Controlling the spread of airborne infectious diseases in health care facilities is a serious concern to patients, staff and visitors. To minimize the spread of airborne infections, certain rooms within a hospital are designed as airborne infectious isolation (AII) rooms with negative-pressure differential or protective environment (PE) rooms with a positive-pressure differential.  

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While isolation rooms must meet the general requirements for a standard medical-surgical patient room, they also have specific requirements. For instance, patient isolation rooms may only have one patient bed, should be provided with an area for gowning and storage of clean and soiled materials directly outside or inside the entry door, and should be provided with a separate toilet room with a hand-washing sink.  

In addition, the isolation room needs to be well-sealed to prevent excess air leakage into or out of the room. The tighter the room is constructed, the more efficiently the air pressure differential can be maintained. In most instances, an isolation room does not require an anteroom. If one is provided, it should offer enough space to don protective equipment before entering the patient room, and the doors must have self-closing devices.

Controls considerations

The latest edition of the American Society of Heating, Refrigerating and Air-Conditioning Engineers' (ASHRAE's) Standard 170, Ventilation of Health Care Facilities, which is integrated into the Facility Guidelines Institute's Guidelines for Design and Construction of Health Care Facilities, requires each isolation room to have a permanently installed visual device or mechanism to constantly monitor the air pressure differential of the room when occupied by a patient who requires isolation.

high plume exhaust multiple fans

The exhaust fan should be located outdoors and placed as far away from intakes and public areas as practical with discharge above the roof.

Image courtesy of RTM Engineering Consultants

While the permanent device can be as simple as a flutter strip or calibrated ball in tube, the most reliable way to monitor room pressure is with the use of an electronic pressure monitor. When properly selected and installed, an electronic room-pressure monitor can provide continuous confirmation of the required pressure differential across the room boundary.

Most electronic monitors consist of two main components: a wall-mounted control panel and a sensor. The control panel usually is mounted on a corridor wall adjacent to the entrance of the isolation room and generally displays the pressure difference in inches of water column (WC).

In addition to providing a continuous readout of pressure differential, the control panel should include both audible and visual alarms to warn staff when room pressurization is lost. The alarm should sound when the measured room pressurization is below the alarm setpoint. For example, in a room designed to maintain a pressure differential of minus 0.03-inch WC, the alarm could be programmed to activate when the pressure differential falls to minus 0.01-inch WC. 

The control panel also should have a programmable, built-in time delay to minimize nuisance alarms. The time delay should be set to allow staff sufficient time to routinely enter and leave the room, and typically is set between 30 and 45 seconds. 

In addition to the alarms integral to the wall control panel, most electronic room pressure monitors include an extra identical signal that allows the pressure differential and alarm signals to be displayed at a remote location. The common location for this remote alarm is either the nurses’ station or the building automation system.

Equipment selection

When designing the mechanical systems to support isolation rooms, the designer must consider not only the airflow required to maintain the proper pressure differential, but also the location of the equipment, the serviceability of the equipment and equipment redundancy. Depending on the number and type of isolation rooms in the facility, it is generally more economical to provide a single larger system to serve multiple rooms than multiple smaller systems.

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The same air-handling system that serves other standard patient rooms may be used for isolation rooms. The air-handling unit serving the isolation rooms requires minimum-efficiency reporting value (MERV) 7 pre-filters, with either MERV 14 or high-efficiency particulate air (HEPA) final filters. MERV 14 filters are adequate for AII negative pressure rooms and for PE positive pressure rooms when terminal HEPA filtration is used at the supply diffusers serving the PE room.

For the exhaust system serving AII rooms, the exhaust fan should be located outdoors, if possible, and be placed as far away from intakes and public areas as practical, but no less than 25 feet with discharge above the roof. For outdoor fans, all exhaust from AII rooms should be exhausted by means of a vertical exhaust stack or exhaust fan with a vertical discharge arrangement.

If the fan must be located inside, welded ductwork should be used downstream of the exhaust fan, and a bag-in/bag-out filter housing with pre-filters and HEPA filters should be installed upstream of the exhaust fan. The exhaust fans shall be served by emergency power, and the fans should be labeled as contaminated air to meet the recommendations of the Centers for Disease Control and Prevention.

Redundancy of the equipment also needs to be considered, and will depend on the facility’s typical census of infectious or immunocompromised patients. On larger systems serving multiple rooms, redundant fans are recommended so the failure of a single fan does not compromise the safety of patients and caregivers. 

hazardous duct labeled

Exhaust ductwork serving AII negative isolation rooms should be permanently labeled as contaminated air.

Image courtesy of RTM Engineering Consultants

Design requirements

These individual control and equipment decisions come together in the designs of the negative- and positive-pressure rooms themselves. They include:

Negative isolation rooms. A negative-pressure AII room is designed to isolate a patient who is suspected of, or has been diagnosed with, an airborne infectious disease. The negative-pressure isolation room therefore is designed to help prevent the spread of a disease from an infected patient to others in the hospital.

Negative-pressure isolation rooms require a minimum of 12 air changes of exhaust per hour and must maintain a minimum 0.01-inch WC negative-pressure differential to the adjacent corridor whether or not an anteroom is utilized. Typically, a setpoint closer to minus 0.03-inch WC is used. When not required for use with an infectious patient, the negative-pressure AII room may be occupied by noninfectious patients. The negative-pressure relationship to the corridor should be upheld; however, it is not required to be maintained at the minimum of minus 0.01-inch WC.   

