ASHRAE Journal - December 2020 - 42

TECHNICAL FEATURE

Absolute Risk (%)

Relative Risk of Infection

Scenarios: Time spent = 6 hours, simulatFIGURE 4 Relative risk of infection in open area.
ing a stay-at-home or school-from-home
Open Area, 6 Hours
SARS-CoV-2
scenario.
1.0
1. Resting (Speaking) = 9.4 quanta/hour
Resting - Speaking, Absolute Baseline Risk = 25%
17%
0.9
Less
Resting - Loudly Speaking, Absolute Baseline Risk = 85%
2. Resting (Loudly Speaking) = 60.5 quanta/
0.8
29%
32%
hour
Less
35%
0.7 Less
40%
Less
Results: Figure 4 shows a significant
45%
Less
47%
0.6
48%
50%
50%
increase in risk from greater activity. As in
Less
Less
53%
Less
Less
Less
0.5
59%
Less
the bedroom scenario, the risks are reduced
63%
Less
65%
67%
0.4
68%
by just running the HVAC fan with a MERV 4
Less
Less
Less
Less
filter. With more infectious quanta expelled
0.3
into the air with the higher activity, the
0.2
HVAC system cannot have as much benefi0.1
cial effect.
0.0
To quantify the impact of different activities for
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
the same exposure time, we run the model for
an exposure time of 2 hours doing the following:
FIGURE 5 Absolute infection risk comparing different activities.
1. Resting (Speaking) = 9.4 quanta/hour
Open Area, 2 Hours
2. Resting (Loudly Speaking) = 60.5 quanta/
SARS-CoV-2
100
hour
3. Light Exercise (Loudly Speaking) = 170
80
quanta/hour
Heavy Exercise +
4. Heavy Exercise (Loudly Speaking) =
60
Loudly Speaking
408 quanta/hour
Results: Figure 5 illustrates the increases in
40
Light Exercise + Loudly Speaking
risk from more activity over a 2-hour period.
20
This could be a neighbor who is socializing
Loudly Speaking
or friends of household members talking.
Baseline No HVAC System Running 0.35 ach, MERV 11
Speaking
0
The volume of the speaking depends on
0
100
200
300
400
500
the friends, their ages or local norms. Light
Quanta (1/h)
exercise could be dance practice, singing or
FIGURE 6 Absolute infection risk comparing different exposure hours.
cheerleading. Heavy exercise could result from some
Open Area, Resting - Speaking
sort of major repair work or a serious workout. Figure
SARS-CoV-2
30
5 illustrates the baseline risk from no HVAC system
Baseline No HVAC System Running 0.35 ach, MERV 11
25
running and with the HVAC running at 0.35 ach and
20
with MERV 11 filters. These risks are shown as per15
cent absolute risk and illustrate that higher quanta
10
emissions result in significantly higher risks. Visit
5
https://tinyurl.com/yxer2364 to see the values that are
0
input to achieve one of the absolute risk values illus0
2
4
6
Exposure Hours
trated in Figure 5 using the Jimenez spreadsheet. The
increase in quanta overwhelm the beneficial ventilation dilution and cleaning of the recirculated air. In the
As expected, spending less time with an infected perextreme situations, residential HVAC cannot overcome
son will decrease the risk of exposure. Figure 6 shows
the intensity of the infectious quanta in the air (i.e., one a comparison between different times of exposure
is more likely than not to become infected). However, in (2 hours, 4 hours, and 6 hours). Figure 6 illustrates the
all cases it is beneficial to have the HVAC running.
baseline risk from no HVAC system running and with
42

ASHRAE JOURNAL

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D ECEM BER 2020

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ASHRAE Journal - December 2020

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