An airborne infection isolation (Aii) room is designed with negative pressurization to protect patients and people outside the room from the spread of microorganisms transmitted by airborne droplet nuclei (small-particle residue of evaporated droplets containing microorganisms that remain suspended in the air for long periods of time) that infect the patient inside the room. Microorganisms carried in this manner can be dispersed widely by air currents and may become inhaled by a susceptible host within the same room or over a longer distance from the source patient, depending on environmental factors; therefore, special air-handling and ventilation are required to prevent airborne transmission. A badly designed and/or incorrectly operating isolation room can place healthcareworkers and other patients at risk for dangerous hospital-acquired infections.

Ventilation can reduce the overall the risk of infection in a room in two ways: dilution and removal. When clean air is supplied to a room, it dilutes the concentration of airborne contaminants within. The removal effect occurs when air from a room is either discharged to outdoors to a safe place or passed through a HEPA filter to trap the particle before recirculation.

While “laminar flow” systems are of proven efficacy in clean room and other applications involving much higher airflow exchange rates, designers cannot expect to achieve or maintain true unidirectional flow in infectious isolation rooms. The preferred design approach emphasizes air mixing effectiveness and dilution ventilation without attempting to establish unidirectional flow.

Diffusers, grilles, and ductwork

Occupant protection is afforded by minimizing the airborne concentration of infectious microorganisms. Ventilation effectiveness is maximized, particularly for perimeter rooms in cooling-dominated climates, by Type A ceiling-mounted, horizontal-throw diffusers, with maximum throw reaching the far wall and with exhaust registers. The exhaust register is a passive device; it simply gathers air that is near.

To encourage air mixing, the exhaust grille should be located at a point remote from supply. Locate supply grilles or registers directly above the patient bed, consider locating exhaust grille near the head of the bed.

Low sidewall grilles, if used, have the potential to become clogged. When the exhaust grille collects room air, dust and lint may be deposited on the grille and on the exhaust ductwork. Over time, these deposits can clog the grille and duct, reducing airflow. To compensate, exhaust grilles, ductwork, and fans should be slightly oversized. Failure to keep low exhaust grilles clean often results in over-pressurization of the Aii room, creating an effect opposite to that of containing infectious disease. If a low exhaust grille is utilized for the design, the bottom of the exhaust grille should be located approximately 6 inches above the floor. The vertical exhaust ductwork should be installed in the Aii room wall. An enlarged cavity will be required and should be coordinated with the architect.

To reduce noise, dampers should be located at a point in the duct far from the outlet. The area in front of the exhaust grille should be kept clear of obstructions, such as furniture and supply carts.

The individual air ducts providing supply and exhaust air for the Aii room should have control dampers to adjust airflow quantity. These dampers are usually manually operated, but may be automatic. Because thetypical Aii room has a hard ceiling, the handles for the dampers should not be above the isolation room ceiling.They should be accessible from above the corridor ceiling, or remote handles should be provided in the ceiling or wall. Provide more exhaust volume than supply in this type of room.

The room needs to be kept under negative pressure. Negative pressure is designed to contain infectious particles within a room by creating a continuous air current going into room under the door. Therefore, when the room is used as designed, airborne particles generated in the room cannot escape to the corridor. This creates ventilation imbalance, called an offset. The rooms make up the offset, continuously drawing in make-up air from outside the room. The room is also sealed to prevent air from being pulled through cracks and other gaps.

When designing HVAC elements of a building, the amount of air supplied to each room is usually selected on the basis of comfort concerns. However, in isolation rooms, infection control concerns are as important as comfort concerns. Engineers should follow federal or state regulations in addition to ASHRAE and AIA guidelines.

Air terminal units

The volume of air supplied to an isolation room should not vary. Therefore, if an isolation room is to be included in a building served by a VAV system, the box supplying air to the isolation room should be set to deliver constant airflow. The mechanical engineer will need to address comfort control in this room separately, such as adding reheat coil to the box.

Exhaust fan

The exhaust fan should be labeled as “Caution-Negative Pressure Isolation Room Exhaust” to help protect maintenance personal from potentially contaminating the isolation room exhaust.

Exhaust air should be directed away from occupied areas or openings into the buildings. To promote dilution, the fan discharge should be directed vertically upward at a speed of at least 2000 FPM. The discharge location should be at least 25 feet away from public areas or openings into a building.


After a new isolation room is constructed and before it’s occupied, the mechanical contractor will adjust the airflow quantities. The most reliable way to monitor negative pressure is to install a permanent electronic room pressure monitor as part of the construction project.

A room pressure sensor measures the differential pressure between the room and adjacent reference area (corridor or anteroom). The sensor and transmitter provide a digital readout or visual indication of room pressure. A remote alarm panel, typically located in the nurses’ station, can be installed and may be connected to the building management system (BMS)  for immediate response by maintenance and system operating personnel.

Specialized sensors and controllers are sometimes used for isolation room pressurization control in hospitals. Some codes now require a minimum space pressure differential of 0.01-inch water gauge in Aii rooms.

Two control strategy options are generally acceptable under hospital codes (with some exceptions): constant volume with volumetric differential between supply and exhaust, or volumetric control (tracking) with direct pressure reset. Constant volume with volumetric differential between supply and exhaust involves a constant volume of supply and exhaust air established when the system is initially tested and balanced. A fixed differential is maintained in isolation room. With volumetric control (tracking) with direct pressure reset, direct pressure controls recognize and compensate for disturbances such a stack effects, infiltration, and exfiltration. This can provide two control functions. It can reset the room point and provide a volume flow differential (supply-exhaust cfm) within a range to maintain room pressurization.


Ardas Sabuncuyan, P.E., HFDP, is an ASHRAE-certified healthcare facility design professional. He is the assistant director of mechanical engineering with Jacobs Engineering Inc. (Fort Worth, Tex.). He can be reached at




  • ASHRAE Special Project #91 HVAC Design Manual for Hospitals and Clinics
  • “Isolation Rooms: Design, Assessment, and Upgrade,” Francis J. Curry National Tuberculosis Center
  • 2007 ASHRAE Handbook HVAC Applic
    ations, Chapter 7: Healthcare Facilities