For the WPF data reported in studies #1 through #6 of Table O, which yielded

the highest upper confidence limits for actual failure rates, the top two and two other of the six data sets were for halfmasks equipped with HEPA filters or organic-vapor sorbent cartridges. 150 The latter cartridges are expected to exhibit zero leakage as is expected for HEPA filters. Thus all the reported mask leakage should have occurred at the halfmask facial seals. NIOSH concludes that there is a serious question whether an APF of 10 is valid for non-powered, HEPA-filter halfmasks, which for over 15 years has been an accept- ed value for professional practice. This conclusion is based on NIOSH evaluation of APF-determination methods used during the 1970s and 1980s 151 considered in combi- nation with this evaluation of nine halfmask-performance studies conducted in the last decade. If the APF of 10 is invalid and is erroneously high, then the six APFS for non-powered, filter halfmasks recommended by NIOSH are erroneously high. For non-powered halfmasks equipped with DM, DFM, HEPA filters, the APFs rec. ommended by NIOSH in Table P of this evaluation have been computed using an APF of 10 to represent faceseal-only leakage. If this value of 10 is invalid and should actually be lower, then several of the APF computations summarized in Ta- bles P and R of this evaluation are in error and six of the Institute's recommended APFs are erroneously high. The potential reductions in NIOSH-recommended APFs that might result from the use of an APF less than 10 can be estimated with the use of Figure II or III provided later in this evaluation. Additionally, NIOSH questions why a failure-rate as high as 5% should be consid- ered acceptable by respirator and public health professionals, as it apparently has since the 1983 proposal of Myers et al. 152 The Institute requests comments whether

150 Galvin et al. (1990), Gaboury and Burd (1989), Lenhart and Campbell (1984), and Colton et al. (1990). 151 Refer to discussion presented in this evaluation under Review and Evaluation of Professional Prac tices Used During the 1970s and 1980s for Respirator Face-Seal Evaluations and APF Determinations. 155 Myers, W. R., S. W. Lenhart, D. Campbell, and G. Provost: The Forum-Letter to the editor, Am.

Ind. Hyg. Assoc. J. 44(3):B25-26 (1983).