ASHRAE Journal - March 2021 - 27

TECHNICAL FEATURE

FIGURE 3 (TOP) Droplet release (face mask). FIGURE 4 (BOTTOM) Droplet release

FIGURE 2 Droplet release (uncovered).

(face shield).

A

A
C

θ0

D

B

*

C

D
B

A.
B.
C.
D.

Thermal body plume.
Large droplets are projected forward, landing on mucous membranes and clothing.
Expelled cloud/jet is directed forward.
Thermal body plume transports pollutants up.

*Initial angle of cloud trajectory.

Note: Figures are based on reference imagery of expiratory flow. See References 29, 31 - 33.

droplets can be formed by the adduction of the vocal
folds and vibration within the larynx during speech.27 In
addition to these smaller droplets, during coughing and
sneezing, droplets can be formed by the laryngeal mode
where shear stress on the mucus-air interface by the
rapid airflow can dislodge mucus from the airways and
produce droplets (d ≥ 1 µm).25,26,28 However, large droplets from the trachea produced during coughing readily
deposit within the oral and nasal cavities and may not be
released into the environment.25
The oral cavity mode of droplet formation is responsible for most of the large droplets (d ≥ 100 µm) during
speech and coughing.25 In this mode, movement and
contact of the tongue and lips can generate salivary
droplets.26 During a sneeze the soft palate and palatine
uvula depress, while the back of the tongue partially
closes the passage to the mouth and directs air through
the nasal cavity to expel mucus, droplets, and irritants.28
When a person coughs, sneezes or exhales, large droplets are projected out of the mouth a short distance following a ballistic trajectory affected by gravity and aerodynamic drag (Figure 2). Intermediate and small-sized
droplets can be dispersed in a multiphase turbulent gas
cloud that traps and carries these droplets and entrains
the ambient air around it. The moist and warm local
atmosphere within this cloud allows the contained droplets to have a longer evaporation time and can extend
the lifetime of the droplets.29 - 31
The turbulence of the gas cloud allows droplets of different sizes to settle out or evaporate based on the cloud
dynamics and the space environmental conditions,
in addition to the droplet size.30 As the larger droplets

A.
B.
C.
D.

Thermal body plume.
Leakage around the top of the mask, crown-ward.
Leakage through the mask.
Backward leakage from the mask.

Note: Figures are based on reference imagery of expiratory flow. See References 29, 31 - 33.
A
C

E
D
B

A.
B.
C.
D.
E.

Thermal body plume.
Leakage around the bottom of the shield.
Leakage from the top of the shield.
Horizontal movement of droplet nuclei.
Airflow diverted backward from the shield.

Note: Figures are based on reference imagery of expiratory flow. See Ref. 29, 31 - 34.

settle out and the average momentum of the cloud
becomes comparable to the buoyancy, the cloud will
start to rise.31 As the jet continues to lose momentum,
the remaining droplets within the cloud evaporate into
droplet nuclei that can remain suspended for hours.30
One's body thermal plume also affects the movement
of droplets and droplet nuclei. The layer of air closest to
the body is warmer and lighter than the surrounding
air, causing it to move upward as a thermal plume.32 The
plume starts near the feet as a laminar flow and grows in
velocity and thickness as it moves up the body, becoming
fully turbulent at the mid-chest level. Maximum velocity (0.2 m/s to 0.3 m/s [0.7 fps to 1 fps]) is reached about
0.5 m (1.6 ft) above the head, and the thickness of the
plume can reach 150 mm (5.9 in.) at the breathing zone.
This rising plume makes up about two-thirds of the total
inhaled air and makes entraining and transporting pollutants near the floor to the breathing zone possible.25
MARCH 2021

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ASHRAE Journal - March 2021

Table of Contents for the Digital Edition of ASHRAE Journal - March 2021

Contents
ASHRAE Journal - March 2021 - Intro
ASHRAE Journal - March 2021 - Cover1
ASHRAE Journal - March 2021 - Cover2
ASHRAE Journal - March 2021 - 1
ASHRAE Journal - March 2021 - Contents
ASHRAE Journal - March 2021 - 3
ASHRAE Journal - March 2021 - 4
ASHRAE Journal - March 2021 - 5
ASHRAE Journal - March 2021 - 6
ASHRAE Journal - March 2021 - 7
ASHRAE Journal - March 2021 - 8
ASHRAE Journal - March 2021 - 9
ASHRAE Journal - March 2021 - 10
ASHRAE Journal - March 2021 - 11
ASHRAE Journal - March 2021 - 12
ASHRAE Journal - March 2021 - 13
ASHRAE Journal - March 2021 - 14
ASHRAE Journal - March 2021 - 15
ASHRAE Journal - March 2021 - 16
ASHRAE Journal - March 2021 - 17
ASHRAE Journal - March 2021 - 18
ASHRAE Journal - March 2021 - 19
ASHRAE Journal - March 2021 - 20
ASHRAE Journal - March 2021 - 21
ASHRAE Journal - March 2021 - 22
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ASHRAE Journal - March 2021 - 24
ASHRAE Journal - March 2021 - 25
ASHRAE Journal - March 2021 - 26
ASHRAE Journal - March 2021 - 27
ASHRAE Journal - March 2021 - 28
ASHRAE Journal - March 2021 - 29
ASHRAE Journal - March 2021 - 30
ASHRAE Journal - March 2021 - 31
ASHRAE Journal - March 2021 - 32
ASHRAE Journal - March 2021 - 33
ASHRAE Journal - March 2021 - 34
ASHRAE Journal - March 2021 - 35
ASHRAE Journal - March 2021 - 36
ASHRAE Journal - March 2021 - 37
ASHRAE Journal - March 2021 - 38
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