Isolation rooms: negative and positive pressure

Isolation rooms use air pressure to control where airborne particles go. There are two basic types, and they do opposite jobs. A negative pressure room keeps infectious particles from leaving the room. A positive pressure room keeps contaminants from entering. Mixing them up, or letting either lose its pressure, defeats the purpose.

This guide explains both room types, the HVAC parameters that define them, how anterooms fit in, and how owners can confirm the rooms are working. In Canada, design is governed by CSA Z317.2, with ASHRAE Standard 170 as a common reference.

The two room types

Airborne infection isolation rooms

An airborne infection isolation room (AIIR) houses patients with diseases spread by airborne particles, such as tuberculosis, measles or varicella. The room is kept at negative pressure relative to the corridor, so air flows into the room through door gaps and is exhausted outdoors. Infectious particles are diluted by high air change rates and removed.

Protective environment rooms

A protective environment (PE) room houses severely immunocompromised patients, such as some transplant or oncology patients. The room is kept at positive pressure, so air flows out to the corridor, and supply air passes through HEPA filters. Fungal spores and other contaminants in corridor air are kept out.

Key design parameters

The values below reflect ASHRAE 170. CSA Z317.2 governs in Canada and its requirements should be confirmed for the edition that applies, as well as any additional health authority requirements.

Owners often design for higher pressure differentials than the minimum, since 2.5 Pa is a small pressure that is easily disturbed.

Room construction matters

Pressure differential depends on the flow offset and the room's leakage area. A room with a lay-in ceiling, unsealed light fixtures and gaps around pipes may leak so much that a reasonable offset produces almost no pressure. Good isolation room construction includes:

• Hard or sealed ceilings.
• Sealed wall and ceiling penetrations for pipes, ducts, conduits and medical gases.
• Self-closing doors with appropriate seals or controlled undercuts.
• Sealed windows that do not open.
• A washroom exhausted as part of the isolation room.

Construction quality should be verified before balancing, because leaks are much easier to seal before finishes are complete.

Anterooms

An anteroom is a small vestibule between the corridor and the isolation room. It allows staff to put on or remove protective equipment, and it preserves the pressure relationship when doors open, since one door can be closed while the other is open.

Pressure in the anteroom is set relative to both spaces. For an AIIR, a common arrangement is corridor, then a slightly negative anteroom, then a more negative patient room. For a PE room, the direction reverses. Some facilities have combination rooms for patients who are both infectious and immunocompromised, using arrangements such as a positive anteroom with a negative or neutral patient room. These must follow the specific requirements of CSA Z317.2 and the facility's infection prevention and control program.

Airflow within the room

Supply air should enter on the clean side of the room and exhaust should be located near the patient's head, low on the wall, so that air moves from staff toward the patient and out. In an AIIR, this pattern carries particles exhaled by the patient toward the exhaust. In a PE room, supply is usually located above the patient to provide clean air at the bed.

Exhaust discharge

AIIR exhaust must discharge outdoors at a location away from air intakes, operable windows and places where people gather. Many designs use dedicated exhaust fans with HEPA filtration in the exhaust where discharge location cannot meet separation requirements. Exhaust fans are typically on emergency power so negative pressure is maintained during utility outages.

Monitoring and alarms

Each isolation room should have a continuous pressure monitor, often with a local display at the door showing the room is in the correct mode and pressure. A typical monitor:

1. Measures the differential pressure between the room and the corridor or anteroom.
2. Shows pressure and status at the entrance.
3. Alarms locally after a time delay when pressure is outside limits.
4. Reports to the building automation system and, often, to nursing stations.

Monitors need periodic calibration. A tissue or smoke test at the door gap provides a quick visual confirmation of direction.

Controls

Most isolation rooms use flow offset control with constant volume or two-state air valves on supply and exhaust. Pressure-independent devices hold flows steady when other zones change. Some use direct pressure control or a combination. See hospital isolation room airflow control with air valves and room pressurization control: offset vs direct pressure.

Switchable rooms

Rooms that switch between normal and isolation modes are convenient but carry risk if left in the wrong mode. If used, they need restricted mode switching, clear indication at the door, and alarms. Some health authorities discourage them.

Surge capacity

Outbreaks can overwhelm the number of permanent isolation rooms. Facilities may plan for surge by designing wards or zones that can be converted to negative pressure, or by using portable HEPA filtration units that exhaust room air, temporarily creating negative pressure. These approaches should be planned with infection control staff and tested before they are needed. See airborne infection control in commercial HVAC.

Testing and verification

Isolation rooms should be verified at commissioning and on a regular schedule, commonly at least annually, plus after any work affecting the room. Typical checks:

1. Measured supply and exhaust airflow, and calculated air change rates.
2. Pressure differential measured with a calibrated instrument, doors closed.
3. Comparison of the room monitor reading with the calibrated measurement.
4. Smoke visualization at the door gap to confirm direction.
5. Alarm testing, including door-open delays.
6. Verification of emergency power operation for AIIR exhaust.
7. HEPA filter integrity testing for PE rooms.

An independent air balancing firm or commissioning provider should document results.

Key takeaways

• AIIRs are negative pressure rooms that contain airborne infectious particles; PE rooms are positive pressure rooms that protect immunocompromised patients.
• ASHRAE 170 sets common minimums of 12 ACH and 2.5 Pa (0.01 in. w.c.); CSA Z317.2 governs in Canada.
• Tight room construction is as important as airflow for achieving pressure.
• Anterooms preserve pressure relationships during entry and exit.
• Continuous monitoring, emergency power on AIIR exhaust, and regular verification keep rooms safe.