ASHRAE Journal - May 2021 - 16

TECHNICAL FEATURE

Figure 6 shows three of the mask
FIGURE 6 Masks considered including (a) four-ply cloth face mask and (b) overhead view of four-ply cloth face
mask showing lack of fit around the nose; (c) procedure mask and (d) overhead view of the procedure mask
types tested and their relative fit
showing better fit around nose; (e) ASTM Level 2 mask and (f) overhead view of ASTM Level 2 showing even
to the faces of the manikins. They
better fit around nose compared to the procedure mask.
include a commercial four-ply knit
A.
B.
cotton mask (Figure 6a and 6b); a
three-ply spunbond polypropylene
mask designed by the University
of Wisconsin-Madison Emergency
Operations Committee (EOC) and
produced by a local custom sewing
manufacturer, referred to as the EOC
C.
D.
mask throughout; a single-use threeply disposable mask with a meltblown
polypropylene center ply, referred to
as the procedure mask; (Figure 6c and
6d); and an ASTM F2100 Level 2 rated
surgical mask (Figure 6e and 6f).
Apart from the filtration ability of
E.
F.
the mask's media, the lack of proper
mask fit to facial contours enables
considerable air bypass, thereby compromising the effective filtration efficiency of the mask as worn. Here we
define effective filtration efficiency
as a measure of aerosol removal
performance that accounts for both
0.949 ±0.026 with the use of the commercial mask brace.
the mask media and the mask fit. This metric is vitally
In all cases, the use of a mask-fitting device significantly
important as we will show what should be obvious-
improved the mask's effective filtration efficiency by
namely that poor fit = poor performance, regardless of
reducing air leakage around the mask.
the filtration rating of the mask media.
It is also noteworthy that while the four-ply cloth mask
As part of the experimental work conducted to meawith a very low effective filtration efficiency of ηf,Minh =
sure mask effective filtration efficiency, we evaluated
0.075 ±0.043 does benefit from the addition of a mask
the use of a mask fitter developed in collaboration
brace, its effective filtration efficiency is still quite low
with Lennon Rodgers at the UW-Madison Makerspace
with the mask brace. Further details on the mask effec(UW-Madison Fitter) and a commercial mask fitter
tive filtration efficiency measurements, including size(Commercial Brace) mask brace. These devices are
resolved data, can be found in Rothamer, et al.18
designed to improve the mask's seal to the user's face
Part 2 of this two-part series will utilize the field-meato minimize inhaled or exhaled air leakage around the
sured data and provide evaluations of various measures
mask. Mask fitters or braces better enable the mask to
aimed at reducing the probability of infection in high
achieve full filtration potential consistently.
Figure 7 shows the measured effective filtration efficien- occupant density indoor environments.
cies for the masks considered in this study, both alone
Conclusions
and in combination with the above-mentioned mask
Field experiments were conducted using a polydisbraces. The most dramatic result is the single-use procedure mask, which had an effective filtration efficiency perse neutralized salt (NaCl) generated in an aerosol size
range consistent with SARS-CoV-2. The concentration
of ηf,Minh = 0.158 ±0.042 when used alone, but a significantly improved effective filtration efficiency of ηf,Minh =
distribution of aerosols was measured for two different
16

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ASHRAE Journal - May 2021

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