Page:NIOSH DM DFM respirator evaluation draft.pdf/50

  For their data, a NIOSH computation of the LTL_1,.90,.05 yields an APF value of 8.9 at the 90% confidence level. However, a 95% confidence level, not 90%, is the accepted value for professional practice in most scientific research work. At the 95% confidence level a NIOSH computation of LTL_1,95,.05 yields an APF value of 7.1, which is substantially lower than the observed WPF point estimate of 18 for the 5th percentile. That is, for these results that best we can conclude with 95% confidence is that the 5th-percentile WPF exceeds 7.1. The difference in WPF values between the 5th-percentile point estimate of 18 and LTL_1,.90,.05 of 7.1 is the margin of error associated with the point estimate.
  Additionally, none of the WPF studies reported in the literature have selected their test subjects according to anthropometric restrictions. As a result, in any given WPF study the test subjects may represent substantially less than 95% of facial sizes in American workers.
  NIOSH has concluded that due to excessive face-seal leakage, while wearing air-purifying, NIOSH-certified respirators under ideal conditions in the workplace, from less than 1% to substantially more than 10% of American workers will not achieve a class APF computed according to the recommendations of Myers et al. These percentages consider only face-seal leakage. Any additional leakage through filters or sorbent elements will increase the percentage of wearers not achieving APF-level protection. These wearers with inadequate respiratory protection would not be identifiable except possibly for those contaminants with adequate warning properties or those very few contaminants for which an overexposure can be biologically detected (e.g., urine- or blood-monitoring techniques).
  Since essentially all respirator-performance data reported since 1983 were measured as WPF values, the 1-sided lower tolerance limit approach might still be a viable means of determining APFs. However, because of public health considerations, the proportion of wearers not achieving the WPF would have to be set substantially lower than 5%. Values such as 0.1% to 1% might be considered. Confidence levels should be set at 95% or 99%. Most importantly, if this APF-determination method is used, both purchasers and users must be fully informed that a given percentage of wearers are not expected to achieve the APF and will not be able in many cases to know who these inadequately-protected wearers are. The issue of informed consent should be investigated if this approach is considered.
  In addition to examining WPF study results for internal validity, it is also essential to examine the external validity of WPF and APF results from any given study. That is, how valid are the research results outside of the research-study sample? Suppose a researcher were to conclude that the WPF-performance of respirator A is better than some performance criterion. If the conclusion was based on activities than are irrelevant to tasks and circumstances in the real world, then the conclusion