Airborne-Infection-Isolation (AII) rooms — the negative-pressure rooms that contain airborne pathogens such as Mycobacterium tuberculosis, measles, varicella, and emerging respiratory viruses — are among the most code-constrained, commissioning-sensitive, and operationally scrutinized spaces in an ICU. This article addresses how many AII rooms to provide, how the room and its optional anteroom are configured to hold a reliable directional airflow, and the governing standards that determine whether the room can be relied upon when an infectious critically ill patient arrives.

This Article is scoped to the isolation function — the pressure relationship, the anteroom, the envelope, and the monitoring that proves it. The broader air-change schedule, filtration, and pressurization of standard ICU rooms is covered by the sibling Article on HVAC and air changes; bedside booms, headwalls, and the general single-patient room are covered by their own Articles. AII is where those general systems are pushed to their isolation-specific extreme.

What an AII room is — and what it is not

An AII room is a single-patient room engineered to keep airborne infectious particles generated by the patient inside the room and out of adjacent corridors and rooms. It achieves this through a sustained negative pressure relationship to surrounding spaces, a high air-change rate, exhaust (not recirculation) of room air, and a sealed envelope. The patient is the contamination source; the room protects everyone outside it.

This is the opposite of a Protective Environment (PE) room, which is positive to its surroundings and protects an immunocompromised patient (for example, a hematopoietic stem-cell transplant recipient) from airborne pathogens in the building. The two functions are mutually exclusive in a single room operating at a single pressure relationship. A room intended to do both for different patients at different times requires an anteroom and a controlled, verified switching protocol — a "combination AII/PE" arrangement that FGI permits only through an anteroom and that most operators avoid because of the failure risk. ICUs typically provide dedicated AII rooms; PE rooms, when needed, are usually located in oncology or transplant units rather than the general ICU.

Common terms used interchangeably with AII include "negative-pressure isolation room," "TB isolation room," and "airborne isolation room." Older facilities may label a single-bed room a "private room with isolation capability," but unless it sustains a verified negative differential and exhausts its air, it is not a true AII room.

How many AII rooms to provide

The minimum number of AII rooms is driven by the FGI Guidelines for Design and Construction of Hospitals, the AHJ's adopted edition, and — critically — by a facility-specific Infection Control Risk Assessment (ICRA). FGI requires that the number of AII rooms be determined by the ICRA rather than set by a single blanket ratio; the ICRA is the document the design team and infection preventionist co-author to justify the count to the AHJ.

Practical planning guidance that the ICRA typically reflects:

• A minimum of one AII room is generally required in a critical-care unit, and most ICUs provide more.
• A frequently cited planning rule of thumb is roughly one AII room per 10–12 ICU beds, adjusted up or down by the ICRA. This is a starting point, not a code mandate.
• The ICRA weighs the facility's patient population, regional TB and airborne-disease prevalence, the unit's role in the regional system (a quaternary or transplant center carries higher demand), and lessons from recent respiratory-pathogen surges.
• Surge planning has become a standard ICRA input. Many systems now design a subset of ICU rooms with the infrastructure to be converted to negative pressure during an outbreak — pre-installed dedicated exhaust, sealable envelopes, and dampers — even if those rooms run neutral or positive day to day. This convertibility is an explicit FGI-recognized strategy and is increasingly written into the program.

The count decision is made early in programming because AII rooms carry outsized cost and routing consequences (dedicated exhaust risers, fan redundancy, monitoring) and because their location within the unit affects nursing workflow and adjacencies.

The pressure relationship — the defining requirement

The single non-negotiable performance attribute of an AII room is a reliable negative pressure differential relative to all adjoining spaces (corridor and, where present, anteroom). Air must flow into the room from those spaces and never out of it.

Governing numbers, per ASHRAE Standard 170 (Ventilation of Health Care Facilities) as referenced by FGI:

• Pressure relationship to adjacent areas: negative.
• Minimum pressure differential: a typical design target of approximately 0.01 inch water gauge (in. w.g.) — about 2.5 Pascals — though designers commonly aim higher (often 0.02–0.03 in. w.g.) to maintain margin against door movement, stack effect, and envelope leakage. The standard sets the relationship and a minimum magnitude; the design margin is an engineering choice.
• Minimum total air changes per hour (ACH): 12, all exhausted, with a defined minimum of outdoor air. (The detailed ACH and outdoor-air schedule, and the relationship to standard ICU rooms, is treated in the HVAC/air-changes sibling Article; what matters here is that an AII room is held at a higher exhaust-dominant rate to dilute and remove airborne contaminants.)