ASHRAE Journal - July 2021 - 23

COLUMN IEQ APPLICATIONS
infl uence on contaminant removal effectiveness than air
changes.7
Air distribution is also included in the ASHRAE
Epidemic Task Force's Core Recommendations, which
state that, " [w]here directional airfl ow is not specifi -
cally required, or recommended as the result of a risk
assessment, promote mixing of space air without causing
strong air currents that increase direct transmission
from person-to-person. " 8
An effective method of limiting occupants' exposure
to contaminants is source capture. Designers apply
this principle when using kitchen hoods, as described
in ASHRAE Standard 154-2016.9 Other common
applications of source capture include manufacturing
tool exhaust systems, decontamination and sterilization
areas and benchtop snorkels in science and
research laboratories. The American Conference of
Governmental Industrial Hygienists publishes a design
manual10 covering many topics related to source capture
systems.
Source capture works best when there is a known and
stationary, or at the very least limited in motion, source
of contaminants, and effective velocities can be achieved
to capture the contaminants. In cases where the contaminants
are widespread or would only be considered
nuisances outside of the space in which they are generated,
containment may be an effective strategy. Two
examples of containment devices include air curtains
and fume hoods. Both devices maintain velocity at a
boundary to inhibit the passage of contaminants.
An alternative to maintaining velocity is to maintain
pressure between spaces, because a fi xed pressure
difference induces air movement in a prescribed
direction to either exclude or contain contaminants.
ASHRAE/ASHE Standard 170-2021 includes exclusionary
pressure relationships, such as the +0.01 in. w.g. (2.5 Pa)
pressure requirement for protective environment
rooms and operating rooms and containment pressure
relationships, as with the -0.01 in. w.g. (-2.5 Pa) pressure
requirements for AII rooms.7ASHRAE Standard
62.1-2019 includes exclusionary pressure relationships
with its positive pressure requirements for occupied
spaces relative to environmental tobacco smoke areas,
attached parking garages and the outdoors, and includes
containment pressure relationships with requirements
for negative pressure of Class 4 exhaust air ducts and all
spaces requiring exhaust air.11
The broad inclusion of containment and source control
requirements within ASHRAE standards is not new
and should not be surprising, but it is not as frequently
discussed as other requirements of the standards that
relate directly to the well-mixed assumption. The wellmixed
model as applied to ventilation leads to the conclusion
that the solution to pollution is dilution. This is
only logical since in an ideally mixed environment the
only way to remove a contaminant is to dilute it. This
way of thinking leads to an emphasis on increasing air
change rates, removing contaminants from the supply
airstream with fi lters or air cleaners or removing
contaminants directly from the space. This emphasis
has been refl ected in government guidance12 and
the media13 on strategies to deal with the COVID-19
pandemic.
Although dilution and fi ltration can often be effective
in improving indoor air quality, they do not refl ect
the entirety of the recommendations of ASHRAE. It is
incumbent upon our community to not only be knowledgeable
of, but conversant in, all the design strategies
for a built environment that is other than ideal.
References
1. 2019 ASHRAE Handbook-HVAC Applications.
2. Straub, H.E., S.F. Gilman, S. Konzo. 1956. " Distribution of Air
Within a Room for Year-Round Air Conditioning-Part I. " University
of Illinois Engineering Experiment Station, Bulletin No. 435.
3. Straub, H.E. M.M. Chen. 1957. " Distribution of Air Within
a Room for Year-Round Air Conditioning-Part II. " University of
Illinois Engineering Experiment Station, Bulletin No. 442.
4. Burkett, J. 2021. " Virus transmission modes and mitigation
strategies, part 2: Airborne transmission and distribution. " ASHRAE
Journal 63(4):10 - 16.
5. ANSI/ASHRAE Standard 55-2020, Thermal Environmental
Conditions for Human Occupancy.
6. ANSI/ASHRAE/ASHE Standard 170-2021, Ventilation of Health
Care Facilities.
7. Memarzadeh, F., W. Xu. 2012. " Role of air changes per hour
(ACH) in possible transmission of airborne infections. " Build Simul
5(1):15 - 28. https://doi.org/10.1007/s12273-011-0053-4
8. ASHRAE Epidemic Task Force. 2021. " Core Recommendations
for Reducing Airborne Infectious Aerosol Exposure. " ASHRAE.
https://tinyurl.com/2m9bmtd7
9. ANSI/ASHRAE Standard 154-2016, Ventilation for Commercial
Cooking Operations.
10. ACGIH. 2019. Industrial Ventilation: A Manual of Recommended
Practice for Design, 30th Edition. Cincinnati: ACGIH.
11. ANSI/ASHRAE Standard 62.1-2019, Ventilation for Acceptable
Indoor Air Quality.
12. CDC. 2021. " Ventilation in Buildings. " Centers for Disease
Control and Prevention. https://tinyurl.com/j3fpertv
13. Mandivilli, A. 2020. " How to Keep the Coronavirus at Bay
Indoors. " NYTimes.com. https://tinyurl.com/57y3yjvv
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https://www.doi.org/10.1007/s12273-011-0053-4 https://www.tinyurl.com/2m9bmtd7 https://www.tinyurl.com/j3fpertv http://www.NYTimes.com https://www.tinyurl.com/57y3yjvv http://www.ashrae.org
ASHRAE Journal - July 2021

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