ASHRAE Journal - October 2019 - 18

TECHNICAL FEATURE

overall occupant satisfacTABLE 1 ASHRAE Standard 170-2017, Table 7.1 Design Parameters-Hospital Spaces.
tion and well-being. As
FUNCTION
PRESSURE MINIMUM MINIMUM ALL ROOM AIR
AIR RECIRCULATED
DESIGN
DESIGN
OF SPACE RELATIONSHIP TO OUTDOOR
TOTAL
EXHAUSTED
BY ROOM UNITS
RELATIVE
TEMPERATURE °F
shown in Table 1, Standard
ADJACENT AREA
ACH
ACH
HUMIDITY %
170 requires temperatures
Operating
Positive
4
20
NR
NO
20 - 60
68 - 75
ranging from 68°F to 75°F
Room
dry bulb (DB) (20°C to
24°C). It is important to note that these recommendaTABLE 2 Typical OR requirements, from the ASHRAE HVAC Design Manual for
Hospitals and Clinics.
tions are considered minimum design values.
OR ROOM TYPE
REQUIREMENTS
Just as critical as temperature, relative humidity
(RH) plays a key role in maintaining a comfortable and
Heart
Low Temperature, Fast Reheat, Large Room
healthy OR environment. Table 1 shows Standard 170
Orthopedic
Low Temperature, Large Room, Extra Filtration
requiring RH to be maintained in a range from 20% to
Cystoscopic
Medium Temperature
60%. Too much humidity can decrease the perspiration
General
Medium Temperature
rate from a person's skin, resulting in an environment
Pediatric
High Temperature
that feels relatively warmer than it actually is. It can also
Neurological
Low Temperature, Large Room
lead to a potential for mold and mildew growth within
Trauma
High Temperature
the built environment. On the other hand, dry air can
Burn
High Temperature
affect people with respiratory problems, and also cause
the capabilities of the HVAC system.
dry skin which can result in discomfort. Dry air below
Many times surgeons will ask for the OR to be made
40% RH has been shown to correlate with an increase in
"colder" when in fact what they really want is a space
health-care associated infections.3
While ORs have an upper humidity limit of 60% RH,
that is relatively drier. Regardless, both temperature and
note that this value is relative to temperature and in
RH may end up being a metric that will simply be "asked
some cases may be too high for the comfort of surgeons
for" by the surgeon or staff at the time of the procedure.
and staff who typically wear various layers of protective
Table 2 may be of some guidance to the design profesgowning that impedes their body's ability to reject heat
sional when trying to pinpoint the proper requirements,
and perspiration into the atmosphere. In this case, lower but unless the engineer is fortunate enough to discuss
RH (and temperatures) values may be necessary in order this matter with the surgeon prior to designing the
to create an environment that can absorb perspiration
HVAC system, any guess may be as right (or as wrong) as
and remove heat through multiple layers of clothing
another.
and into the surrounding dry air. Certain surgery clasIn my own experience, I have found that surgeons
sifications may require specific RH levels to help cure
performing more intensive procedures prefer cooler,
orthopedic cements and adhesives or to keep the space,
less humid surroundings. The point where the facility
equipment and delicate medical devices free from conmanager's cell phone stops ringing with complaint calls
densation on surfaces.
from the surgical staff, or what I refer to as "the point
of diminishing complaints" occurs when OR condiDetermining Proper OR Environmental Conditions
tions are capable of being held in a range between 60°F
With regard to the temperature and relative humidity
and 64°F (15.5°C/17.7°C) and 40% to 50% RH. Without
range listed in Standard 170-2017, Table 7.1, normative
a clear understanding of what the procedural requirenote "l" states: "Systems shall be capable of maintaining
ments dictate (or what the staff desires) it may behoove
the rooms within the range (listed in the table) during
the facility design professional to default on the safe
normal operation. Lower or higher temperature shall
side, designing a more flexible HVAC system that can
be permitted when patients' comfort and/or medical
achieve cooler and drier environments, should it ever
conditions require those conditions." The HVAC Design
be expected. Regardless of what space conditions are
Manual for Hospitals and Clinics2 states that it is essential to
deemed acceptable, the HVAC system must be capable of
determine the desires of the doctors and staff for temdelivering the supply air conditions necessary to mainperature and humidity and to match those desires with
tain the combined temperature and RH.
18

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ASHRAE Journal - October 2019

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