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TECHNICAL FEATURE

The thermal plume can be affected by body position
(sitting vs. standing) and clothing, but is still significant
while wearing pants and long sleeves.33

Droplet Release with Masks and Shields
When looking at face masks (Figure 3, page 27), there are
all types of masks from commercial to homemade. A study
by Viola, et al., found that all of the face covers they tested
reduced the frontal jet by more than 90%.32 A similar
study by Verma, et al., found that for a simulated cough a
stitched mask limited the frontal respiratory jet to 64 mm
(2.5 in.); a commercial mask to 203 mm (8 in.); a folded
handkerchief to 381 mm (1 ft, 3 in.); and a bandana to 1 m
(3 ft, 7 in.).29 During normal breathing, when the expelled
flow is weak, the air is slowed down by the mask or redirected through the narrow gaps such that air can be
captured by the upward convection of the thermal plume
without being displaced significantly horizontally.29,32
The face shield (Figure 4) has been shown to deflect the
expelled frontal respiratory jet downward and limit the
horizontal travel of large droplets. Smaller droplets, however, can move around the bottom of the shield and leak
around the gaps on the sides and top of the shield. As
these droplets move forward and laterally, they begin to
rise upward after a few seconds, since they are warmer
than the ambient air, and have the potential to spread
approximately 0.9 m (3 ft) in these directions. Studies also
showed that masks, shields and ambient disturbances can
cause the airflow to spread toward a person's back.32,34

Conclusions
This article has taken a look at what makes up a virus,
how it can be released into the space through droplets
and aerosols and the effect of certain environmental
conditions like temperature and humidity on the droplets. The next article will cover airborne virus transmission and the effect of air distribution.

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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
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