ASHRAE Journal - May 2023 - 51

TECHNICAL FEATURE
table " used today was expanded and changed (Table 1).
The earliest air change tables appeared in ASHRAE
Guidebooks in 1959 and 1962. The 1966 version contained
17 lines of requirements. By 1982, the air change
table contained over 40 lines. Changes in rates were
common from version to version, diminishing over
time. By 1997, all the air change rates used today were in
the table.
In the modern era, 2008 to present, the air change
tables continue to grow. However, changes to the rates
are rare. The 2021 air change tables contain 155 lines of
requirements, but they are highly repetitious. Fortythree
of the 155 lines call for 2 ach outdoor air and 6 ach
total air. The 19th century legacy requirement, 2 ach, is
still widely used; it is the outdoor air requirement for 114
of 155 lines (74%).3
State of the Art: Evidence Basis of the Air Change Rates
In 2019, an ASHRAE research project, in collaboration
with American Society of Healthcare Engineers (ASHE)
and Facilities Guidelines Institute (FGI), gathered the
available literature and reviewed the evidence-basis
behind the air change requirements.4 They found very
little available evidence. Appendix A of the report lists
144 ach rates. For 120 of 144 (83%), there was no available
evidence. Where evidence was available, it was inconclusive.
The researchers concluded only that further
investigation is necessary.
Traditionally, infection control or disease transmission
is sometimes implied to be the reason underlying
certain air change rates. However, the known evidence
to support this is sparse. The 2019 research project
acknowledged the potential importance of ventilation
but could not identify evidence to support the prescribed
rates. This echoes numerous meta-research
studies across decades.2
Based on the lack of strong evidence, an opportunity
exists to evaluate the effects of ventilation rates on both
FIGURE 1 Theoretical relationship between contamination levels and ach.
No Generation Rate
" Low " Generation Rate
" Moderate " Generation Rate
" High " Generation Rate
2
4
6
8
Ventilation Rate (ach)
air quality and energy consumption.
Project Description
This demonstration project took place at a U.S. hospital
tower that was constructed in 2015 located in Harbor
City, Calif. (south of Los Angeles). The tower holds 140
patient beds and is 280,000 ft2(26 000 m2). The HVAC
system is 100% outdoor air, with two-pass fi ltration and
constant volume reheat zones.
Planning
The design process included HVAC design, multidisciplinary
development of the indoor air quality thresholds,
development of the hospital and infection control
monitoring protocols and agency approvals.
A multidisciplinary team of engineers, department
leadership, hospital leadership, infection control and
industrial hygienists developed the project air quality
thresholds and action levels, which are shown in Table 2a
and Table 2b.
All project plans were reviewed and approved by the
state authority having jurisdiction (Offi ce of Statewide
Health Planning Design). The approvals process was
lengthy, since several novel variances from California
code were involved. The engineering design process
TABLE 1 Historical air change rates in selected non-operating or isolation spaces (total ach/outside air ach).
1959 1962 1964 1966 1968 1971 1974 1978 1982 1987 1991 1993 1997 2001 2006 2008 2013 2017
Recovery
Nursery
Intensive
Care
LDRP
4
Patient Room 1.5
8-12 12
1.5
4
12
2
15/6 15/6 15/6 6/2 6/2 6/2 6/2 6/2 6/2 6/2 6/2 6/2 6/2 6/2
12 15/5 15/5 15/5 12/5 12/5 12/5 12/5 6/2 6/2 6/2 6/2 6/2 6/2 6/2
4/2 4/2 4/2 2/2 2/2 2/2 4/2
4
6/6 6/6 6/2 6/2 6/2 6/2
4/2
2/1 2/2 6/2 6/2 6/2 4/2 4/2
6/2 6/2 6/2 6/2 6/2 6/2 6/2
2
2/2 6/2 6/2 6/2 6/2 6/2
M AY 2 0 2 3 ashrae.o rg ASHRAE JOURNAL
51
10
12
Space Contaminant Concentration
(Contaminants /Volume)

http://www.ashrae.org

ASHRAE Journal - May 2023

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