ASHRAE Journal - July 2020 - 18

TECHNICAL FEATURE

FIGURE 1 The Wells evaporation-falling curve for droplets (reformatted from source2).

Nomenclature
Concentration of contamination
(genome copies/m3)

CCE

Contamination containment efﬁciency

Exposure

The period during which the
occupants are exposed to infection, h

Virus retention efﬁciency of air ﬁlter

Zone air distribution efﬁciency

The ratio between the ﬂoor area of
source space and total ﬂoor area served
by the system

HIIDR

Human inhalation infectious dose risk
factor

Medium tissue culture infectious dose,
genome copies (in virology known as
TCID50 )

Human inhalation infectious dose
(in virology known as ID50 )

The percent of outdoor air in total supply airﬂow rate

rate varies from 8 to 20. Therefore, the minimal upward
air velocity is 8 × 2.5 m/3,600 s = 0.005 m/s or (1 ft/min).
The horizontal and vertical air velocity vectors created by the system are greater than the settling velocity.
This simple calculation allows us to make a conclusion
that the air movement entrains the aerosol and transfers it through the ductwork from the infected space
to the other spaces. Swabs taken from the exhaust air
outlets in a hospital room with a SARS-CoV-2 (COVID19) infected patient tested positive,5 supporting the
conclusion.
Recently, Finnish researchers6 used a supercomputer
to simulate the ﬂuid dynamics of a 20 µm virus aerosol
in a grocery store. The simulation demonstrated that
the aerosol can transfer from one aisle to another with
upward air currents.
The "ASHRAE Position Document on Airborne
Infectious Diseases"7 has general recommendations to
increase outdoor air volume and install UVC germicidal
lights to better protect the indoor environment from airborne virus diseases.
The simpliﬁed engineering analysis and the

Droplet Diameter (µm)
0

ASHRAE JOURNAL

ashrae.org

J U LY 2 0 2 0

160

Time to
Evaporate
Completely

publications indicate that virus spread by air-handling
systems is a valid concept.

Existing Ventilation Standards
ASHRAE Standard 62.1-20198 provides minimum ventilation air requirements for nonresidential buildings,
assuming a consistent source of contamination in the
building. The standard allows recirculating a portion
of the indoor air for most occupancies. For spaces with
hazardous contamination, the standard excludes the
recirculation. Virus-laden aerosol is not a consistent
contamination and is not addressed by the standard.
The standard requires air ﬁlters to have a minimum
efﬁciency reporting value (MERV) of not less than 8 in all
systems with a cooling coil. The MERV-8 ﬁlter9 retains
70% of particle sizes 3 µm to 10 µm. Viruses may have
smaller sizes. For example, the inﬂuenza A virion has an
average size of 2.5 µm.10 The SARS-CoV-2 virion has sizes
in the range of 0.6 µm to 1.4 µm.11
Many systems in public and institutional buildings are
equipped with a MERV-4 to MERV-8 ﬁlter.12 These ﬁlters
have no tested capacity to capture particles of the virion
sizes below 3 µm. Systems for hospitals and other special
facilities typically have ﬁlters rated MERV-13 or higher.
As per ASHRAE Standard 52.2-2017,9 a MERV-13 ﬁlter
should remove only 50% or less of 0.3 µm to 1 µm particles at the standard air velocity across the ﬁlter bank.
The air recirculation and limited air ﬁlter efﬁciency
make potentially infectious particles/droplets spread
from space to space highly probable.
The next section describes a simple mathematical
model estimating the virus spread.

Renat Manassypov, P.Eng., Ph.D., is a solutions development engineer at the Honeywell Energy Services Group Canada, Markham, Ontario.
18

120

2m
Time (s)

http://ashrae.org

ASHRAE Journal - July 2020

Table of Contents for the Digital Edition of ASHRAE Journal - July 2020