ASHRAE Journal - July 2020 - 22

TECHNICAL FEATURE

temperature. The source space ceiling height is 3.5 m
(11 ft). For these conditions, the zone air distribution
efficiency (Ez) is 0.7.8
The reference system has the same air distribution
pattern, the ceiling height of 2.75 m (9 ft), and the supply air temperature is always less than the indoor temperature. For these conditions, Ez = 1.0.
The calculated virus containment efficiency of the
system in question (CCE) is 69%, while for the reference
system the value is 96%. To achieve the 96% efficiency of
the reference system, the system needs modification. It
can be achieved by increasing the outdoor air ratio from
15% to 75%.
The size of the SARS-CoV-2 virus is much smaller than
the size of the influenza A virus. Therefore, the MERV-8
retention efficiency and, subsequently, the containment
efficiency for the SARS-CoV-2 virus should be lower than
the one for the influenza A virus. The high virulence of
the SARS-CoV-2 virus requires an increase of the reference containment efficiency, for example, to 99%. To
achieve this value, additional modifications to the system would be required.

Example Two
Estimate the Engineering Risk of Acquiring the Infection in Example One
Assume the occupants in the classrooms in Example
One are exposed to the infection agent for 4 hours.
To complete the estimate, a reference virus concentration in the source space (CS ) is required.
Concentration of the influenza A virus was measured
in hospitals, day-care facilities and commercial passenger airplanes.10 The average value was found as
(1.6 ±0.9) × 104 gc/m3. The measurements were taken
in spaces with an estimated average footprint area
of 40 m2 (430 ft2), estimated average occupant density
of 60 people/100 m2 (60 people/1,076 ft2), average air
change rate of 10, air temperature of 22°C (72°F) and
humidity of 40%.
Given the occupant density in the classrooms is
30 students/100 m2 (30 people/1,076 ft2), the concentration CS was selected to be proportionally lower than the
measured value and equal to 8,000 gc/m3.
The virus concentration in the recipient spaces was
calculated by Equation 1:
CR = 8,000 (1 - 0.69) = 2,480 gc/m3

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For the influenza A virus, the estimated average value
of K1 is 452 genome copies (gc) and K2 = (0.6 to 3.0) K1 for
the aerosol inhalation route.10 For this example, assume
K2 = 1.8 K1.
The risk factor of inhaling infectious doses by the occupants in the recipient spaces during the exposure time
was estimated using Equation 3:
HIIDR = 0.67 × 2,480 gc/m3 × 4 h/452 gc/1.8 = 8.2 doses
For the reference system, the HIIDR is 1.01. Comparison
of the HIIDR values shows the engineering risk of acquiring the infection spread by the system in question is
substantially higher than the risk related to the reference system. To mitigate the risk, the system needs
modifications.
The inputs in Equation 3 carry measurement uncertainties. The standard deviation of the HIIDR for the system
in question was evaluated using the input uncertainties
and the common formula used for indirect measurements.14 The resulting 90% confidence uncertainty
interval around the HIIDR value of 8.2 was estimated as
±2.4. The estimate gives a hypothetical value that could
be achieved if the input data were obtained by multiple
repetitive experiments.

Conclusions
The analysis of existing ventilation standards, the
basics of bioaerosol behavior and the design of standard
air-handling systems demonstrated that during viral
infection outbreaks, air-handling systems may become
a source of infectious agent spread. Proposed herein is a
mathematical model of the virus-laden aerosol propagation through air-handling systems.
The model has been used to develop a simple engineering method for evaluating the contamination
containment efficiency of air-handling systems and
the engineering risk of acquiring infection in recipient
spaces based on standard units used in virology.
Acceptable contamination containment efficiency
and associated engineering risk of acquiring infection
should be provided by health-care authorities in consultation with an engineering board. The acceptable levels
for specific types of contamination should be included
in relevant standards. The method applies to any type
of contamination and multiroom enclosures including
cruise ships.

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ASHRAE Journal - July 2020

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