ASHRAE Journal - June 2021 - 18

TECHNICAL FEATURE
infection reduction of only a factor of 2. Doubling the
ventilation rate from 5 ach to 10 ach results in a conditional
probability of infection reduction of only a factor
of 1.7. This diminishing benefit agrees with the trend
noted by Pantelic and Tham.8
Technologies such as in-room recirculating HEPA
filtration units might be an option; however, in cases
where the units maintain a well-mixed room airflow
condition, these units have the same net effect as
increasing room ventilation rates on decreasing the
infection probability. Engineers need to begin rethinking
traditional airside designs that function to achieve
well-mixed room conditions and pursue alternative
approaches that can deliver occupant comfort, acceptable
indoor air quality and decreased risk of transmission
of airborne pathogens.
The combination of modest increases in ventilation
rates with masks is synergistic, as both are factors in
the infectious quanta dose, Dq, given by Equation 2. For
example, the combination of having occupants wearing
knit cotton masks with either mask fitter and increasing
the ventilation rate from 1.34 ach to 5.0 ach reduces the
conditional infection probability by a factor of 3.4 relative
to the no-mask baseline. This reduction is equal to
the product of the reductions in infection probability
for each individual measure. It is interesting to note
that this combined reduction is greater than could
be achieved by increasing room supply airflow from
1.34 ach to 10 ach.
The prospect of increasing ventilation rates to reduce
infection probability is attractive because it can be " centrally "
controlled, unlike ensuring that each occupant in
the space has an appropriate mask that is properly fitted.
Yet as we have already shown, occupants equipped
with higher effective filtration efficiency masks can
reduce the probability of infection far more than ventilation
can. The real challenge with achieving and
sustaining lower infection probability by relying on
high mask filtration efficiency is that it is dependent on
ensuring that all occupants properly and consistently
don their own masks. Even with mask materials that
have modest filtration capability, the use of mask fitters
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ASHRAE Journal - June 2021

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