ASHRAE Journal - October 2023 - 14

INFECTIOUS AEROSOL CONTROL
Column
Richard Bruns
Cost-Benefit Analysis of
ASHRAE Standard 241
BY RICHARD BRUNS, PH.D.
The risk assessment committee for ASHRAE Standard 241, Control of Infectious Aerosols,
produced a model to predict the reduction in infections as airflow increased. But
what are the estimated costs and benefits of operating in Standard 241's infection
risk management mode (IRMM), which is " the mode of operation in which measures
to reduce infectious aerosol exposure documented in a building readiness plan are
active " ? I extended the model to find out.
Because I did not have time or resources to do a
detailed cost analysis for each space type, I aimed for
a reasonable high cost estimate. Based on discussions
with engineers on the committee, I used a standard cost
estimate of about $1 per year per additional cfm of clean
air required for operating Standard 241's infection risk
management mode (IRMM) for four months. This value
includes the cost of an HVAC upgrade or commercialgrade
air purifier amortized over seven years, plus labor
and material operation and maintenance costs, filter
replacement and electricity, including the social cost of
carbon emissions generated by electricity production,
valued at $260/ton ($0.11/kWh).1
This is likely to be an overestimate for three reasons.
First, in many spaces, filter upgrades and recirculation
will be able to supply much or all of the required
equivalent clean airflow (ECAi) increase, without
requiring any increase in outdoor air. Second, meeting
the ECAi requirement using ultraviolet germicidal
irradiation (UVGI) may have lower costs than increasing
ventilation and filtration. Third, building operators
can choose to meet Standard 241's per-person airflow
rate requirements by reducing occupancy instead of
14
ASHRAE JOURNAL ashrae.org O CTO B ER 2023
increasing the equivalent clean airflow rate, if that is
more profitable. In addition, this cost estimate will be
too high for infrequently used places. Although much
of the cost estimate is due to equipment installation,
variable costs exist that will not be paid when the
building is not in operation.
The calculation of monetized benefits was based on
an infection risk model used by the risk assessment
committee. The committee used the Wells-Riley
model to estimate the change in infection probability
in each space type as the equivalent clean airflow
rate increased, using a procedure similar to Jones,
et al.,2 and Iddon, et al.3 The number of infectors
was generated according to a binomial distribution
according to the number of occupants and the
community infection rate.
To calculate the expected community infection rate,
we assumed future years would be similar to previous
respiratory virus seasons. Clarke, et al.,4 shows that
the seroprevalence of infection-induced SARS-CoV-2
antibodies in the U.S. increased by 25% between Dec.
Richard Bruns, Ph.D., is a senior scholar at the Johns Hopkins Center for Health
Security and an economist specializing in cost-benefit analysis of public health policy.
http://www.ashrae.org
ASHRAE Journal - October 2023

Table of Contents for the Digital Edition of ASHRAE Journal - October 2023
