ASHRAE Journal - December 2020 - 10

LETTERS

(5) Comparable Airflow Rates
The HEPA filters' effectiveness is
determined by the percentage of
the space that air flows through the
filter. In an airplane, this volume
is 50%, with the other 50% from
outside airflow, which is virus free.
Conversely, in a typical office building, large air volumes result in lower
air change rates.17 As such, localized
concentrations of virus in buildings
can be far higher. A building's MERV
13 filter is far less impactful in terms
of concentration in the space than
what would be found in an airplane
recirculation system due to airflow
patterns and filter efficiencies.
Buildings typically rely on outside
airflow for microbial control and
have limited effectiveness in controlling highly localized spread. The
airplane cabin has high air change
rates (typically 20-30 ACH) and airflow patterns that limit the spread
in a localized environment. The
aircraft's seating arrangement and
" shield-like " high seat backs limit
exposure to nearby susceptible passengers from a perturbation event
like a cough.

(6) Comparable Air Change Rates.
Despite being a conditioned air
source, the aircraft air change rate
is substantially higher than in other
enclosed spaces. The air change
rates for subway cars, classrooms,
auditoriums and offices are not
comparable to that of an aircraft.
A more accurate analysis would
compare total air changes per hour
between the spaces while accounting for filter efficiency and the
directionality of the airflow. In the
case of viral loading, HEPA filtered
recirculated air is effectively a virusfree supply air. Additionally, the
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FIGURE 5 Mass of expiratory material inhaled by susceptible subjects in different seats for different ventilation
conditions (a-c) and different index seat positions at 100% airflow (a, d-e). Error bars in (a) represent the range
over three different initial conditions.

aircraft's equilibrium conditions
are also vastly different due to the
airflow patterns that do not act as a
uniformly mixed system.

(7) Modeling Analysis Summary and
Conclusion. The uniformly-mixed

assumption is more applicable to
systems that have low air exchange
rates (i.e., offices, classrooms etc.)
where the timescale to complete
an air exchange is long enough to
allow the infectious aerosol to more
uniformly disperse across the entire
environmental volume. This is not
the case in an airplane cabin.
Any application of a uniformlymixed assumption fails to consider
the aircraft's unique airflow design.
The cabin design provides continuous air supply and exhaust paths,
maintains high air exchange rates
and precludes re-distribution
of infectious aerosols due to the
HEPA filters. In terms of aerosol

D ECEM BER 2020

concentration levels, there is also a
very high decay of aerosol concentration levels as a function of distance from the source as confirmed
by USTRANSCOM.13
The airflow systems in Boeing's
aircraft go well beyond the current ASHRAE recommendations for
providing a safe environment from
the standpoint of airborne disease
transmission.17 Aircraft designs
exceed those recommendations
when considering the direction,
velocity, and volume of cabin air,
the HEPA filters' efficiency and the
seating systems in the cabin. The
result is a low risk of aircraft virus
transmission.

Dan Freeman
Engineering Leader, Boeing Confident Travel Initiative

References

1. International Air Transport
Association (IATA), " Restarting aviation

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ASHRAE Journal - December 2020

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