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An airborne infection isolation room (AIIR) — also called a negative-pressure isolation room — is a single-patient room engineered to contain airborne pathogens and prevent them from migrating to adjacent spaces. It is used for patients with suspected or confirmed infections that spread by the airborne route: tuberculosis, measles, varicella (chickenpox), disseminated herpes zoster, and, on a case-by-case basis set by facility policy and current public-health guidance, novel or high-consequence respiratory pathogens. Unlike standard contact or droplet isolation, which rely primarily on personal protective equipment and work practice, an AIIR is fundamentally a building engineering control — its protective effect depends on the room’s air-handling design working correctly and being verified as working, not just on staff following a protocol.
Before you use these figures: the ventilation values on this page are drawn from widely-cited secondary summaries of ASHRAE Standard 170 and the FGI Guidelines, not from a direct reading of either standard — both are paywalled, and CDC’s own guideline pages were unreachable when this page was written. They are given to explain how the requirement is structured, not as design values. Confirm the current ACH, pressure-differential and exhaust requirements against the edition of ASHRAE 170 and the FGI Guidelines your jurisdiction has adopted before using them for design, commissioning or a compliance determination. Requirements also differ between new construction and existing rooms, and state adoption of a given edition varies.
The Core Engineering Specification
Three elements define whether a room functions as an AIIR:
- Negative pressure relative to the corridor and adjacent spaces, so that when the door is opened, air flows into the room rather than out of it. CDC and HICPAC guidance, incorporated into ASHRAE 170 (the ventilation standard for healthcare facilities) and the Facility Guidelines Institute (FGI) Guidelines for healthcare construction, commonly specify a minimum pressure differential in the range of roughly 2.5 pascals (about 0.01 inch of water column) negative to the corridor, with a minimum airflow differential between exhaust and supply of at least 10 percent, or 100 cubic feet per minute, whichever is greater. Treat this range as the widely cited baseline rather than a fixed universal figure — the exact number governing a specific project is set by the current edition of ASHRAE 170 and the FGI Guidelines, both of which a facility’s engineering and infection prevention teams should confirm directly rather than relying on a remembered figure, since these editions are periodically revised.
- A minimum air change rate — commonly cited guidance calls for at least 12 total air changes per hour (ACH) for new construction or renovation, and at least 6 ACH for existing AIIRs that predate the current construction-era requirement. Air changes per hour, not just negative pressure alone, are what actually dilute and remove airborne contaminant concentration over time; a room can technically be negative-pressure and still not meet the ACH threshold that determines how quickly it clears infectious particles after a patient leaves or an aerosol-generating procedure ends.
- Direct exhaust handling. Air from an AIIR is exhausted directly to the outdoors, away from air intakes, pedestrian areas, and operable windows, or — where direct outdoor exhaust is not feasible — passed through a HEPA filter before being introduced to the general ventilation system. Air from an AIIR is never simply recirculated untreated into the general hospital air supply.
Because these figures sit inside standards that are periodically revised (ASHRAE 170) and inside the FGI Guidelines, which are a paywalled, purchased document, a facility’s engineering, infection prevention, and facilities management teams should confirm the current specification against the current edition rather than a secondary summary — including this one — before finalizing a construction or commissioning specification.
Continuous Monitoring, Not a One-Time Test
An AIIR’s negative pressure is expected to be monitored continuously, not verified once at commissioning and assumed stable afterward — door position, adjacent HVAC changes, and mechanical drift can all cause a room to lose negative pressure without any visible sign. In practice this monitoring takes two complementary forms:
- Continuous electronic pressure monitoring, typically a differential-pressure sensor with a visible display (often at the room entrance) and, in many facilities, a building-management-system alarm if pressure drifts out of range.
- Periodic manual verification, such as a smoke-tube test at the door threshold, used to visually confirm airflow direction, particularly as a check on the electronic monitor itself or where continuous monitoring is not installed on an older room.
Facilities are expected to document a defined monitoring frequency and to have a documented protocol for what happens operationally if a room is found out of specification — including a process for taking the room out of service for isolation use until corrected.
