ASHRAE Journal - April 2021 - 11

TECHNICAL FEATURE

and inﬂuenza, appear to have the capability for longrange airborne transmission under certain conditions.3

Viral Load and Infectious Dose
In 1955 Wells suggested the concept of quantal infection as a unit of measure of the infectious dose.4 A
quantum of infection is the minimum number of
infectious airborne particles required to produce
infection in a susceptible host.5,6 Infectious quanta
shouldn't be confused with the number of infectious
particles released from the source (viral shedding).
Quantum of infection is a measurement of the inhaled
particles able to cause infection.7 For some diseases,
such as tuberculosis or smallpox, the infectious dose
required to produce infection is low, even down to one
organism.8
The idea of quantum of infection was used in the
Wells-Riley equation, which assumes a well-mixed
room (i.e., droplet nuclei are instantaneously and
evenly distributed in a space). Wells deﬁned the
quantum of infection as being 63.2% of occupants
infected when each occupant breathed one infective
particle.7 The major limitation for the Wells-Riley
equation is in the estimation process of the quanta
generation rate. This rate must be estimated from an
outbreak case in which the attack rate is substituted
back into the equation. This backward estimation
assumes all infection cases are caused by airborne
infections; inﬂuencing factors, such as survival rate,
deposition rate, etc., can cause the rate to vary widely
across different cases.9
A dose-response type model is a toxicological approach
to assess infection risk that addresses some of the
shortcomings of the Wells-Riley equation.9 In the doseresponse model, infectious dose data is required to
construct the dose-response relationship.10 This infectious dose data is sometimes only available for animals
and requires extrapolation to adapt to humans. The
dose-response model is more ﬂexible than the WellsRiley equation in that it can be used to model transport
methods other than the airborne route, but it requires
the dose data, which may not be known early on in an
outbreak.
The basic reproduction number (R0) is another popular metric (Table 1). R0 is an indicator of the contagiousness or transmissibility of an infectious agent and is
deﬁned as the mean number of infections caused by an

TABLE 1 Basic reproduction number (R0 ) for various diseases.

Measles (Pre-Vaccination:

DISEASE

REPRODUCTION NUMBER

1912 - 1928)12,32

12 - 18

Mumps32

4-7

Outbreak)33

1.51 - 2.53

(COVID-19)34

1 - 2.8

Inﬂuenza35

0.9 - 2.1

Ebola (2014
SARS-CoV-2

Seasonal

infected individual in a susceptible population.11 Values
of R0 greater than 1 indicate the infectious agent can
start spreading in a population. In general, the higher
the R0 the harder it is to control the spread of disease if
an epidemic breaks out.12 While R0 can be a useful metric, care must be taken to use the most recent data available to calculate it. Many reported R0 still use obsolete
data from outbreaks in the early 20th century, which
may not be valid today.12
While not fully understood, some people can act as
superspreaders; these people infect a disproportionately
large number of susceptible contacts.13 It is believed
superspreading is a normal feature of disease spread,11
and it has been linked to several outbreaks, such as the
2003 SARS-CoV outbreak in Hong Kong and the 2015
MERS-CoV outbreak in South Korea.14 Not taking into
account these superspreaders can skew an R0 value
based on population estimates.11

Additional Transmission and Transport Modes
An occupied space is a dynamic environment full
of complex interactions between occupants, thermal plumes, vortices created by movement of people
within the space, the ventilation equipment, and other
environmental conditions. As a person moves around
a space, the layer of air closest to the body is comparable to their walking speed.5 This pushing of the front
layer creates a volume ﬂux of air and a wake bubble
behind the person. At walking speeds above 0.2 m/s
(0.45 mph), the thermal plume gives way to the created
wake, which mixes strongly with the surrounding air,
entraining room air and transporting it by the wake.15
A person walking forward at 1 m/s (2.2 mph) would create a volume ﬂux of about 255 L/s (540 cfm) with an
attached wake of 76 L to 230 L (2.7 ft3 to 8.1 ft3) behind
them.5 Walking on carpet can also resuspend pathogens
attached to dust from the surface.10 These ﬂoor-level
contaminants would now be smaller after evaporation
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ASHRAE Journal - April 2021

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