Operating room temperature and humidity standards (ASHRAE 170 and AORN)
In the US, operating room design conditions come from ANSI/ASHRAE/ASHE Standard 170, which sets a temperature range of 68°F to 75°F (20°C to 24°C) and relative humidity of 20 to 60 percent. CMS lowered its minimum humidity from 35 to 20 percent in 2013. The UK's NICE guideline adds a patient-focused rule: at least 21°C (69.8°F) while the patient is exposed.
What temperature range does ASHRAE 170 require for operating rooms?
ANSI/ASHRAE/ASHE Standard 170, Ventilation of Health Care Facilities, is the design standard adopted through the FGI Guidelines and referenced by accreditors and state regulators. For operating rooms, the 2021 edition specifies a design temperature range of 68°F to 75°F, roughly 20°C to 24°C.
The range is a design capability, not a single set point. The HVAC system must be able to hold any temperature within it, and the surgical team chooses where to operate within that band. Which edition applies to a given room depends on when it was built or renovated and what the state has adopted.
What humidity is required in the operating room?
ASHRAE 170 sets relative humidity for operating rooms at 20 to 60 percent. The lower limit was once higher. In April 2013, CMS issued a categorical waiver lowering its operating room humidity requirement from at least 35 percent to at least 20 percent, according to the American Society for Health Care Engineering.
The waiver came with conditions. It does not override stricter state or local rules, hospitals must document their decision to use it, and they must keep monitoring to maintain at least 20 percent. The original 35 percent floor dated from the era of flammable anesthetics, when static discharge was the main concern.
Why do both high and low humidity matter?
A 2019 article in the APSF Newsletter explained the trade-offs. Humidity above 60 percent increases the risk of mold and mildew growth on surfaces and reduces staff comfort. Humidity below 20 percent can shorten the shelf life of some sterilization indicators and increase electrostatic discharge risk to equipment.
The same article noted that some device manufacturers specify a minimum of 30 percent humidity for their equipment, and cited FDA data indicating that levels below 30 percent have contributed to device malfunctions. That is why many facilities aim well inside the 20 to 60 percent band rather than at its edges.
Temperature and humidity are linked. For the same amount of moisture in the air, cooler air has a higher relative humidity, a point the APSF article illustrates. Turning a room down sharply can therefore push relative humidity above the upper limit, which is one reason set point changes should be made within a written policy.
How do temperature and humidity standards compare?
The key numbers from each source are summarized below.
| Source | Temperature | Relative humidity |
|---|---|---|
| ASHRAE 170 (2021 edition), operating rooms | 68°F to 75°F (20°C to 24°C) | 20 to 60 percent |
| CMS categorical waiver, 2013 | Not changed | Minimum lowered from 35 to 20 percent |
| NICE CG65 (UK) | At least 21°C (69.8°F) while the patient is exposed | Not specified |
| AORN evidence table | Can be set for team comfort when the patient is actively warmed | Not specified |
What do NICE and AORN say about room temperature and the patient?
Engineering standards describe what the room must be able to do. Clinical guidance describes what the patient needs. NICE CG65 recommends that the ambient temperature be at least 21°C while the patient is exposed, meaning during positioning, skin preparation and draping, before active warming covers the body.
AORN's full temperature guidance is member content, but its published evidence table includes a 2018 randomized trial concluding that ambient temperature can be set to the comfort of the perioperative team when patients are actively warmed. Together these support a common protocol: keep the room warmer while the patient is uncovered, then cool it once the patient is draped and actively warmed.
Why are operating rooms kept so cold?
Several pressures push temperatures toward the low end of the range. Surgeons work under lights in gowns and gloves, often with lead aprons, and are usually the warmest people in the room. Surgical teams also generate heat from equipment and lighting, and the scrubbed team cannot easily remove layers once gowned.
The result is a room that can be comfortable for the scrubbed team and cold for everyone else, including the patient and the circulating nurse and anesthesia staff. Heat loss from patients is driven mainly by radiation to cool surfaces, followed by convection, which is why room temperature and covering the patient both matter.
How should hospitals write an OR temperature and humidity policy?
State the design ranges that apply to each room, by edition of ASHRAE 170 or state code.
Define who adjusts set points and when, including a warmer setting while the patient is exposed.
Monitor and log temperature and humidity, and define actions for excursions outside range.
Check equipment and supply manufacturers' humidity requirements, which may be narrower than the standard.
Pair room settings with patient warming, so a cooler room never replaces active warming.
Account for staff comfort, since the circulating and anesthesia teams are often colder than the scrubbed team.
What does this mean for patients?
A cool operating room is normal and is managed through warming devices, warmed fluids and monitoring rather than by making the whole room warm. Patients who feel cold should tell staff, as NICE encourages. Anyone with questions about their own care should talk to a clinician on their surgical team.
Key takeaways
ASHRAE 170 (2021) sets operating room design conditions of 68°F to 75°F and 20 to 60 percent relative humidity.
CMS lowered its minimum OR humidity from 35 to 20 percent through a 2013 categorical waiver.
Very low or very high humidity carries equipment, sterilization and mold risks, so many facilities aim mid-range.
NICE recommends at least 21°C while the patient is exposed; AORN evidence supports a cooler room once active warming is on.
Room temperature complements patient warming; it never replaces it.
Frequently asked questions
What temperature should an operating room be?
ASHRAE 170 requires operating rooms to be capable of 68°F to 75°F (20°C to 24°C). The team chooses a set point within that range, and NICE recommends at least 21°C while the patient is uncovered.
What is the humidity requirement for operating rooms?
ASHRAE 170 specifies 20 to 60 percent relative humidity for operating rooms. CMS accepts a 20 percent minimum under its 2013 categorical waiver, unless state or local rules are stricter.
Why was OR humidity lowered to 20 percent?
The older 35 percent minimum dated from the use of flammable anesthetics, when static sparks were the main concern. Updated codes found 20 percent acceptable, and CMS adopted it by categorical waiver in 2013.
Does AORN set an operating room temperature?
AORN's patient temperature guideline focuses on keeping patients normothermic. Its evidence table includes a trial showing the room can be set for team comfort when the patient is actively warmed.
Why do staff feel cold in the operating room?
Rooms are often run at the cool end of the range for the comfort of gowned surgeons and for ventilation performance. Circulating nurses and anesthesia staff, who are not gowned, often feel the cold most.
Related reading
Sources
ASHRAE 170 by Room: Air Changes, Pressure Relationships, Humidity, and Filter Minimums, Building and Air (summary of ANSI/ASHRAE/ASHE Standard 170)
Issue Brief: CMS lowers OR humidity requirement (April 2013), American Society for Health Care Engineering (ASHE)
Humidity Levels in ORs (Loeb and Pollitt, APSF Newsletter, 2019), Anesthesia Patient Safety Foundation
Hypothermia: prevention and management in adults having surgery (CG65), Recommendations, National Institute for Health and Care Excellence (NICE)
AORN Guideline for Patient Temperature Management: Evidence Table, AORN
Intraoperative Hypothermia, OpenAnesthesia (International Anesthesia Research Society)
About HEATJAC. HEATJAC is a thermal architecture company founded by an anesthesiologist. We design patented garment systems that capture, conduct and broaden warmth across the body, and we publish this knowledge hub because the science of staying warm should be public. HEATJAC products are not medical devices and are not intended to diagnose, treat, cure or prevent any disease.