ASHRAE Journal - June 2021 - 14

TECHNICAL FEATURE
λS = loss rate due to quanta settling, which can be
expressed as the aerosol's settling velocity, VTS,
divided by the height where quanta are emitted,
h-1
k = rate of inactivation of the quanta, h-1.
Additional loss mechanisms that may be applicable to
a given situation can be added to Equation 5.
We use this modified version of the Wells-Riley model
to predict the conditional probability of infection for
susceptible individuals being exposed to infectious
SARS-CoV-2 quanta generated by a single individual
(Nl = 1) actively shedding virus-laden aerosols into a
classroom space. The efficacy of various protective measures
that aim to reduce infection risk are then quantified
by determining how they individually and in combination
impact the predicted conditional probability of
infection. Further details on the analysis are provided by
Rothamer, et al.5
Assessment of Measures to Prevent Airborne
Transmission of COVID-19
Using the aerosol behavior characterized in the
classroom, mask effective filtration efficiency data
and other measured mechanical system data as input
to the Wells-Riley model, we can quantify the conditional
probability of infection for various protective
measures using scenarios where one of the classroom's
occupants is COVID-19-positive and actively
shedding. The conditional probability of infection
represents the probability for any one person being
infected during any one event (e.g., a scheduled class
meeting period). The conditional probability of infection
is evaluated for the following three distinct exposure
scenarios:
Scenario A: Infectious instructor with a quanta emission
rate, qI
, of 110 quanta/h and the corresponding
calculated conditional probability of a student being
infected when the student's breathing rate,
VbS,
, is
0.540 m3/h (0.318 cfm).
Scenario B: Infectious student with a quanta emission
rate of 19.1 quanta/h and the corresponding calculated
conditional probability of the instructor being infected
when the instructor's breathing rate,
VbI,
, is 1.38 m3/h
(0.818 cfm).
Scenario C: Infectious student with quanta emission
rate, qS
14
ASHRAE JOURNAL ashrae.o rg
J U N E 2021
, of 19.1 quanta/h and the corresponding
calculated conditional probability of another student
Types of Masks and Braces Tested
They include a commercial four-ply knit cotton
mask (KCM); a three-ply spunbond polypropylene
mask designed by the University of Wisconsin-
Madison Emergency Operations Committee
(EOCM), a single-use three-ply disposable mask
with a meltblown polypropylene center ply medical
procedure mask (PM); and an ASTM F2100 Level 2
rated surgical mask (SM).
In addition, external braces tested include the UW
fitter (U) and a commercial brace (C).
being infected when that student's breathing rate is
0.540 m3/h (0.318 cfm).
As noted in the three scenarios, the quanta emission
rate, q , is higher for the instructor who is speaking
loudly and frequently compared to the student who is,
generally, seated and speaking infrequently. Similarly,
the breathing rate for individuals is proportional to
their metabolic activity level, so the breathing rate,
Vb
, will be higher for the instructor and lower for the
student.
For each of the three exposure scenarios, a range of
protective measures are evaluated for their ability to
decrease the likelihood of an infection via airborne
route. The protective measures include:
a. Total air change rate for the room expressed as air
ACH SAR= 
changes per hour (NV V );
/
b.Airflow rate circulating through in-room air purifying
filtration unit(s); and
c. All occupants equipped with:
i. No mask;
ii. One of the previously mentioned masks
(alone); and
iii.One of the previously mentioned masks + mask
fitter (see " Types of Masks and Braces Tested "
sidebar).
Table 1 provides values of both fixed and variable
parameters used to determine the conditional probability
of infection.
Figure 1 shows the conditional probability of infection
as a function of mask effective filtration efficiency
for the three scenarios described above, assuming one
infectious individual present in the room for a 60 minute
duration and at a baseline room air exchange rate
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ASHRAE Journal - June 2021

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