ASHRAE Journal - December 2020 - 43

TECHNICAL FEATURE

0.8
0.6

Bedroom, Caregiver 30 Minutes
SARS-CoV-2

Baseline 1 ach, Isolation Room

0.4
0.2
0.0

+Air Cleaner CADR 170

+50%
Efﬁcient Mask

+Air Cleaner CADR 170
+50% Efﬁcient Mask

FIGURE 8 Relative risk of infection in rest of house.

Relative Risk of Infection

Case 3: Infection is Known, and
The Infected Person is in an
Isolated Room (Bedroom 200 ft2
[18.6 m2]) Per ASHRAE COVID-19
Recommendations. a Caregiver Goes
Inside the Infected Person's Room for
30 Minutes

FIGURE 7 Relative risk of infection when infected in isolation room, Resting - Speaking.

Relative Risk of Infection

the HVAC running at 0.35 ach and with MERV 11 ﬁlters.
These risks are shown as percent absolute risk and illustrate that higher exposure time results in signiﬁcantly
higher risks. All risks decrease when adding ventilation
and ﬁltration. For the cases evaluated (including all the
ﬁltration/ventilation options), doubling the exposure
time from 2 hours to 4 hours leads to a 2.2 times increase
in risk. Compared to the risk at 2 hours, 6 hours leads to
about 3.4 times increase in risk.

1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0

Infected in Isolation Room, Risk to Household Members, Awake 16 Hours
SARS-CoV-2
Baseline 0.35 ach, MERV 4 in Rest of House
50%
56%
Less
Less

35%
Less

2.8%
Less
48%
Less

58%
Less

63%
Less

90%
What is the risk to the
93%
95%
96%
Less
Less
Less
Less
caregiver when entering
the isolation room? This
0.35 ach 0.35 ach 0.35 ach 0.35 ach 0.35 ach 0.35 ach 0.35 ach 0.35 ach 0.35 ach 0.35 ach 0.35 ach
MERV 7 MERV 11 MERV 14 MERV 4 MERV 7 MERV 11 MERV 14 MERV 4 MERV 7 MERV 11 MERV 14
would be isolation accordCADR 170 CADR 170 CADR 170 CADR 170 CADR 170 CADR 170 CADR 170 CADR 170
ing to the CDC. Assuming
Infected Infected Infected Infected Rest of
Rest of
Rest of
Rest of
Room
Room
Room
Room
House
House
House
House
the bedroom is set up per
the recommendations of
ASHRAE for an isolation space, the risks to the noninexhaust air pathways are closed. The HVAC in the rest
fected individual(s) improve by isolating the room from
of the house is always on.
the HVAC system and exhausting the room.
In the case of the portable air cleaner, it is added to the
Scenarios: Time spent = 30 minutes
infected person's room. Although it may be counterin1. Resting (Speaking) = 9.4 quanta/hour
tuitive to clean the bedroom air, it does seem logical to
2. Resting (Speaking + 50% Efﬁcient Mask) =
contain the infectious particles at or near the source.
4.7 quanta/hour
The air cleaner is assumed to have a ﬂow of 191 cfm
3. Resting (Speaking + Air Cleaner With CADR 170) =
(90 L/s) and an efﬁciency of 89%.
9.4 quanta/hour
The results show using a portable air cleaner in the
4. Resting (Speaking + 50% Efﬁcient Mask + Air Cleaner infected person's room will halve the risk. Wearing a 50%
With CADR 170 = 4.7 quanta/hour
efﬁcient mask will have the same impact as operating
Results: Figure 7 illustrates a comparison of decrease
a portable air cleaner with a CADR of 170. Operating a
in risk when adding a portable air cleaner that has a
portable air cleaner and wearing a mask will decrease
CADR of 170 to the infected person's room and/or using the risk by four times. This emphasizes that even with
50% efﬁcient masks. For the baseline, we assume a
isolation modiﬁcations, additional protective actions
separate HVAC system is used for the isolation zone.
including face coverings are essential to protecting the
Any grille connecting the isolation space to the rest of
caretaker from airborne aerosols. More information is
the home is sealed. The space includes a private reston the CDC website (see " Caring for Someone Sick at
room with an exhaust fan; that fan is running continu- Home, " https://tinyurl.com/y3u6w3kq).
ously at high speed. We estimate an air exchange rate
Case 4: Infection is Known, and the Infected Person is in an Isolated Room
of 1 ach: a 25 cfm (12 L/s) bathroom fan will provide
about 1 ach for the bedroom assuming air is exhausted (Bedroom 200 ft2 [18.6 m2]) Per ASHRAE COVID-19 Recommendations
What is the risk to the other household members (the
directly through the attached bedroom and other
D ECEM BER 2020

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ASHRAE Journal - December 2020

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