Short answer
ASHRAE 170 requires surgical suites to hold 68 to 75 degrees Fahrenheit, 20 to 60 percent relative humidity, positive pressure of at least 0.01 inches water gauge relative to adjacent spaces, and at least 20 total air changes per hour with a minimum of 4 outdoor. Filtration is MERV 16 at minimum. CMS and the Joint Commission require continuous monitoring of theatre temperature and humidity, and the standard requires individual control per operating room, which means zone based monitoring does not demonstrate compliance.
The published requirements
| Parameter | Requirement | What it is protecting |
|---|---|---|
| Temperature | 68 to 75 F, that is 20 to 24 C | Surgical team performance and patient thermoregulation, which pull in opposite directions |
| Relative humidity | 20 to 60 percent | Below the band, static and drying risk. Above it, microbial growth and condensation risk |
| Differential pressure | Positive, at least 0.01 inches water gauge to adjacent spaces | Keeps unfiltered corridor air out of the sterile field |
| Air changes per hour | 20 or more total, at least 4 outdoor | Dilution and removal of airborne contamination generated during the case |
| Filtration | MERV 16 minimum, HEPA typical in practice | What is actually in the supply air reaching the field |
The three words that create the gap
"Continuous"
CMS and the Joint Commission require continuous monitoring of theatre temperature and humidity when the suite is in use for its intended purpose. A logbook entry taken on a round is a sample. Between one sample and the next there is no record at all, and it is precisely in those gaps that an excursion occurs and recovers unseen.
This is the single most common finding on a hospital survey of environmental records: not that the readings were bad, but that they were sparse.
"Individual"
The standard requires individual temperature and humidity control for each operating room. That has a monitoring consequence people miss: a sensor in the AHU or in the corridor serving a group of theatres cannot demonstrate that any one of those theatres was in band. Monitoring has to be per room, because control is per room.
"Relative to adjacent spaces"
Pressure is not a property of a room, it is a relationship between two rooms. A single pressure reading is meaningless without knowing what it is being compared against, which means the monitoring has to model room pairs and the cascade between them rather than list gauges.
From seven readings to one verdict
There is a practical problem with monitoring all of this properly, which is that it produces a lot of numbers, and the person who needs the answer is about to start a case.
The useful design is to score each parameter against its own limit and its trend, then roll them into a single readiness state for the room. Humidity climbing towards the upper limit with everything else in band is not a failure, it is a specific condition with a specific recommendation, such as holding the next case until it recovers. The verdict then updates live as the air handling unit responds, so the team can see the room coming back.
A note on India and the humidity band
ASHRAE 170 gives 20 to 60 percent relative humidity. Indian practice under NABH commonly holds theatres to a tighter 40 to 60 percent. Both are defensible and they are not in conflict, since the tighter band sits inside the wider one.
The practical point for a group operating in more than one market is that the monitoring layer has to carry the limits per room class per site, rather than assume one global set. That is a configuration question, and the hospital protocol is what decides it, not the vendor.
How many air changes per hour does an operating room need?
ASHRAE 170 requires a minimum of 20 total air changes per hour, of which at least 4 must be outdoor air.
What humidity range is required in an operating theatre?
ASHRAE 170 specifies 20 to 60 percent relative humidity. Indian practice under NABH commonly applies a tighter 40 to 60 percent band, which sits inside the ASHRAE range.
Is zone monitoring enough for operating room compliance?
No. The standard requires individual temperature and humidity control for each operating room, which means monitoring has to be per room. A sensor in the AHU or a shared corridor cannot demonstrate that a specific theatre held its limits.
What does continuous monitoring mean in practice?
A record with no gaps in it, produced automatically, for the period the suite was in use for its intended purpose. Rounds produce samples with gaps between them, and the gaps are where excursions hide.
It also implies alerting, because a record that nobody reads until a survey does not protect the patient in the room today.
