Comparing the fit of N95, KN95, surgical, and cloth face masks and assessing the accuracy of fit checking

Abstract

Introduction: The COVID-19 pandemic has made well-fitting face masks a critical piece of protective equipment for healthcare workers and civilians. While the importance of wearing face masks has been acknowledged, there remains a lack of understanding about the role of good fit in rendering protective equipment useful. In addition, supply chain constraints have caused some organizations to abandon traditional quantitative or/and qualitative fit testing, and instead, have implemented subjective fit checking. Our study seeks to quantitatively evaluate the level of fit offered by various types of masks, and most importantly, assess the accuracy of implementing fit checks by comparing fit check results to quantitative fit testing results.

Methods: Seven participants first evaluated N95 and KN95 respirators by performing a fit check. Participants then underwent quantitative fit testing wearing five N95 respirators, a KN95 respirator, a surgical mask, and fabric masks.

Results: N95 respirators offered higher degrees of protection than the other categories of masks tested; however, it should be noted that most N95 respirators failed to fit the participants adequately. Fit check responses had poor correlation with quantitative fit factor scores. KN95, surgical, and fabric masks achieved low fit factor scores, with little protective difference recorded between respiratory protection options. In addition, small facial differences were observed to have a significant impact on quantitative fit.

Conclusion: Fit is critical to the level of protection offered by respirators. For an N95 respirator to provide the promised protection, it must fit the participant. Performing a fit check via NHS self-assessment guidelines was an unreliable way of determining fit.

Fig 1. Experimental setup for quantitative fit testing.

Particle counter measures particle concentrations in the mask from mask intake valve (clear) and from the ambient air by ambient intake (blue) while mask is worn by participant. Research assistant records readings and instructs participants in testing procedures. Not shown in this image: TSI model 8026 particle generator.

Fig 2. N95 and KN95 respirators tested.

Top row, from left to right: 3M 8511, 3M 8200, Aero Pro AP0028. Bottom row, from left to right: Makrite 9500, Xiantao Zong ZYB-11, Zhong Jian Le KN95.

Fig 3. The fit factor achieved by a set of volunteers while wearing different varieties of masks.

Protection when wearing an N95 respirator was high only if the respirator properly fit the participant. Fit factors for KN95 respirator, surgical masks, and cloth masks were similar.

Fig 4. Percentage of particles outside of the mask and percentage of particles measured inside of the mask during the last cycle of normal breathing during the fit test.

Fig 5. The 3M 8511 mask on participants F-28 and F-68.

In both cases, the material of the respirator makes contact with the wearers’ skin close to the tip of the chin, leaving what appears to be a visible gap. However, there was enough subcutaneous fat to consistently seal the mask in participant F-28, achieving the maximum score of 200+ for all tests. In the case of F-68, there was not enough subcutaneous fat to consistently ensure a seal between the respirator and the skin, leading to consistently low fit testing results.

Fig 6. Participants’ fit-check predictions with the quantitative mask fit factor organized by participant.

Fit factor results are color coded to represent the participants’ fit check results with green representing a belief of fit and red representing a belief the mask did not fit. Depth of color represented confidence with lighter shades representing low confidence in the fit check results and darker shades representing a high confidence.