ASHRAE Journal - October 2023 - 16

COLUMN INFECTIOUS AEROSOL CONTROL
All numbers are rounded to two significant figures.
Given the average daily amount of time Americans
spend in various spaces,10 if every space improved its
equivalent clean airflow per person for infection risk
mitigation (ECAi) following Standard 241, an average
person's risk of catching a respiratory virus via longrange
aerosol transmission would decrease by about
25%. If future winters are similar to December 2022
through March 2023, this could prevent about 7,000
COVID deaths annually, in addition to the many other
benefits of clean indoor air.
To calculate the total costs, I first estimated the
number of these spaces in the U.S. with brief Internet
searches of public estimates and then made adjustments
to account for occupancy. If all spaces in the U.S. were
actually operating at the default occupancy at all
times, there would have been many more infections.
In reality, some spaces are at their default occupancy,
but most have fewer people. Many places with fewer
people will already be meeting the required airflow
rates and will make no changes, generating no costs or
benefits. To account for this, I adjusted the numbers of
spaces downward so the number of predicted infections
matched the number we observe.
The annual cost of all spaces operating according to
Standard 241's IRMM for 16 weeks of the year, during
the time of peak respiratory virus transmission in the
winter, would be at most $4 billion. The total monetized
COVID-reduction benefit of 16 weeks of IRMM per
year is about $40 billion, about 10 times the total cost.
Monetized values of other benefits, such as increased
productivity and reduction in other virus infections,
would likely be another $20 billion to $40 billion.
However, the ratio of costs to benefits is different in
different spaces. The risk assessment committee set
rates so the absolute per-hour risk in various spaces
would be roughly similar. The annual benefits are
based on the per-hour risk reduction, multiplied by the
expected number of hours per year people will be in
the space when it is operating in IRMM. Infrequently
used spaces will have relatively lower benefits compared
to costs, although as noted above the costs are likely
to be overestimated for these spaces. Additionally,
manufacturing spaces are the most sparsely populated,
and require much more volume of air per person
protected, which lowers the benefit/cost ratio.
Much of the annual benefit comes from residential
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ASHRAE JOURNAL ashrae.org O CTO B ER 2023
dwelling units. Even though the risk reduction is only
about 8%, many units nationwide have six or more
occupants, and a large fraction of disease spread
happens in homes. Upgrading home air filters to
MERV 13 or better and circulating air through them,
especially in the winter virus season, is a very costeffective
public health measure.
Despite large benefits, these costs may appear high in
many spaces. I argue we should adjust our expectations
of what systems are needed and what costs should
be paid. Historically, airflow rates have been set at
the minimum required for comfort, with no regard
for benefits of preventing infection. Additionally, in
recent years we have learned the chronic harm of air
pollution, especially fine particulates, is much higher
than previously estimated. People's expectations of
system capacity and costs are not well-calibrated to what
is needed to offer even a minimal level of protection to
people in indoor spaces.
References
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https://tinyurl.com/26wmwyjd

https://www.doi.org/10.1038/s41467-021-24487-w https://doi.org/10.1016/j.buildenv.2021.107617 https://doi.org/10.1016/j.buildenv.2021.107617 https://doi.org/10.1016/j.buildenv.2022.109309 https://doi.org/10.1016/j.buildenv.2022.109309 http://dx.doi.org/10.15585/mmwr.mm7117e3 https://www.doi.org/10.1016/j.cmi.2022.07.009 https://tinyurl.com/4pyhfwd4 https://www.doi.org/10.1002/hec.4208 http://www.OurWorldInData.org https://tinyurl.com/58w2nct9 https://tinyurl.com/yhd3r5tj https://tinyurl.com/26wmwyjd http://www.ashrae.org

ASHRAE Journal - October 2023

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