ASHRAE Journal - July 2020 - 20

TECHNICAL FEATURE

The dimensionless coefficient CCE reflects the overall
ability of the system to suppress potential spread of the
contamination. It is a newly introduced characteristic of
the air-handling system that will be referred to as contamination containment efficiency.
The efficiency is calculated by the following formula
derived from the equations:
CCE = 1 − {1 + [Ez FA (1 − OA) (1 − EF )]-1 − (Ez FA)-1}-1 (2)
Note: the same system may exhibit different containment efficiency for different contamination. The
term "contamination" has been used to emphasize
that Equations 1 and 2 can be used for any airborne
contamination.

Sequence of Evaluating the Virus Containment Efficiency

Proposed Method of Predicting Virus Spread by
Air-Handling Systems
In the engineering practice, Equation 2 can be used to
compare the contamination containment efficiency of
air-handling systems as well as the impact of protective
engineered measures. The comparative analysis does not
require biological characteristics of the virus in question.
If the characteristics as described below are available,
the method can be extended with an optional procedure
of evaluating the contamination concentration in recipient spaces measured in human inhalation infectious
dose risk (HIIDR)10 factor and the engineering risk of a
human acquiring the infection. The term "engineering"
implies that the risk value depends only on the system's
performance.
The procedure replicates the standard approach10 that
requires the following biometric characteristics of the
virus received from virologists:
* K1: the number of virus particles that infect 50% of
cultured cells in a tissue culture plate; and
* K2: the amount of virus inhaled by a human that
signifies a 50% infection risk.
The concentration in recipient spaces (CR ) calculated
by Equation 1 is converted from the genome copies/m3
(gc/m3) units to the human inhalation infectious doses
in proportion to the infection exposure period and the
human air inhalation rate of 16 m3/day for an adult:15
HIIDR = 16 m3/24 h CR Exposure/K1 /K2
= 0.67 CR Exposure/K1 /K2
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To evaluate the engineering risk of acquiring the infection transferred by the air-handling system, the following criterion has been established: if the calculated
human inhalation infectious dose risk factor is greater
than one, the risk is unacceptably high (more than
50%), and the system should be modified to mitigate the
risk. The calculated HIIDR and actual risk of acquiring
the infection is not a linear correlation. For example,
HIIDR = three doses does not mean that the actual risk is
150%.

(3)

The input data is:
* Virus average particle size; and
* Design and operational conditions of the system.
The procedure includes the following steps:
1. Select a reference system and an acceptable virus
containment efficiency (CCEREF).
2. Use ASHRAE Standards 62.1-20198 and 52.2-20179
and building drawings to determine the zone air distribution efficiency (Ez ), the percent of outdoor air (OA)
and the air filter efficiency (EF ) related to the virus.
3. Use Equation 2 to calculate CCE.
4. If CCE < CCEREF, modify the system and repeat the
steps.

Example One
Evaluate Influenza A Virus Containment Efficiency of an
Air-Handling System in a Public School

An air-handling system serves five classrooms.
Evaluate the efficiency of the system to control potential
spread of the influenza A virus if one of the spaces is
infected. The reference system serves an emergency area
in a hospital. The average size of the influenza A virion is
assumed to be 2.5 µm.10
The system in question has a MERV-8 air filter with an
estimated efficiency to retain the virus (EF ) of 10%.9 As
per Standard CSA-Z317.2-15,13 the reference system has
two air filters, MERV-8 and MERV-14, with an estimated
retention efficiency of 75%.9 The source space-to-total
floor area ratio (FA) is 0.2 for both systems. For the system in question, the outdoor air ratio (OA) is 15%.8 As per
Standard CSA-Z317.2-15,13 the OA ratio for the reference
system is 33%.
The system in question is designed with a ceiling-toceiling air distribution pattern (Figure 2). In winter, the
supply air temperature is higher than the indoor air

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ASHRAE Journal - July 2020

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