ASHRAE Journal Supplement - AMCA InMotion - November 2023 - 23

Centers for Disease Control and Prevention (CDC) and
other advisory agencies repeatedly stressed that the safest
place for people to gather to reduce the risk of communicable-disease
transfer was outdoors7-the furthest thing
from a sealed chamber. Similarly, the CDC's primary
recommendation for improving building ventilation to
reduce the risk of transmissible disease is to increase
the amount of air circulated from outdoors.8
But what to do when the quality of outdoor air is
dangerous? While spending time outdoors is recognized
as an effective means of preventing the spread of respiratory
diseases such as COVID-19, the U.S. Environmental
Protection Agency (EPA) recommends remaining
indoors on days of particularly low air quality, such as
those when levels of ambient smoke from wildfi res are
high.9 Indeed, results of a 2021 Harvard study suggest
increased airborne particulates from wildfi re smoke
were responsible for an 11.7-percent increase in the rate
of COVID-19 contraction and an 8.4-percent increase in
case mortality across three western U.S. states.10 Meanwhile,
a 2023 University of Michigan study demonstrates
wide-ranging risks, including accelerated cognitive
decline,11 posed by particulate pollution from wildfi re
smoke. Such data imply that, at times, it is prudent to
keep outside air outside. Air curtains are known for
their ability to prevent the intrusion of exterior air into
conditioned spaces.
Experiment. A study
was conducted to determine
an air curtain's effectiveness
in preventing
airborne particulates from
infiltrating a building.
To simulate ai rborne
particulates from a wildfire,
a commercial haze
machine was used to
generate a light fog of fi ne
water droplets outside of
a doorway. Outside and
inside the doorway, airqual
ity sensors were
deployed to estimate the
www.amca.org
concentration (in micrograms per cubic meter [μg/m³]) of
airborne particles with diameters less than 2.5 microns
(PM2.5). The door was actuated with an automatic opener,
which, in turn, was controlled with a programmable timer
(photos C and D).
" Outside " the doorway, the fog generated by the haze
machine typically reached concentrations of 1,200 μg/
m³ to 1,500 μg/m³, which served as a rough analog to the
greater-than-800-μg/m³ concentrations of PM2.5 observed
during extreme wildfi re events.12 The concentrations
occasionally dipped as low as 1,100 μg/m³ or went as high
as 2,000 μg/m³ because of shifting wind patterns around
the outside air sensor; this variance was counteracted by
the use of fans at the doorway applying a controlled light
wind load and ensuring a constant stream of particulates
in the direction of the " inside " environment. Tests were
performed with the air curtain off and the air curtain on
and the door open 25 percent of the time and with the
air curtain off and the air curtain on and the door open
50 percent of the time.
Results. Running the air curtain reduced particulate
concentration inside the room (Figure 2). As might be
expected, absolute indoor particulates increased with
the amount of time the door spent open; percent reduction
in indoor particulates, however, appeared largely
PHOTOS C and D. Test equipment, airborne-particulates experiment.
Photos courtesy of Mars Air Systems
2023 AMCA inmot ion 23
http://www.amca.org
ASHRAE Journal Supplement - AMCA InMotion - November 2023

Table of Contents for the Digital Edition of ASHRAE Journal Supplement - AMCA InMotion - November 2023