When an anteroom is provided, airflow should be from the corridor into the anteroom, and from the anteroom into the patient isolation room. To maintain the required pressure differential, the exhaust air quantity must always be higher than the supply airflow. Depending on such factors as room size and the room’s heating and cooling loads, more than 12 air changes per hour may be necessary. Typically, a minimum airflow difference of 150 to 200 cubic feet per minute (CFM) is adequate to maintain pressure differential in a well-sealed room.

Exhaust from negative-pressure isolation rooms, associated anterooms and associated toilet rooms must be discharged directly to the outdoors without mixing with exhaust from any non-AII rooms. However, multiple AII isolation rooms may be connected to the same exhaust system. The exhaust ductwork serving AII negative isolation rooms also should be permanently labeled as contaminated air within the facility at a maximum of 20 intervals and at all wall or floor penetrations.

Supply air for the room generally is located in the ceiling at the foot of the patient bed, with exhaust air taken from exhaust grilles or registers located directly above the patient bed on the ceiling or low on the wall near the head of the bed. When the head wall exhaust grilles are mounted lower than 7 feet above the floor, NFPA 90A requires that the opening be protected by a grille or screen through which a half-inch sphere cannot pass.

Positive isolation rooms. A positive-pressure isolation room is designed to keep contagious diseases away from patients with compromised immune systems, such as those with cancer or transplants. These rooms require a minimum of 12 air changes per hour of supply air and must maintain a minimum 0.01-inch WC positive-pressure differential, ensuring that the patient is protected from airborne contamination regardless of whether an anteroom is used. Typically, positive-pressure rooms are designed to maintain an even stricter setpoint of positive 0.03-inch WC.

Like the AII room, the PE positive room may be occupied by general patients when not in use. Similarly, the positive-pressure relationship to the corridor should be preserved; however, it is not required to be maintained at the minimum of positive 0.01-inch WC.

When an anteroom is used, airflow must be from the patient room into the anteroom and from the anteroom out into the corridor. As with the AII negative room, typically a minimum airflow difference of 150 to 200 CFM is adequate to maintain pressure differential in a well-sealed room. Positive-pressure rooms are required to be supplied with HEPA-filtered air, with the filters installed at the main air-handling unit or at the supply terminals in the room. 

Supply air for the room must be located in the ceiling above the patient bed, with return air taken from the ceiling near the patient room door. The supply diffuser shall be a non-aspirating, laminar-flow device and should be designed to limit the air velocity at the patient bed to reduce the possibility of patient discomfort. In addition, ASHRAE 170 requires that airflow to the PE isolation room be maintained at a constant volume to provide consistent ventilation in the room.

Combination AII/PE rooms. In the past, some isolation rooms were designed to be switchable between negative and positive isolation; however, this type of isolation room is no longer allowed. To address the need to protect an immunocompromised patient with a known infectious disease, ASHRAE 170 now includes guidelines for a combination AII/PE room. Unlike separate AII and PE isolation rooms, the combination isolation room must be used with an anteroom.

Supply air for the room must be located in the ceiling above the patient bed, with return air taken from the ceiling near the patient room door similar to a standard PE isolation room. The pressure relationship for the anteroom shall either be positive in relation to the AII/PE room and corridor or negative in relationship to the AII/PE room and corridor.

In addition, ASHRAE 170 requires two separate permanently installed visual devices or mechanisms to constantly monitor the air pressure differential. One device monitors the pressure relationship between the anteroom and AII/PE room and the second checks the pressure relationship between the anteroom and corridor. The exhaust from the combination AII/PE room, associated anteroom and associated toilet room must be discharged directly to the outdoors without mixing with exhaust from any non-AII rooms.

Room maintenance

After construction is completed, but prior to occupancy, the mechanical or balancing contractor typically will adjust the airflow quantities as directed by the design engineer to ensure that the isolation room is operating as designed. In addition, isolation rooms should be commissioned to prove correct pressure relationships, proper operation of room controls and the functionality of the pressure monitor and alarms. ASHRAE 170 also requires that the room be tested daily while it is being used as an isolation room. 

Given that mechanical systems drift out of balance over time, it is important to regularly check that an isolation room still is maintaining the proper pressure relationship and the pressure-monitoring device is working correctly. The room pressure should be checked monthly with smoke trail or similar testing. Most manufacturers of pressure monitors also recommend that the pressure monitor be recalibrated annually. The results of this periodic testing should be recorded because the authority having jurisdiction may request the data during a survey.

In addition to routine testing of the isolation room, the hospital staff who will be utilizing or maintaining the room should be trained on the proper use of the room, including how the pressure monitor works. A benefit of using a continuous pressure monitor connected to the building automation system is that the pressure differential for the isolation room can be monitored, trended and reviewed. It also can be used to alert hospital staff if the room is not performing as designed.

Vitally important

Given that a common deficiency cited by the Joint Commission relates to a ventilation system that is unable to provide appropriate pressure relationships, air exchange rates and filtration efficiencies, the proper design and maintenance of isolation rooms is vitally important. Whether the isolation rooms are designed during new construction or renovation of an existing space, careful planning is key.

Martin Herrick, PE, HFDP, is an associate at RTM Engineering Consultants, Schaumburg, Ill. He can be reached at