ASHRAE Journal - February 2021 - 9

LETTERS

clean cabin but not prevent transmission by fomites. But
aircraft cabins are not cleanrooms, and infectious aerosols emitted by an infected person travel in the air many
rows as well as laterally.1,2 Further, airborne infection
risks are not small.3,4 A recent review consistently found
that humans produce pathogens predominately as aerosols or small respirable particles (<5 microns), with PCR
studies identifying infectious aerosols in the air of rooms
with persons ill with COVID-19, the common cold, inﬂuenza A and B, tuberculosis, measles, herpes and chicken
pox.5 Low humidity exacerbates the risk.6
Many " authorities " saying high air change rate is the
criteria for good air quality as far as airborne aerosol infections are concerned does not make it true.
Infectious aerosols originate with infected persons creating and shedding virus-not with visible and hidden
building or aircraft cabin materials creating and shedding virus, not with equipment creating and shedding
virus and not with the frost and moisture behind the
insulation creating and shedding virus. Given that fact,
the rate of infectious virion-free air supply to each person, including each ill person and the persons around
them, governs virion concentration in the air around the
ill persons-not air change rate.
While high air change rates provide some protection,
they also result in the virions in the air being inhaled by
others being fresher and more virulent, with less settling
having occurred and the concentrations higher than
in low air change settings. High air change rates can be
the antithesis of low infectious aerosol exposure when
they are the result of tightly packed together persons
in spaces with low ceilings-the situation with aircraft
passenger cabins, for example. The total virion-free air
being supplied is large, but the amount per person is
not. Speaking of low ceilings, spaces with high ceilings
are naturally safer, because body heat and warm breath
rise to the ceiling, taking with them human-generated
aerosols, and stay there if they are not pushed back
down by ceiling fans and the like.
Equation 2, then, in my paper predicts the total number of virion aerosols inhaled by persons in the same
space but not their dispersion or individual inhalation.
It does this prediction by accounting for the per occupant
outdoor air supply rate and ﬁltration rate of particles from
the recirculation air supply. It will also accommodate an
estimation of the ﬁltration beneﬁt of mask wearing on
aerosol exposure reduction, which should eliminate viral

TABLE 1 Narrow body and wide body passenger aircraft influenza A virions inhaled
and infections per ill person, I = 9 L/min·p, t = 5 h, N = 76,000 v/h, HID50 =
900 v. Narrow body: OD = 1 m3 /p, V = 9.4 L/s·p, Ve = 0.65. Wide body: OD =
1.6 m3 /p, V = 11.8 L/s·p, Ve = 1.
NARROW
BODY VIRIONS
INHALED

NARROW BODY
PREDICTED
INFECTIONS

WIDE BODY
VIRIONS
INHALED

WIDE BODY
PREDICTED
INFECTIONS

No Mask

9,244

≤5

4,794

≤3

Mask 50%

2,311

≤1

1,199

≤1

infection risks from surfaces contaminated with fomites
(except for eye touching) and greatly reduce risk from
airborne droplets, both emitted and inhaled. Eye exposure might be important only in hospital-related viral
transmission during ophthalmic practice.3 Mask wearing
will not reduce the infection risks completely, as a justcompleted case study indicates. That study identiﬁed a
9.8% to 17.8% COVID-19 attack rate on persons spread
throughout the cabin, some wearing masks, during a 17%
full, seven-hour international ﬂight to Ireland.7
I will assume in the calculations that follow regarding
the August 3 ﬂight from Delhi to Hong Kong that masks
remove 50% of the virions from both our exhaled and
inhaled air. In place of COVID-19 for which there are no
shedding and infectiousness criteria, I will use inﬂuenza
A (virions are similar in size to those of COVID-19),3
virion (v) shedding rate and its lowest infectiousness
criteria, which seems appropriate for aircraft due to the
very dry conditions being more favorable to airborne
infection spread due to impairment of nasal mucociliary clearance, innate antiviral defense and tissue repair
function. Table 1 here shows Equation 2 predictions and
all parameters used.
So if inﬂuenza A is a valid surrogate and everyone was
wearing masks, it is possible that ﬁve COVID-19 infections occurred during that ﬂight from exposure to six
persons with false-negative preﬂight tests if they were
all shedding.8 It is also possible that if masks were being
worn, no COVID-19 infections occurred during the ﬂight
or, if there were infections, that these persons had not
yet begun shedding at the time of the post-ﬂight tests.
To these predictions must be added any virions inhaled
prior to the ﬂight and those of the 24,000 v/h coarse
(>5 microns) particles shed by ill persons that were aerosolized and inhaled before settling or exiting the cabin.

References

1. Bennett, J.S., B.W. Jones, M.H. Hosni, Y. Zhang, et al. 2013.
FEBRUARY 2021

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