ASHRAE Journal - June 2021 - 20

TECHNICAL FEATURE
results in significant improvements in effective filtration
efficiency and corresponding reductions in infection
probability.
The time duration in the space is one of the factors
that determine the infectious quanta dose, Dq, in
Equation 2. The results shown thus far are for a fixed
time duration in the space of 60 minutes. Increasing
dwell time in the space increases the conditional probability
of infection. Varying the time duration in the
space by a factor of 2 with the 60 minute period as the
base case proportionally changes the probability of
infection proportionally (doubling the time duration
in the space to 120 minutes doubles the probability of
infection, while cutting the time duration to 30 minutes
cuts the infection probability in half). At much
longer duration times, the probability of infection
increases nonlinearly.
Conclusions and Recommendations
We prepared an assessment of the various measures
that offer the potential to reduce airborne transmission
of the virus that causes COVID-19, SARS-CoV-2, for
high occupant density spaces. The measures considered
include both facility-related and occupant-related factors.
Facility-related factors include physical distancing
of occupants, room ventilation rates and the use of inroom
recirculating HEPA filtration unit(s). Occupantrelated
factors include the use of various types of masks,
both with and without external braces that function to
improve mask fit. Face shields were evaluated early in
the present study, but because they showed little efficacy
in decreasing exposure to respiratory droplets, they
were eliminated from further consideration.
Increased ventilation rates alone, or in tandem with
in-room recirculating HEPA units, were not able to
provide greater than approximately a 4× reduction in
infection probability, for ventilation rates up to 10 ach.
Mask effective filtration efficiency was found to have
the single greatest impact for reducing the probability
of infection via aerosol transmission. Occupants with
masks having an effective filtration efficiency of 0.9
(90%) achieved a reduction in infection probability
by a factor of 100× compared to the no-mask baseline.
Although masks with high filtration efficiency ratings
can provide significant reduction of infection probability,
their fit to an individual is crucial to achieving
performance.
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The impacts of mask and HVAC control measures are
independent, and combined reductions in infection
probability are multiplicative. For example, the reduction
in infection probability achieved with occupants
wearing masks having a modest effective filtration
efficiency of 0.55 (55%) (EOCM reusable spunbound
polypropylene mask without a fitter) vs. the no-mask
baseline is a factor of 4×. Increasing room airflow from
1.34 ach to 3 ach reduces infection probability by a factor
of 2×. Both measures together yield a reduction in
infection probability of a factor of ~8×. In-room recirculating
HEPA air filtration devices have a similar effect at
reducing the infection probability as increasing room air
changes per hour.
Finally, it is important to note that our analysis does
not include any medical assessment or appropriateness
of the fitness of a given mask to be worn by an individual
based on their physiology or medical condition, including
their state of health.
References
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https://www.tinyurl.com/s7xtc8tm https://www.tinyurl.com/s7xtc8tm https://www.doi.org/10.1111/ina.12751 https://www.doi.org/10.1111/ina.12751 https://www.medrxiv.org/content/10.1101/2020.12.31.20249101v1 https://www.ashrae.org/
ASHRAE Journal - June 2021

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