In designing HVAC for hospital operating theatres (OT), what percentage of outdoor (fresh) air should be supplied to the space supply stream?

Introduction / Context:
Operating theatres require stringent infection control and indoor air quality standards. Ventilation design must minimize contamination risk, control odours and anaesthetic gases, and maintain positive pressure relative to adjacent areas.

Given Data / Assumptions:

• Healthcare OT with aseptic requirements.
• Supply air is filtered (often HEPA) and pressurized.
• Air change rates are high, and exhaust is controlled.

Concept / Approach:
Best practice and many codes require 100% outdoor air (no recirculation into the OT) to prevent build-up of contaminants and to ensure high dilution of any airborne pathogens or anesthetic gases. Positive pressurization is maintained to push clean air outwards, limiting ingress of contaminants from adjacent spaces.

Step-by-Step Reasoning:

Verification / Alternative check:
Healthcare guidelines and standards (varies by jurisdiction) consistently recommend or mandate 100% outdoor air for OTs to maintain asepsis and to control anaesthetic gas accumulation.

Why Other Options Are Wrong:

• (a) Zero outdoor air undermines IAQ and infection control.
• (b) and (c) Partial outdoor air still allows contaminant accumulation.
• (e) Even with HEPA, recirculation into the OT is typically avoided; HEPA is used on the supply, not to justify recirculation.

Common Pitfalls:
Assuming that high-grade filtration alone permits recirculation. Dilution with 100% outdoor air and proper pressurization are critical.

Final Answer:
100% outdoor air