ASHRAE Journal - May 2021 - 11

TECHNICAL FEATURE

emphasis was placed on direct and indirect contact as
major modes of transmission, with recommendations
focused on hand hygiene4 and physical distancing.5 In
July 2020, Morawska and Milton, with 239 scientists
and medical professionals as signatories, summarized
a signiﬁcant and growing body of evidence supporting
airborne transmission of COVID-19 and made an urgent
appeal to professionals to pursue the development and
implementation of intervention measures to mitigate or
interrupt airborne transmission pathways.6 Subsequent
study results aligned closely with Morawska and Milton's
call to action, although for brevity's sake further published work supporting airborne transmission will not
be cited here. One aim of this study is to provide recommendations for interventions that can decrease the
likelihood of COVID-19 transmission in high occupant
density indoor environments like classrooms.

Aerosol Particles and Airborne Transmission of COVID-19

As Nardell and Nathavitharana note, the term " airborneÓ and the related lexicon describing infectious
bioaerosols have not been uniformly applied in the literature regarding the person-to-person spread of respiratory pathogens.7 Here, we deﬁne a respiratory droplet as
an aqueous droplet composed of saliva, mucous, infectious agent(s), and other biomatter. Respiratory droplets
can be generated directly and dispersed to the air by a
person coughing, sneezing, talking, singing or vomiting; and indirectly by the aerosolization of feces during sewage removal and treatment.8 When an infected
individual is actively shedding virions being replicated
within their body, these dispersed respiratory droplets
serve as transport vehicles, with each respiratory droplet
potentially carrying a viral payload into the air, where
it can infect other individuals in the vicinity (see sidebar " What Viral Payload Could Be Contained Within a
Respiratory Droplet? " ).
Respiratory droplets can range from submicron to
tens or hundreds of microns in diameter. Large droplets
(approximately 10 µm or larger) and very large droplets
(much greater than 10 µm) tend to either quickly fall
to the ground by gravity within about 1.5 m (5 ft) of the
generation source or rapidly evaporate.10 If these large
respiratory droplets are inhaled, they tend to deposit
into the upper respiratory tract,11 where viable infectious pathogens within the droplet can infect a susceptible individual. Aerosol particles are respiratory droplets

What Viral Payload Could Be Contained
Within a Respiratory Droplet?

The answer to this question is not known with
certainty, and one can expect the
actual number to be highly variable and ranging from a single
virion to a maximum number of
virions on the limits of volume
packing of spheres within a sphere. Illustration of the SARSCoV-2 virus (CDC) .
The diameter of SARS-CoV-2 is
9
approximately 100 nm. If we assume that a 1 µm
spherical droplet has a packing factor of 0.3, its viral
payload could be 300 SARS-CoV-2 virions.

of smaller diameter, in the 200 nm to 5 µm range.
Particles smaller than 10 µm are easily inhaled and able
to penetrate farther into the respiratory tract than larger
ones, depositing deeper into the lungs.
Our focus in this article is on inhalable virus-laden
aerosol particles (bioaerosols) in the 200 nm to 5 µm
range. Aerosol particles in this size range have been
demonstrated to carry viable SARS-CoV-2,12 and they
are consistent with the particle sizes humans generate
through activities that are common in a classroom environment, including speaking, coughing and sneezing.8
More importantly, aerosol particles in this size range
readily breach the current 2 m (6 ft) physical distancing
guidelines because of their ability both to remain airborne for extended periods and to be carried by air currents within the indoor environment in what is referred
to as " long-range transport. "
Beyond recommendations for hand hygiene, cleaning/
sanitizing solid surfaces and physical distancing, CDC
guidelines aiming to limit the spread of COVID-19 in
indoor spaces also recommend the following:13 - 15
* Wearing of face masks. In recommending masks,
the CDC excludes medical procedure masks, respirators, or other medical personal protective equipment.
* Ventilation of indoor spaces to the greatest extent
possible. This includes increasing total supply air and
outdoor air rates when possible and pre- and post-occupancy ventilation purging of occupied spaces.
In this ﬁrst part of the series, we show that cloth
face coverings provide modest protection from
virus-laden aerosol particles based on ﬁeld measurements of the actual effective ﬁltration efﬁciency for
M AY 2021

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