ASHRAE Journal - December 2020 - 41

TECHNICAL FEATURE

FIGURE 2 Relative risk of infection in bedroom, sleeping for 8 hours. Data labels
show the comparison relative to the baseline case (no HVAC system running, no
ventilation, no filtration, no infiltration).

Relative Risk of Infection

exposed to the infectious particles and subsequently
succumbing to infection. In this work, we used the
COVID-19 aerosol transmission estimator tool developed and published by Jose Jimenez of the University
of Colorado-Boulder.7 The calculator is calibrated to
COVID-19 per recent literature on quanta emission
rates. The model does not include droplet or contact/
fomite transmission and assumes that 6 ft (2 m) social
distancing is respected. This model does not include
transmission to the people present when they are in
locations other than the one analyzed here.

1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0

Case 1: Susceptible Person Shares a 200 ft2 (18.6 m2) Bedroom With
Infected Person (Infection is Not Known)
What is the risk to the susceptible household member?
Scenario 1: Sleeping-8 hours.
1. Resting (Oral Breathing) = 2 quanta/hour
Scenario 2: Awake before and after sleeping-2 hours
1. Resting (Speaking) = 9.4 quanta/hour
2. Standing (Loudly Speaking) = 65.1 quanta/hour
Results: Figure 2 shows the relative risk of airborne
infection from sleeping in a bedroom with an infected
person (Scenario 1). Merely running the fan on the
air-conditioning system lowers the risk by one-third.

33%
Less

Sleeping, Baseline Absolute Risk = 20%
49%
Less

54%
Less

59%
Less

64%
Less

66%
Less

68%
Less

0.35 ach 1 ach 0.35 ach 1 ach 0.35 ach 1 ach 0.35 ach 1 ach
MERV 4 MERV 4 MERV 7 MERV 7 MERV 11 MERV 11 MERV 14 MERV 14

Results

FIGURE 3 Relative risk of infection in bedroom, awake for 2 hours. Data labels show
the comparison relative to the baseline case (no HVAC system running, no ventilation, no filtration, no infiltration).

Bedroom, Awake 2 Hours
SARS-CoV-2
Relative Risk of Infection

We show the results of infection risk in absolute numbers (top of the graph or tables), and we explored trends
in risk reductions for each case using a measure of relative
risk as shown in the bar charts. Relative risk values use
the baseline of the HVAC system not running (case where
thermostat is satisﬁed), no ventilation, no ﬁltration, no
inﬁltration. Each relative risk is calculated as the probability of infection with a particular ﬁlter installed and for
a certain ach divided by the probability of infection for the
baseline. For example, a relative risk of infection of 0.8
provides 20% reduction of risk over the baseline.
The following improvement over baseline is considered for the different scenarios: HVAC system running
100%, 0.35 ach and 1 ach and MERV 4, MERV 7, MERV 11
and MERV 14. For some scenarios, we explore masks
and portable air cleaners' effect on risk reduction. We
are assuming people will not wear masks at home when
no known infection exists (Case 1 and Case 2). In all
cases the highest risk of infection is in the space with the
infected person (the source). The risks for other than
those spaces addressed are not the same.

Bedroom, Sleeping 8 Hours
SARS-CoV-2

1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0

22%
Less

Resting - Speaking, Baseline Absolute Risk = 15%
36%
Less

41%
Less

47%
Less

52%
Less

54%
Less

56%
Less

57%
Less

0.35 ach 1 ach 0.35 ach 1 ach 0.35 ach 1 ach 0.35 ach 1 ach
MERV 4 MERV 4 MERV 7 MERV 7 MERV 11 MERV 11 MERV 14 MERV 14

Improving the ﬁltration in the system can lower the risk
by two-thirds.
Figure 3 shows the effect of awake time in the bedroom
(Scenario 2). In this case the infected person is assumed
to be speaking during the two-hour time period.
Improvements in ﬁltration can cut the risk in half.
The baseline risk of not running the HVAC system at
all is 20% for sleeping for 8 hours and 15% for talking for
2 hours. More activity by the infected person increases
risk. In this case talking loudly in the bedroom for 2
hours would increase the risk to 67%-or more than four
times higher risk to the well person than normal talking.

Case 2: Infected Person Spends Time With Other Household Members or a
Visitor in a 500 ft2 (46 m2) Open Area (Infection is Not Known)
What is the risk to the other household members? Open
or common areas are in most houses built in the last 50
years, and they usually feature the kitchen opening into a
family room or a living room. The open area is assumed to
be 500 ft2 (46 m2) although it could be larger or smaller.
D ECEM BER 2020

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