FIT TEST STUDIES
“This study found that the overall fit factors obtained by the pressure-based system of the Fit Tester 3000 were significantly lower than those obtained by the particle-based system of the Portacount. Additionally, the standard deviation for the Portacount was found to be much greater than that of the Fit Tester 3000. The dramatic differences between the two methods may be due to aerosol streamlining and other biases associated with Condensation Nuclei Counter (CNC) technology. Another observation that can be drawn from this study is that the Controlled Negative Pressure (CNP) method provides a more conservative measure of respirator fit. These findings were in agreement with Crutchfield et al. (1995) and Oestenstad and Graffeo (1994).”
“This comparison found that FFs measured by the NPS method were significantly lower than those measured by the CNC method and were not correlated. Both the parametric 2-sample t-test and the nonparametric Mann-Whitney test found that the mean and median of the measured FFs by the CNC were significantly higher than those measured by the NPS. The difference in FFs was about an order of magnitude.”
“ It was the conclusion of this study the CNC and the dynamic pressure method are not comparable, in their ability to assess respirator fit. However, the dynamic pressure method would provide a more conservative estimate of respirator fit than the aerosol method.”
An analysis of variance showed significant differences in aerosol system measurements of leakage as a function of leak location and mask type. No significant differences in Leak% were detected as a function of respirator brand or breathing rate. These data were pooled for subsequent analysis. The percentages of the known leakage (Leak%) detected by each fit-test system were calculated for each test respirator Mean values of Leak% at each fixed leak location are shown, along with measurement variation. The ambient aerosol system detected an overall average of 37.2% of the known leakage introduced into test respirators, with a coefficient of variation of 44.7%. The CNP system detected an overall average of 97.9% of the known leakage through the same hypodermic needles, with a coefficient of variation of 4.3%. CNP system measurements of Leak% were not affected by leak location.
REGULATIONS
Guidance
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Two of the specified quantitative fit-test methods measure aerosol leakage as an indicator as to how well an RPD fits. The third recommended quantitative fit test (controlled negative pressure) method measures the air exhaust rate required to hold the in-RI pressure constant as an indicator as to how well an RPD fits. Several studies have shown that the controlled negative pressure (CNP) technique consistently results in lower fit factors compared to the same RPD fit measured using QNFT methods using aerosols. While there has been considerable variability in the difference between the aerosol-derived fit factors and the ones derived using the CNP method when measured on the same person with the same RPD donning (i.e. the RPD was not removed or adjusted between fit tests), these studies have shown that the CNP derived fit factors are conservatively lower than the aerosol fit factors. While the CNP fit factor is consistently lower than the aerosol derived fit factors, the lack of consistent correlations may be due to the huge amount of variability in repeated measurements (repeat donnings on the same person) using the aerosol QNFT methods. For these reasons, a lower required fit factor is recommended when using the CNP method on higher performing negative-pressure RPD. This recommendation is believed to result in providing adequate-fitting RPD.