ASHRAE Journal - December 2023 - 22

INFECTIOUS AEROSOL CONTROL
Column
Benjamin Jones
Models for ASHRAE Standard 241
I Need How Much Clean
Air to Control Infection?
BENJAMIN JONES, DR.ENG.
New ASHRAE Standard 241, Control of Infectious Aerosols, establishes minimum
requirements aimed at reducing the risk of disease transmission through exposure to
infectious aerosols in new buildings, existing buildings and major renovations. The
group I vice-chaired considered the minimum equivalent clean airflow (ECA) rates for
infection control for a range of spaces. The details of the modeling we used to inform
our decision-making are extensive and will be part of future peer-reviewed articles;
this column gives an overview of key assumptions and expected utility.
Why Model and What to Model?
We started by examining some existing models in
the academic literature but found they didn't meet our
needs or our understanding of the problem. Therefore,
we constructed our own mathematical model of the
complex interactions between people and the airborne
pathogens contained in respiratory particles. Figure 1
shows a system diagram. With it, we predicted the
likelihood of an infection occurring from long-range
airborne transmission. We knew it was impossible to
make any model completely accurate, but ours had
to have utility. We made sure we understood how
assumptions we used to create it-and how the quality of
the data we gave it-might affect predictions.
We started by considering model output. Meeting
airflow rates given in the latest version of ASHRAE
Standards 62.1, 62.2 and 170 is a prerequisite, but we
wanted additional ECA to be delivered using a range of
technologies and by reducing the number of occupants
in a space. As we built the model, the ECA rate per
22
ASHRAE JOURNAL ashrae.org D ECEMBER 2 0 2 3
person-termed equivalent clean airflow rate per person
(ECAi)-emerged as the appropriate metric because it
naturally matches the physics more than oversimplistic
and unphysical air change rates and is clearly logical.
People
Our first assumptions were about the occupants
of any space who might be susceptible to infection.
We assumed they have no immunity, which occurs
at the beginning of a pandemic but may not be true
for endemic pathogens, and that they are equally
susceptible. We then considered how many infected
people might be present in each space. This is obviously
a function of the number of occupants-more infected
people will be in a full 1,000-person auditorium than
in a four-person family home-and of the prevalence
of the disease in the local community indicated by
a community infection rate. Many existing models
Benjamin Jones, Dr.Eng., is an associate professor in the department of architecture and
built environment at the University of Nottingham in the U.K.

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ASHRAE Journal - December 2023

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

Contents
ASHRAE Journal - December 2023 - Intro
ASHRAE Journal - December 2023 - BB1
ASHRAE Journal - December 2023 - BB2
ASHRAE Journal - December 2023 - Cover1
ASHRAE Journal - December 2023 - Cover2
ASHRAE Journal - December 2023 - 1
ASHRAE Journal - December 2023 - Contents
ASHRAE Journal - December 2023 - 3
ASHRAE Journal - December 2023 - 4
ASHRAE Journal - December 2023 - 5
ASHRAE Journal - December 2023 - 6
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ASHRAE Journal - December 2023 - Cover3
ASHRAE Journal - December 2023 - Cover4
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