Airborne infection isolation (AII) rooms

An AII room isolates a patient with a disease that spreads through the air — tuberculosis, measles, and similar — so the infectious particles they exhale are contained and removed rather than drifting out into corridors and other patient areas. It protects everyone outside the room from what is inside.

This is the opposite job from a protective-environment room, which protects a vulnerable patient from the outside. AII contains; protective environment excludes. Both rely on precise pressure relationships.

The defining feature is negative pressure: the room is kept at slightly lower pressure than the corridor and anteroom, so when a door opens, air flows in toward the patient rather than out. That inward flow keeps infectious particles from escaping into the rest of the facility.

The negative pressure is created by exhausting more air from the room than is supplied to it. Maintaining that offset reliably — even as filters load and doors open — is what keeps the containment intact.

AII rooms require high air change rates to rapidly dilute and remove infectious particles — the faster the room’s air is replaced, the faster pathogens are cleared. And critically, the room’s air is exhausted, not recirculated back into the building, so the contained particles are removed from the facility entirely.

The exhaust is typically discharged outdoors away from intakes and people, sometimes with additional filtration, so the contained pathogens are safely removed rather than reintroduced anywhere.

Many AII rooms have an anteroom — a small buffer space between the isolation room and the corridor — that strengthens containment by creating an additional pressure step and an airlock-like transition. Staff don and doff protective equipment there, and the anteroom reduces the direct exchange of air between the isolation room and the corridor.

The anteroom is part of a pressure cascade arrangement — corridor, anteroom, isolation room stepping down in pressure — so air always flows in the protective direction.

Because a failed AII room is a containment breach that could spread infection, the negative pressure is continuously monitored, with a visible indicator and alarm at the room so staff can confirm at a glance that it is holding negative. If the relationship fails, staff must know immediately.

This continuous verification — not set-and-trust — is essential. The monitoring is commissioned and maintained as a patient- and public-safety system, the same rigor applied to all critical pressure relationships.

Reliable AII performance requires careful airflow design, a well-sealed room (leaks undermine the pressure), good controls to hold the offset, proper exhaust, and continuous monitoring — then commissioning to verify it all works and periodic re-verification because performance drifts.

We design, control, and commission airborne infection isolation rooms to ASHRAE 170 and FGI requirements, and support the ongoing verification that keeps them reliable — as the installing contractor, with a Florida PE of record on the sealed engineering. It is among the most consequential HVAC any building does.