A Brief Introduction To Passenger Aircraft Cabin Air Quality
Are we to believe that the stingy airlines are changing the HEPA filters on a regular basis? or at all? Clearly, this is an area in need of oversight by the Dept. of Transportation.
BY DOUGLAS STUART WALKINSHAW, PH.D., P.ENG., FELLOW ASHRAE
The passenger aircraft industry says passenger cabin air quality is exceptionally good
compared with that of other public settings. Some airlines claim the air in aircraft
cabins is cleaner than that in offices and is on par with the air in hospitals. Another
airline says the air is particularly good because it is very dry, creating a sterile cabin
environment. Some say virus particles will only travel one or two rows. Nearly all say
the air change rate is high and recirculated air is passed through HEPA filters that
remove nearly 100% of airborne viruses.
Dry Air in Passenger Cabins
The air in passenger cabins is dry, with a rela-
tive humidity (RH) of 10% as the flight progresses.
Meanwhile, a portion of the cabin air with its ventila-
tion components (very dry outdoor air plus filtered,
recirculated air) and humidity components, passes
from the cabin to behind the cabin insulation, drawn
there through liner leaks and openings by stack pres-
sures. Some of this air is not lost as useful ventilation
air. However, all the air drawn there (perhaps 25% of the
cabin ventilation air) loses its humidity prior to recircu-
lation, depositing its moisture as condensation on the
very cold fuselage behind the insulation. There it freezes
during flight, adding nonproductive dead weight. When
the frozen water melts when the plane is back on the
ground, this moisture causes metal corrosion, hastening
metal fatigue and creating microbial growth.7
However, in addition to air at 10% RH being uncom-
fortable, it has been shown to impair nasal mucociliary
clearance, innate antiviral defense and tissue repair
function in mice and is, therefore, postulated to do so
in humans.8 Additionally, RH this low rapidly turns
droplets into aerosols,9 which disperse more widely, five
rows longitudinally either way (Figure 1).10 Aerosols are
more likely to inoculate the respiratory system, where
the minimum dose requirement to inoculate is lower
and the symptoms more severe than if the inoculation
occurs in the nasal system where the larger droplets are
more likely to rest.11 In the past more limited longitu-
dinal transport has been postulated.12 However, more
recent research on a wide body airplane indicates that a
10% concentration of droplet nuclei remains after travel-
ing 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 win-
ter.13 This is when relative humidity indoors with a heat-
ing 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 air-
borne 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 occu-
pants 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 poten-
tially 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 sup-
plied to the passengers is one-eighth the amount circu-
lated 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 sup-
ply 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 out-
door 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 concentra-
tion 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 distanc-
ing 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 mois-
ture 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
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