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,
26

ASHRAE JOURNAL

ashrae.org

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


http://ashrae.org

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
ASHRAE Journal - March 2021 - 23
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
ASHRAE Journal - March 2021 - 39
ASHRAE Journal - March 2021 - 40
ASHRAE Journal - March 2021 - 41
ASHRAE Journal - March 2021 - 42
ASHRAE Journal - March 2021 - 43
ASHRAE Journal - March 2021 - 44
ASHRAE Journal - March 2021 - 45
ASHRAE Journal - March 2021 - 46
ASHRAE Journal - March 2021 - 47
ASHRAE Journal - March 2021 - 48
ASHRAE Journal - March 2021 - 49
ASHRAE Journal - March 2021 - 50
ASHRAE Journal - March 2021 - 51
ASHRAE Journal - March 2021 - 52
ASHRAE Journal - March 2021 - 53
ASHRAE Journal - March 2021 - 54
ASHRAE Journal - March 2021 - 55
ASHRAE Journal - March 2021 - 56
ASHRAE Journal - March 2021 - 57
ASHRAE Journal - March 2021 - 58
ASHRAE Journal - March 2021 - 59
ASHRAE Journal - March 2021 - 60
ASHRAE Journal - March 2021 - 61
ASHRAE Journal - March 2021 - 62
ASHRAE Journal - March 2021 - 63
ASHRAE Journal - March 2021 - 64
ASHRAE Journal - March 2021 - 65
ASHRAE Journal - March 2021 - 66
ASHRAE Journal - March 2021 - 67
ASHRAE Journal - March 2021 - 68
ASHRAE Journal - March 2021 - 69
ASHRAE Journal - March 2021 - 70
ASHRAE Journal - March 2021 - 71
ASHRAE Journal - March 2021 - 72
ASHRAE Journal - March 2021 - 73
ASHRAE Journal - March 2021 - 74
ASHRAE Journal - March 2021 - 75
ASHRAE Journal - March 2021 - 76
ASHRAE Journal - March 2021 - 77
ASHRAE Journal - March 2021 - 78
ASHRAE Journal - March 2021 - 79
ASHRAE Journal - March 2021 - 80
ASHRAE Journal - March 2021 - 81
ASHRAE Journal - March 2021 - 82
ASHRAE Journal - March 2021 - 83
ASHRAE Journal - March 2021 - 84
ASHRAE Journal - March 2021 - 85
ASHRAE Journal - March 2021 - 86
ASHRAE Journal - March 2021 - 87
ASHRAE Journal - March 2021 - 88
ASHRAE Journal - March 2021 - 89
ASHRAE Journal - March 2021 - 90
ASHRAE Journal - March 2021 - 91
ASHRAE Journal - March 2021 - 92
ASHRAE Journal - March 2021 - 93
ASHRAE Journal - March 2021 - 94
ASHRAE Journal - March 2021 - 95
ASHRAE Journal - March 2021 - 96
ASHRAE Journal - March 2021 - 97
ASHRAE Journal - March 2021 - 98
ASHRAE Journal - March 2021 - 99
ASHRAE Journal - March 2021 - 100
ASHRAE Journal - March 2021 - 101
ASHRAE Journal - March 2021 - 102
ASHRAE Journal - March 2021 - 103
ASHRAE Journal - March 2021 - 104
ASHRAE Journal - March 2021 - Cover3
ASHRAE Journal - March 2021 - Cover4
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_VJKSRY
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_SDHUTC
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_JPPKRR
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_SDLTTH
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_CKLLES
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_SLDOX
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_HJETUK
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_OLUHGE
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_amca_2020october
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_amca_2020october_v2
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_ZERDGH
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_QVMNEO
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_RTPOKE
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_BBATRE
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_STUBMW
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_TPEMPE
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_JNMKDS
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_FBTTPA
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_WQMMNE
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_TVBRYN
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_showguide2020
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_KTUZMA
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_ABEDGD
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_201910
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_201909
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_amca_2019septmeber_v2
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_amca_2019september
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_201908
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_201907
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_201906
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_201905
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_201904
https://www.nxtbook.com/nxtbooks/ashrae/meetingplanner_2019april
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_201903
https://www.nxtbook.com/nxtbooks/ashrae/meetingplanner_2019march
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_201902
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_201901
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_showguide2019
https://www.nxtbook.com/nxtbooks/ashrae/meetingplanner_2018december
https://www.nxtbook.com/nxtbooks/ashrae/meetingplanner_2018november
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_amca_2018fall_v2
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_amca_2018fall
https://www.nxtbook.com/nxtbooks/ashrae/meetingplanner_2018october
https://www.nxtbook.com/nxtbooks/ashrae/ashraemexico_2018
https://www.nxtbook.com/nxtbooks/ashrae/meetingplanner_201810
https://www.nxtbook.com/nxtbooks/ashrae/ashraeinsights_201806
https://www.nxtbook.com/nxtbooks/ashrae/meetingplanner_201805
https://www.nxtbook.com/nxtbooks/ashrae/meetingplanner_201804
https://www.nxtbook.com/nxtbooks/ashrae/meetingplanner_201803
https://www.nxtbook.com/nxtbooks/ashrae/meetingplanner_201712
https://www.nxtbook.com/nxtbooks/ashrae/meetingplanner_201711
https://www.nxtbook.com/nxtbooks/ashrae/meetingplanner_201710
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_amca_2017fall_v2
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_amca_2017fall
https://www.nxtbook.com/nxtbooks/ashrae/meetingplanner_201709
https://www.nxtbook.com/nxtbooks/ashrae/meetingplanner_201705
https://www.nxtbook.com/nxtbooks/ashrae/ashrae_meetinginsert_201610
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_amca_2016fall_v2
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_amca_2016fall
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_acrexindia
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_amca_2015summer_v2
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_amca_2015summer
https://www.nxtbook.com/nxtbooks/amca/2014summer2
https://www.nxtbook.com/nxtbooks/amca/2014summer
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_acma_2014summer
https://www.nxtbook.com/nxtbooks/ashrae/meetingplanner_201311
https://www.nxtbook.com/nxtbooks/ashrae/meetingplanner_201309
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_acmasupp_2013fall
https://www.nxtbook.com/nxtbooks/ashrae/meetingplanner_201305
https://www.nxtbook.com/nxtbooks/ashrae/meetingplanner_201303
https://www.nxtbook.com/nxtbooks/ashrae/pubcatalog_2013winter
https://www.nxtbook.com/nxtbooks/ashrae/meetingplanner_201211
https://www.nxtbook.com/nxtbooks/ashrae/achr_expo_mexico2012
https://www.nxtbook.com/nxtbooks/ashrae/meetingplanner_201209
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_201208_v3
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_201208_v2
https://www.nxtbook.com/nxtbooks/ashrae/pubcatalog_2012summer
https://www.nxtbook.com/nxtbooks/ashrae/meetingplanner_201205
https://www.nxtbook.com/nxtbooks/ashrae/meetingplanner_201203
https://www.nxtbook.com/nxtbooks/ashrae/pubcatalog_2012winter
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_201111_v2
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_201109_v2
https://www.nxtbook.com/nxtbooks/ashrae/pubcatalog_2011summer
https://www.nxtbook.com/nxtbooks/ashrae/meetingplanner_201105
https://www.nxtbook.com/nxtbooks/ashrae/meetingplanner_201103
https://www.nxtbookmedia.com