ASHRAE Journal - October 2020 - 13
TECHNICAL FEATURE
more likely to rest.11 In the past more limited longitudinal transport has been postulated.12 However, more
recent research on a wide body airplane indicates that a
10% concentration of droplet nuclei remains after traveling 4.39 m (14.4 ft) or five rows.10
In terms of the quantifiable increased severe infection
risk from COVID-19 and other coronaviruses due to cabin
humidity this low, all we know for sure is that influenza
in the United States occurs primarily in the fall and winter.13 This is when relative humidity indoors with a heating system operating is perhaps 20% - 35% as opposed to
being 50% - 65% in summer air-conditioning weather.
In the case of COVID-19 with its person-to-person airborne infection risk, offsetting factors may be in play in
buildings. For example, outside air can enter buildings
naturally via open windows and envelope leakage, and
through door opening in ground-based public transit
vehicles. This cannot happen in aircraft. Further, in
buildings social distancing is more the norm and occupants in ground-based public transit vehicles often can
move around more freely, whereas in aircraft occupants
may have to remain in one place for hours with a potentially ill person nearby.
Air Change Rates and Filtration
While aircraft HEPA filtration removes almost 100% of
the 0.3 micron and larger particles circulating through
them (and supposedly, therefore, all viruses), the
amount of air recirculated through these filters and supplied to the passengers is one-eighth the amount circulated through MERV 13 office air filters, which remove at
least 30% of 0.3 micron particles and larger. Thus, with
their eight times larger airflows through less efficient
filters, building filters can remove twice the number
of viruses from the air supplied to each office occupant
than aircraft HEPA filters remove from the air they supply to aircraft cabin occupants.14,15
Aircraft cabin outdoor air changes per hour (ach) are
indeed high-perhaps 15 ach for a narrow body aircraft
and 13 ach for a wide body aircraft. However, a high outdoor air change in the case of densely occupied spaces
like an aircraft cabin or a subway car is not an indicator
of a high supply of virus-free air to the occupants. Three
parameters govern airborne virus exposure concentration in any space-occupancy density (spatial volume
FIGURE 1 Aircraft cabins are high occupancy density, with air currents moving
aerosols along four or more rows longitudinally either way, making social distancing impractical and infectious aerosol exposures more likely, while the low cabin
humidity weakens our immune system's defense against infections. Humidity is
kept low by ventilating with very dry outdoor air that needs to be humidified and
also by the continual loss of cabin humidity from the recirculation air due to the
movement of a portion of the cabin air to behind the insulation where the moisture in it condenses and freezes on the cold skin and fuselage.
divided by the number of persons in the space), outdoor
air supply per person and the rate of virus-filtered air
supply per person.
The latter two parameters set the maximum airborne
virus concentration, C, while the first parameter (OD)
governs how quickly the airborne virus concentration
reaches the maximum concentration in a uniformly
mixed system. The higher the occupancy density, the
faster the airborne virus concentration or any other
occupant-generated bioeffluents, such as human breath
carbon dioxide and perspiration, perfume, clothing and
skin oil volatile organic compound emissions, rise to
their maximum value. The governing equation is14
N
−VVe t
C = p
1 − exp
OD
VVe
(1)
where
C = Bioeffluent infectious aerosol concentration in
the space at time t, virus/L
p = Fraction of infected persons
N = Rate of bioeffluent infectious aerosol generation/person in the space, virus/s per person
t = Duration of infectious aerosol generation, s
OD = Spatial volume/person, L/person
V = Infectious aerosol-free ventilation rate per person
(HVAC outdoor air + virus-filtered recirculation
air + envelope infiltration air), L/s per person
Douglas Stuart Walkinshaw, Ph.D., P.Eng., is president, Indoor Air Technologies Inc., VEFT Aerospace Inc., and ECHO Air Inc, Canada and USA.
O CTO B E R 2020
ashrae.org
ASHRAE JOURNAL
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ASHRAE Journal - October 2020
Table of Contents for the Digital Edition of ASHRAE Journal - October 2020
Contents
ASHRAE Journal - October 2020 - Intro
ASHRAE Journal - October 2020 - Cover1
ASHRAE Journal - October 2020 - Cover2
ASHRAE Journal - October 2020 - 1
ASHRAE Journal - October 2020 - Contents
ASHRAE Journal - October 2020 - 3
ASHRAE Journal - October 2020 - 4
ASHRAE Journal - October 2020 - 5
ASHRAE Journal - October 2020 - 6
ASHRAE Journal - October 2020 - 7
ASHRAE Journal - October 2020 - 8
ASHRAE Journal - October 2020 - 9
ASHRAE Journal - October 2020 - 10
ASHRAE Journal - October 2020 - 11
ASHRAE Journal - October 2020 - 12
ASHRAE Journal - October 2020 - 13
ASHRAE Journal - October 2020 - 14
ASHRAE Journal - October 2020 - 15
ASHRAE Journal - October 2020 - 16
ASHRAE Journal - October 2020 - 17
ASHRAE Journal - October 2020 - 18
ASHRAE Journal - October 2020 - 19
ASHRAE Journal - October 2020 - 20
ASHRAE Journal - October 2020 - 21
ASHRAE Journal - October 2020 - 22
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ASHRAE Journal - October 2020 - 24
ASHRAE Journal - October 2020 - 25
ASHRAE Journal - October 2020 - 26
ASHRAE Journal - October 2020 - 27
ASHRAE Journal - October 2020 - 28
ASHRAE Journal - October 2020 - 29
ASHRAE Journal - October 2020 - 30
ASHRAE Journal - October 2020 - 31
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ASHRAE Journal - October 2020 - 33
ASHRAE Journal - October 2020 - 34
ASHRAE Journal - October 2020 - 35
ASHRAE Journal - October 2020 - 36
ASHRAE Journal - October 2020 - 37
ASHRAE Journal - October 2020 - 38
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ASHRAE Journal - October 2020 - 40
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ASHRAE Journal - October 2020 - 46
ASHRAE Journal - October 2020 - 47
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ASHRAE Journal - October 2020 - 49
ASHRAE Journal - October 2020 - 50
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ASHRAE Journal - October 2020 - 53
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ASHRAE Journal - October 2020 - 72
ASHRAE Journal - October 2020 - HR1
ASHRAE Journal - October 2020 - HR2
ASHRAE Journal - October 2020 - HR3
ASHRAE Journal - October 2020 - HR4
ASHRAE Journal - October 2020 - HR5
ASHRAE Journal - October 2020 - HR6
ASHRAE Journal - October 2020 - HR7
ASHRAE Journal - October 2020 - HR8
ASHRAE Journal - October 2020 - HR9
ASHRAE Journal - October 2020 - HR10
ASHRAE Journal - October 2020 - HR11
ASHRAE Journal - October 2020 - HR12
ASHRAE Journal - October 2020 - HR13
ASHRAE Journal - October 2020 - HR14
ASHRAE Journal - October 2020 - HR15
ASHRAE Journal - October 2020 - HR16
ASHRAE Journal - October 2020 - HR17
ASHRAE Journal - October 2020 - HR18
ASHRAE Journal - October 2020 - HR19
ASHRAE Journal - October 2020 - HR20
ASHRAE Journal - October 2020 - HR21
ASHRAE Journal - October 2020 - HR22
ASHRAE Journal - October 2020 - HR23
ASHRAE Journal - October 2020 - HR24
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ASHRAE Journal - October 2020 - Cover3
ASHRAE Journal - October 2020 - Cover4
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