ASHRAE Journal - March 2021 - 26

TECHNICAL FEATURE

developed dose-response models and used numerical
modeling techniques to provide more complete risk
assessments.17 There has been a lot of discussion about
where the line is between droplet and airborne transmission. The World Health Organization (WHO) and the
U.S. Centers for Disease Control and Prevention (CDC)
both use 5 microns to make this distinction, considering
particles with a diameter greater than 5 microns to be
droplets and those less than or equal to 5 microns to be
aerosols.12,18
Wells showed large droplets would deposit on the
ground before total evaporation.5 The size this occurs at
is referred to as the critical size. In 2007 Xie, et al., revisited the Wells evaporation-falling curve and found that
for free-falling droplets at a relative humidity (RH) of 0%,
50%, 70% and 90%, the critical sizes were 125 microns, 100
microns, 85 microns and 60 microns, respectively. These
values were smaller than those Wells had determined.12
As shown above, relative humidity has a great effect
on droplet evaporation and falling rate. When relative
humidity increases, the critical droplet size decreases.
Since large droplets will settle out of the airstream by
gravitational forces and small droplets will evaporate
quickly, maximum horizontal distance is achieved by
droplets of medium diameter.12 Xie, et al., found that
exhaled air carried droplets less than 1 m (3 ft) away
during normal breathing (1 m/s [3 fps]) and more than
6 m (20 ft) away during sneezing (50 m/s [164 fps]).12
Relative humidity not only plays a role in the evaporation and carriage of the droplet, but it can also affect
the survival rate of the pathogen. Research has shown
that viruses that have a lipid envelope (influenza, parainfluenza, coronaviruses, etc.) are more stable in dry
air (20% to 30% RH), while viruses without a protective
envelope (rhino, entero, adeno, etc.) are more stable
in moist air (70% to 90%).19,20 There is also indication
that for both lipid-enveloped and non-lipid-enveloped
viruses, minimal survival occurs at an intermediate RH
(40% to 70%).20 Some researchers have proposed that at
high relative humidity the initial salt concentrations are
maintained, and at low relative humidity rapid evaporation causes the salts to crystallize out of solution, yielding low salt concentrations and higher virion stability.21
Intermediate relative humidity results in increased salt
concentrations, resulting in virus inactivation.21
Humidity also has an effect on your nasal passages.
Mucous membranes lose water through evaporation,
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ASHRAE JOURNAL

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MARCH 2021

TABLE 1 Droplets produced.
ACTIVITY

NUMBER OF DROPLETS PRODUCED

Projectile Vomiting35

3 × 107

Sneeze25,36

40,000 to 1 million

Toilet Flush (High Pressure)37

145,000 to 287,000

Toilet Flush (Low Pressure Tank)37

14,500

Cough25,36

3,000 to 5,000

Loudly Speaking (5 min)38

3,000

words)36

250

Breathing (In Through Nose and Out
Through Mouth)39

98/L

Loudly Speaking (100

and low relative humidity can desiccate the nasal
mucosa (altering the mucus viscosity), lead to epithelial
damage and reduce mucociliary clearance, increasing
susceptibility to infection.22,23
In addition to humidity, temperature also has an effect
on the viral survival rate. Lower temperatures (7°C to
8°C [44.6°F to 46.4°F]) are thought to be ideal for airborne influenza survival, with survival decreasing as
temperatures increase.18,24 Moderate temperatures
(20.5°C to 24°C [68.9°F to 75.2°F]), and to a greater
extent high temperatures (>30°C [>86°F]) are associated with an increase in the rate of protein and nucleic
acid inactivation.18,24 Tang states, " Maintaining temperatures above 60°C [140°F] for more than 60 min is
generally sufficient to inactivate most viruses, though
this can be very dependent on the presence of any surrounding organic material (e.g., blood, feces, mucus,
saliva, etc.), which will tend to insulate the virus against
extreme environmental changes. " 20 The effect of low
temperatures and low humidity on viruses has led some
researchers to conclude that these factors may account
for the seasonal nature of the influenza virus.19,21

Droplet Formation and Release
Droplets can originate in different parts of the respiratory tract, and the number and size of droplets produced
can vary by activity (Table 1). During normal breathing
and speech, small droplets (d ≤ 1 μm) can be formed by
the bronchiolar fluid film burst mode.25 In this mode
fluids lining the airways of the lungs can form a film
across smaller airways at the end of exhalation (much
like how a soap solution can form a film across the end of
a child's bubble wand). As the bronchioles reopen during inhalation, these films can burst and release droplets into the alveoli to be expelled.25,26 Similarly, small
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