ASHRAE Journal - November 2023 - 52
TECHNICAL FEATURE
temperature is not analyzed. Similarly
the impact of supply air volumes and the
resulting diffuser throws is not analyzed.
FIGURE 2A Airflow patterns in an office space showing large recirculation zones in all the cases.
Spread Index (SI)TC
Ventilation performance was analyzed using
the Spread Index, which is the ratio of the
volume of the space in which the infection
risk level is above a certain value (10%) to the
total volume of the space.8 The infection risk
level is determined by the Wells-Riley model
as described in the next section. Ideally,
the ventilation systems should minimize
the spread of contaminants and reduce the
probability of infection everywhere in the
space. Assuming the target concentration
(TC) is one that results in low risk of infection,
the ideal (SI)TC should always be close to zero.
Acceptable concentrations depend on
several established risk factors including
the type and quantity of pathogens in the space and
exposure limits. Therefore, for each space (SI)TC may
need to be evaluated for risk from multiple pathogens.
The UV-C dose, design of the HVAC system and the
resulting flow path of airborne contaminants play
an important role in determining the (SI)TC levels
in a space. The Spread Index provides a normalized
comparison of UVGI and ventilation effectiveness for
various cases. In this study, the Spread Index of the
probability of infection is set arbitrarily at 10%, and
hence, (SI)10 is evaluated.
Probability of Infection
Infection risk assessment is performed with the aid
of the Wells-Riley model, which is established for
prediction of respiratory infection probability in indoor
spaces.11,12 This model, as stated in Equation 2, considers
probability of infection in terms of the number of
infectious quanta (dose that will cause an infection in
63% of a susceptible population) inhaled to evaluate the
probability of infection
PI = =− −
C
S
1 exp
Iqpt
Q
(2)
where PI is the probability of infection (a ratio of the
number of infection cases to the number of susceptible),
I is the number of infectors, p is the pulmonary
52
ASHRAE JOURNAL ashrae.org N OVEMBER 2 0 2 3
Case 1
Case 2
Case 3
Case 4
FIGURE 2B Spread Index of mean age of air (SI)1,200 indicating the extent of the
space volume at or above the 1,200 seconds of mean age of air.
SI: 30%
SI: 64%
Case 1
Case 2
SI: 30%
SI: 23%
Case 3
Case 4
ventilation rate of a person, q is the quanta generation
rate per infector, t is the exposure time interval and Q
is the room ventilation rate with clean air. This form of
the model assumes well-mixed conditions and predicts
a single number for the infection probability for an
entire indoor space. Other implementations have
used various methods to address spatial and temporal
http://www.ashrae.org
ASHRAE Journal - November 2023
Table of Contents for the Digital Edition of ASHRAE Journal - November 2023
Table of Contents
ASHRAE Journal - November 2023 - Intro
ASHRAE Journal - November 2023 - Cover1
ASHRAE Journal - November 2023 - Cover2
ASHRAE Journal - November 2023 - 1
ASHRAE Journal - November 2023 - Table of Contents
ASHRAE Journal - November 2023 - 3
ASHRAE Journal - November 2023 - 4
ASHRAE Journal - November 2023 - 5
ASHRAE Journal - November 2023 - 6
ASHRAE Journal - November 2023 - 7
ASHRAE Journal - November 2023 - 8
ASHRAE Journal - November 2023 - 9
ASHRAE Journal - November 2023 - 10
ASHRAE Journal - November 2023 - 11
ASHRAE Journal - November 2023 - 12
ASHRAE Journal - November 2023 - 13
ASHRAE Journal - November 2023 - 14
ASHRAE Journal - November 2023 - 15
ASHRAE Journal - November 2023 - 16
ASHRAE Journal - November 2023 - 17
ASHRAE Journal - November 2023 - 18
ASHRAE Journal - November 2023 - 19
ASHRAE Journal - November 2023 - 20
ASHRAE Journal - November 2023 - 21
ASHRAE Journal - November 2023 - 22
ASHRAE Journal - November 2023 - 23
ASHRAE Journal - November 2023 - 24
ASHRAE Journal - November 2023 - 25
ASHRAE Journal - November 2023 - 26
ASHRAE Journal - November 2023 - 27
ASHRAE Journal - November 2023 - 28
ASHRAE Journal - November 2023 - 29
ASHRAE Journal - November 2023 - 30
ASHRAE Journal - November 2023 - 31
ASHRAE Journal - November 2023 - 32
ASHRAE Journal - November 2023 - 33
ASHRAE Journal - November 2023 - 34
ASHRAE Journal - November 2023 - 35
ASHRAE Journal - November 2023 - 36
ASHRAE Journal - November 2023 - 37
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ASHRAE Journal - November 2023 - 53
ASHRAE Journal - November 2023 - 54
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ASHRAE Journal - November 2023 - 57
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ASHRAE Journal - November 2023 - 67
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ASHRAE Journal - November 2023 - 72
ASHRAE Journal - November 2023 - Cover3
ASHRAE Journal - November 2023 - Cover4
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