AIIR vs. Protective Environment (PE) Room: Opposite Pressure, Different Purpose
It is worth being precise about the distinction, since the two room types are easy to confuse and serve opposite purposes:
| Airborne Infection Isolation Room (AIIR) | Protective Environment (PE) Room | |
|---|---|---|
| Pressure relative to corridor | Negative | Positive |
| Purpose | Contain airborne pathogens the patient may be shedding, protecting others | Protect a severely immunocompromised patient from airborne pathogens (including fungal spores such as Aspergillus) present in the general hospital environment |
| Typical filtration | HEPA-filtered exhaust | HEPA-filtered supply air |
| Typical patient population | Suspected/confirmed TB, measles, varicella, etc. | Allogeneic hematopoietic stem cell transplant patients and similarly severely immunocompromised patients, per facility policy |
A patient who is both severely immunocompromised and suspected of having an airborne infectious disease presents a genuine engineering conflict — negative pressure protects others from the patient, positive pressure protects the patient from the environment — and cannot be resolved by a single standard room; facilities address this with specialized anteroom configurations or by prioritizing transmission-based precautions based on clinical risk assessment, which is beyond the scope of a standard AIIR specification and should be handled per facility infection-control policy.
Anterooms
Many AIIRs, particularly in new construction, include an anteroom — a small buffer space between the corridor and the patient room, itself pressure-controlled, used for staff to don and doff personal protective equipment without directly opening onto the corridor or the patient room simultaneously. Anterooms are recommended in current FGI Guidelines for new AIIR construction but are not present in every existing AIIR built under an earlier code cycle; where a facility relies on isolation rooms without anterooms, work-practice controls (deliberate door discipline, PPE staging just outside the room) become a more direct substitute for the anteroom’s function.
Where AIIR Requirements Intersect With Construction (ICRA)
Building or renovating an AIIR is itself a construction project subject to a facility’s infection control risk assessment (ICRA) process before and during the work — and because the space under construction will itself become a negative-pressure isolation room, the construction-phase infection controls and the room’s eventual commissioning verification are closely related but distinct steps: passing ICRA construction precautions does not substitute for commissioning the completed room’s actual pressure differential and ACH against the engineering specification.
Frequently Asked Questions
What is an airborne infection isolation room used for?
Patients with suspected or confirmed airborne-transmissible infections — tuberculosis, measles, and varicella are the classic indications — plus, per facility policy and current public health guidance, other pathogens judged to spread by the airborne route.
How is an AIIR different from a standard negative-pressure room description used casually?
“AIIR” is a specific, defined engineering configuration (negative pressure to a defined differential, a minimum air change rate, and controlled, filtered or direct-outdoor exhaust), not just any room a facility informally calls “negative pressure.” A room has to meet and maintain the specification, verified through monitoring, to function as an AIIR.
Does an AIIR need an anteroom?
Current guidance recommends an anteroom for new AIIR construction, but many existing AIIRs, built under earlier code cycles, do not have one and rely on work-practice controls instead. It is a recommended feature, not a universal requirement for every existing room.
Who verifies that an AIIR is actually working correctly?
Facilities management and clinical/infection-prevention engineering typically share this responsibility — facilities management for the mechanical commissioning and ongoing monitoring system, and infection prevention for confirming the room is used correctly and that monitoring alerts are acted on.
Where do the exact AIIR numbers (pressure differential, air changes per hour) come from?
They derive from CDC/HICPAC infection-control guidance as incorporated into ASHRAE Standard 170 (Ventilation of Health Care Facilities) and the Facility Guidelines Institute’s Guidelines for Design and Construction of Hospitals and Outpatient Facilities. The FGI Guidelines are a purchased, copyrighted document rather than a free public standard, so a project’s exact current-edition figures should be confirmed directly against that source rather than a secondary summary.
See also our companion guide to infection control risk assessment (ICRA) for construction, the infection preventionist role, and the patient safety and infection prevention pillar.
