Operating room HVAC design

An operating room's HVAC system is part of the surgical environment. It delivers clean air over the surgical site, dilutes airborne particles shed by staff, keeps contaminants from entering from corridors, and holds temperature and humidity at levels suited to both patients and surgical teams. It must do this reliably, day after day, often with little margin for downtime.

This guide covers the core design principles, typical parameters and the checks that confirm an operating room performs as intended. In Canada, CSA Z317.2 governs health care HVAC, as outlined in hospital HVAC and CSA Z317.2, and ASHRAE Standard 170 is a common reference.

Design goals

An operating room HVAC system aims to:

• Supply clean, filtered air directly over the surgical field.
• Sweep airborne particles away from the patient and toward returns.
• Keep the room positive to adjacent corridors and spaces.
• Maintain temperature and humidity within a defined range, with surgeon-adjustable setpoints.
• Provide high reliability and maintain conditions during equipment failure.
• Keep noise and drafts at acceptable levels for the surgical team.

Typical design parameters

The values below reflect ASHRAE 170 and are widely used as a starting point. CSA Z317.2 sets the governing requirements in Canada, and some of its values differ, so confirm against the edition that applies to the project.

Airflow patterns

Unidirectional supply over the table

Modern operating rooms typically use a primary supply diffuser array directly above the operating table. These are non-aspirating diffusers that deliver a gentle, downward column of clean air, washing over the surgical field and pushing particles down and away. ASHRAE 170 sets requirements for the array's size relative to the table and limits on supply velocity.

The goal is a stable column of clean air, not high velocity. Excessive velocity can cause turbulence, induce room air into the clean zone, and cause discomfort or patient cooling.

Low returns

Return or exhaust grilles are usually located low on the walls, near the corners of the room and away from the door. Low returns draw air down through the room and away from the surgical field. Placing returns high or near the table can disrupt the downward flow.

Interference from lights and equipment

Surgical lights, booms and equipment hang into the supply column and disrupt airflow. Coordination of the diffuser array, booms and lights is essential during design. Computational fluid dynamics modelling is sometimes used to evaluate layouts in complex rooms.

Pressurization

Operating rooms are kept positive so that air flows out to the corridor when doors open, preventing corridor air from entering. Supply exceeds return and exhaust by an offset sized for door leakage and room tightness. Many owners install continuous room pressure monitors with local display and alarm.

Door management matters. Frequent door openings during surgery disrupt pressurization and airflow. Design features such as sliding doors with good seals, and operational practices that limit traffic, help maintain conditions.

Temperature and humidity control

Surgical teams often want cooler conditions, especially during long procedures in heavy gowns, while patients risk hypothermia. The system must give staff control within the allowed range and respond quickly to setpoint changes. Some procedures, such as paediatric or burn surgery, need warmer rooms.

Each operating room usually has its own terminal unit with reheat and often a dedicated humidifier or humidity trim. In Canadian winters, humidification is essential to meet minimum humidity requirements. In summer, the air handling system must dehumidify enough to keep humidity below the maximum, which can require low supply air temperatures. See humidification in cold climates.

Filtration

Operating room air passes through multiple filter stages: pre-filters at the air handler and high-efficiency final filters, located downstream of fans and humidifiers so they stay dry. Some owners specify HEPA filtration at terminal diffusers for orthopaedic, transplant or similar procedures. Filter pressure drop should be monitored, and filters must be replaced before airflow falls below design. Filtration, MERV ratings, HEPA and pressure drop covers the details.

Reliability

Operating rooms need continuity of service. Common features include:

• Redundant supply and exhaust fans or dedicated air handlers per suite, with N+1 capacity.
• Emergency power for fans, controls and critical heating and cooling.
• Redundant chilled water and heating sources.
• The ability to maintain conditions in one room while another is serviced.

Unoccupied setback

When operating rooms are unoccupied, many health authorities permit reducing air changes to save energy, provided positive pressure is maintained and full airflow resumes before use. The standard and health authority policy define what is allowed. Controls must make the return to occupied mode quick and reliable, often triggered by an occupancy sensor or a switch at the door.

Commissioning and verification

At commissioning and during periodic re-verification, typical tests include:

1. Supply, return and exhaust airflow measurement and calculation of air change rates.
2. Diffuser array velocity and airflow uniformity.
3. Pressure measurement relative to the corridor and adjacent rooms.
4. Smoke visualization of airflow over the table and at doors.
5. Temperature and humidity performance across setpoint range and under load.
6. Filter installation checks and, where HEPA is used, installed leak testing.
7. Unoccupied setback and return-to-occupied sequences.
8. Alarm function for pressure, temperature and humidity.

These tests are part of thorough building commissioning for health care projects.

Frequently asked questions

Do operating rooms need HEPA filters?

Requirements depend on the standard edition, procedure type and owner policy. High-efficiency final filters are required; HEPA is often specified for specific procedure types.

Can operating rooms use 100 percent outdoor air?

They can, but it is not usually required. Recirculation through the central system with appropriate filtration is generally permitted and saves significant energy in cold climates.

Why does humidity matter in surgery?

Low humidity can affect staff comfort, static electricity and some equipment, while high humidity can support microbial growth and fogging. The standard sets a range that balances these.

Key takeaways

• Operating room HVAC delivers clean air over the surgical field, maintains positive pressure and controls temperature and humidity.
• ASHRAE 170 values such as 20 total ACH and 4 outdoor ACH are common starting points; CSA Z317.2 governs in Canada.
• Non-aspirating diffuser arrays over the table and low returns create a protective downward airflow.
• Reliability, redundancy and emergency power are essential.
• Verify airflow, pressure, temperature, humidity, filters and alarms at commissioning and periodically.