ASHRAE Journal - April 2021 - 10
TECHNICAL FEATURE
Virus Transmission Modes and Mitigation Strategies, Part 2
Airborne Transmission
And Distribution
BY JONATHAN BURKETT, P.E., HFDP, MEMBER ASHRAE
Part 1 of this article in last month's ASHRAE Journal looked at what makes up a virus, how
it can be released into a space through droplets and aerosols, and the effect of certain
environmental conditions like temperature and humidity on the droplets. Part 2 will
look at airborne transmission and the effect of common air distribution methods on
virus propagation.
Airborne Transmission
As mentioned in Part 1, as droplets evaporate the
residual material contained in the droplet (mucus, dust,
solids, virions, etc.) form droplet nuclei. These droplet
nuclei can then remain suspended for long periods of
time and be transmitted long distances from their origin. Airborne transmission can be further broken up
into short-range and long-range transmission. Shortrange airborne transmission occurs when two people
are in close proximity (typically <2 m [6.6 ft]) and the
aerosols are directly inhaled.
Long-range airborne transmission occurs when droplets
evaporate to a size where they can remain suspended in
the air for prolonged periods of time. These droplets can
then be carried by air currents, thermal plumes, electrostatic charges and pressure differentials throughout the
room or to connected rooms through the HVAC system.
A study by Chen, et al., found that when comparing
large droplet transmission and short-range airborne
transmission, the short-range airborne mode dominated the exposure risk for both talking and coughing.1,2
The large droplets only beat out the short-range airborne mode when the droplets were larger than 100 µm
and the subjects were close (≤0.2 m [7.9 in.] for talking
and ≤0.5 m [20 in.] for coughing).1
The proximity to the infected person greatly affects
the exposure risk.2 Research has shown that at distances
within 1 m to 1.5 m (3 ft to 5 ft) a substantial increase in
exposure to droplet nuclei exhaled by the source patient
exists.2 General dilution ventilation does not directly
affect these large droplets or short-range airborne
modes of transmission when a susceptible person is in
close proximity to the source.2
The importance of long-range airborne transmission
in the spread of pathogens has been debated over the
years, but viruses such as measles and chicken pox are
thought to be capable of infections by long-range airborne transmission.3 Other viruses, like coronaviruses
Jonathan Burkett, P.E., is a mechanical engineer at Buford Goff & Associates in Columbia, S.C.
This peer-reviewed article does not represent official ASHRAE guidance. For more information on ASHRAE resources on COVID-19, visit ashrae.org/COVID19.
10
ASHRAE JOURNAL
ashrae.org
APRI L 2021
http://www.ashrae.org/COVID19
http://www.ashrae.org
ASHRAE Journal - April 2021
Table of Contents for the Digital Edition of ASHRAE Journal - April 2021
Contents
ASHRAE Journal - April 2021 - Intro
ASHRAE Journal - April 2021 - Cover1
ASHRAE Journal - April 2021 - Cover2
ASHRAE Journal - April 2021 - 1
ASHRAE Journal - April 2021 - Contents
ASHRAE Journal - April 2021 - 3
ASHRAE Journal - April 2021 - 4
ASHRAE Journal - April 2021 - 5
ASHRAE Journal - April 2021 - 6
ASHRAE Journal - April 2021 - 7
ASHRAE Journal - April 2021 - 8
ASHRAE Journal - April 2021 - 9
ASHRAE Journal - April 2021 - 10
ASHRAE Journal - April 2021 - 11
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ASHRAE Journal - April 2021 - 14
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ASHRAE Journal - April 2021 - Cover3
ASHRAE Journal - April 2021 - Cover4
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