ASHRAE Journal - September 2024 - 32

COLUMN INFECTIOUS AEROSOL CONTROL
FIGURE 4 Equivalent air changes per hour from 241 Appendix C tests versus
occupant density for various school spaces.
Does not meet minimum Standard 241-2023 target
Meets Standard 241-2023 target but does not meet CDC target
Meets both Standard 241-2023 and CDC targets
Target eACH for meeting minimum Standard 241-2023 target for offices
Classroom
20
18
16
14
12
10
8
6
4
2
16
14
12
10
8
6
4
2
5
10
20
(40 cfm/person)
CDC target (5 eACH)
30
40
Occupant Density (#/1,000 ft2)
(80 cfm/person)
Gym
50
60
70
either the Standard 241-2023 or CDC targets. The
median occupant density was 32 people per 1,000 ft2
(32 people/100 m2), close to the Standard 62.1-2022
recommended density of 35 people per 1,000 ft2
(35 people/100 m2).
* Gyms. Nearly half of the tests (53%) conducted
in school gymnasiums met the Standard 241-2023
target of 80 cfm/person (40 L/s/person). However, the
majority of these tests did not meet the CDC target of
5 ach.
* Libraries and Staff Offices. The majority of these
spaces (77%) met the Standard 241-2023 targets.
Additionally, significant gaps in ventilation systems
were discovered during testing, which contributed
to the failure to meet the standards in classrooms
in many cases. These gaps included issues such as
unchanged filters, under-ventilation, and portable
air cleaners or ultraviolet germicidal irradiation that
were either not turned on or improperly used.
CDC target (5 eACH)
10
Occupant Density (#/1,000 ft2)
in-place testing procedure can serve as an energysaving
measure by adjusting clean airflow rates to the
clean air target and relying more on filtration rather
than on outdoor air.
Schools
In 197 Appendix C tests conducted across 20 schools,
four distinct space types were evaluated. Figure 4
illustrates eACH in relation to occupant density
(number of people per 1,000 ft² [100 m²]) specifically
for classrooms and gyms. Additional figures with
data for libraries and offices are available at https://
tinyurl.com/JournalExtras. The libraries tested were
repurposed as classrooms for specialized courses like
arts and science, which required a ventilation target
of 40 cfm per person (20 L/s/person).
Tests revealing the following trends:
* Classrooms. Most classrooms (72%) did not meet
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ASHRAE JOURNAL ashrae.org S E PTEM B ER 2024
References
1. CDC. 2023. " Ventilation in Buildings. " U.S. Centers for Disease
Control and Prevention. https://tinyurl.com/yckr2y5z
2. ASHRAE Standard 241-2023, Control of Infectious Aerosols,
Normative Appendix C.
3. Occupancy Management. " 2023 Office Occupancy Management
Benchmarks. " Tableau Public. https://tinyurl.com/48j2prb8
4. Zaatari, M, A. Goel, J. Maser. 2023. " Impact of new ventilation
guidance, Standard 241 on energy costs, carbon emissions: why
equivalent clean airflow doesn't have to be expensive. " ASHRAE
Journal 65(9):18-24.
5. Zaatari, M, J. Maser. 2024. " ASHRAE Standard 241: Energy
Implications of Different Strategies to Meet Equivalent Clean Air
Requirements for Commercial and Educational Spaces. " 2024
ASHRAE Winter Conference-Chicago.
15
20
Conclusions
The ASHRAE Appendix C method is an efficient
approach to test the equivalent clean airflow rate
(ECAi) for infection risk mitigation in a single
occupied space, considering all physical removal
mechanisms. Offices can adopt Standard 241-2023
as a year-round health standard and use it with the
equivalent clean airflow testing process to save money
and energy by adjusting their clean airflow rates to
their true peak use rates. Schools, on the other hand,
require additional interventions to comply. Appendix
C tests can assist in the effective placement and
optimization of ventilation and filtration systems for
occupant health and building energy use.
eACH (h-1)
eACH (h-1)
https://tinyurl.com/yckr2y5z https://tinyurl.com/48j2prb8 https://tinyurl.com/JournalExtras https://tinyurl.com/JournalExtras http://www.ashrae.org
ASHRAE Journal - September 2024

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