The mechanical design elements of a new hospital isolation room should, at a minimum, meet all local code requirements, as well as OSHA requirements and CDC recommendations.

Many mechanical systems do not provide a constant airflow rate.  These are called variable air volume (VAV) systems.  They are designed to continually vary the amount of cooling or heating air delivered to a room in response to the amount of cooling or heating required.  Supply air varies between a fixed minimum and a fixed maximum using a VAV box installed in the ductwork.  VAV systems are generally not found in hospitals, but are common in buildings that include clinics.

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 of this room separately.

Supply and Exhaust Locations and Ductwork

The supply and exhaust location should be chosen to maximize air mixing and to optimize directional airflow from the staff member towards the patient. Exhaust should be removed near the possible contamination source (the patient's head).

The best arrangement is to supply air at the ceiling above the foot of the bed. If this is not feasible then the supply air should be on the wall near the foot of the bed.   Other options may be considered based on the geometry of the room, but the intra-room airflow dynamics should be carefully considered when positioning the supply air.  Ideally, the best method of supplying air is to position the diffuser(s) in a location that sweeps fresh air into all parts of the room in one pass and is then exhausted.

The individual air ducts providing supply and exhaust air for the isolation room suite should have control dampers to adjust the airflow quantity.  These dampers are usually manually operated, but may be automatic.  Because of the hard ceiling, the handles for the dampers should not be above the isolation room ceiling.  They should be either accessible from above the corridor ceiling, or remote, tamper-proof handles should be provided in the ceiling or wall of the isolation room.

For new construction, the exhaust grille does not direct air, its face pattern is not as important as that of the diffuser.  If the APS is a wall mounted unit, the vertical exhaust duct should be installed in the isolation room wall.  An enlarged wall 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.

For upgrading of existing rooms, a wall unit should be located directly behind the patient's head an exhausted either straight through the wall or up to the ceiling.  A ceiling unit should directly above the patient.  HEPA filtered air may be exhausted to the house HVAC return or preferably, to the outside.

Negative pressure is achieved when exhaust exceeds supply and the room is well sealed except for a gap under the door (normally ½").  The CDC Guidelines note that negative pressure can be established if exhaust exceeds supply by 100 cubic feet per minute (CFM) or by 25%+ of the supply air quantity, whichever is greater.  These values are chosen to provide a negative pressure differential of at least 0.003 inches of water column (" W.C.).

Effectively sealing an existing room can increase the negative pressure thereby reducing the threat of contaminated air escaping the room.

Different groups have different regulations or guidelines for the pressure in negative pressure rooms.  OSHA has a -0.001 minimum, JCAHO has a -0.003 minimum and CDC recommends -0.005 for existing rooms and -0.05 for newly constructed rooms.

It should be noted that the AIA recommends that the exhaust from a dedicated bathroom not be included when calculating the total exhaust for the isolation room.   This is still up to the certifying technician as to whether to include it when testing a "grandfathered" infectious isolation room.  With newly constructed rooms (not grandfathered), the exhaust from the bathroom should not be considered when certifying the room.

The bathroom should have a minimum for fifty (50) CFM exhaust depending on the size of the room.   Bathroom exhaust should always be a dedicated exhaust going directly to the outdoors at least 25" away from any building entrances, windows, work or pedestrian areas.

Isolation room exhaust ducts and fan systems should be sized for the expected airflow plus an extra 50%.  MAS normally uses a 25% safety factor when sizing APS units.  Having a unit give more that a 100% increase in flow would be cost prohibitive.

JCAHO, OSHA and the CDC all recognize the fact that, in many cases, the exhaust from the isolation room may not be exhausted to the outdoors.  In that case it is acceptable to exhaust into the HVAC air return provided the air is HEPA filtered.  MAS strongly recommends adding UVGI within the APS to permit the unit to "capture and kill" all airborne pathogens.  UVGI is a viable "second line of defense".

Permanent Room Pressure Monitor

The most reliable way to monitor negative pressure is to install a permanent electronic room pressure monitor as part of the construction project.  When properly selected and installed, a room pressure monitor can provide continuous qualitative and quantitative confirmation of negative pressure across a room boundary.  This is in contrast to routine periodic smoke testing, which merely provides an indication of directional airflow at the moment of testing.

Continuous monitoring can provide instant notification if the pressurization fails or fluctuates during the day.  Most monitors consist of two main components: a wall-mounted panel and a sensor.  The panel is usually mounted on the corridor wall just outside the isolation room suite and displays the pressure difference in units of " W.C.

In addition to providing a continuous readout of the pressure difference, the wall panel should include an audible and visual alarm to warn staff when pressurization is lost. 

The audible alarm is usually a beeping sound, which will stop when negative pressure is restored or when a "mute" button on the panel is pressed. The visual alarm usually consists of a red warning light. Most wall panels also have a green "normal" or "safe" light, which indicates that the monitor is operating and negative pressure is within programmed parameters. Unlike the audible alarm, the visual alarm will not reset when the "mute" button is pressed. After negative pressure is restored, the lights will either automatically reset or the "reset" button must be pressed, depending on the brand of the monitor. In case no one was present, the latter option will indicate that negative pressure was temporarily lost.

Most manufacturers recommend that each monitor be recalibrated annually. The recalibration procedure will depend on the monitor type and should be available from the manufacturer. ICS recommends that a new monitor checklist be completed at the same time.

If space and budget permit, an anteroom should be provided between the negative pressure isolation room and the corridor.  This will help prevent infectious particles in the isolation room from escaping to the corridor.

When an isolation room door is open, negative pressure is immediately lost.  If there is an anteroom that is negative to the corridor, then the overall integrity of the suite is maintained.  The anteroom provides an "air lock" between the isolation room and the rest of the facility.

An anteroom should be at positive pressure with respect to the isolation room, and at either neutral or negative pressure with respect to the corridor.  Because smoke may migrate from the corridor if there is a fire, some codes and regulations mandate that the anteroom be neutral to the corridor, rather than negative.  However, in practice this is very difficult to accomplish.  It is not easy to balance airflow to a space so that it will be positive at one door and neutral at the other.  Local codes should be consulted regarding other design elements of anterooms for isolation rooms.

Air movement within a room is neither constant nor consistent.  There are always eddies and swirling in its’ movement caused by obstructions and the movement of people.  Generally, the placement of the supply air registers/diffusers and the exhaust grilles should be placed so that the air will continually move across the patient’s head and away from any HCWs.

For a single patient isolation room, the best solution is the ceiling system.  A ceiling unit will exhaust the air flowing just above the patient’s head thereby removing the majority of droplet nuclei immediately from the room.  The next best solution is a wall exhaust that is located directly above the head of the patient’s bed.

Ventilation air supplied to a room by a mechanical system will mix with air already in the room. This air mixture is removed by exhausting to the outdoors via the HVAC return system or by individual air purification systems (APS).  The effectiveness of dilution and removal depends on the effectiveness of the mixing process: the better the mixing, the better the dilution and removal. Stagnation and short-circuiting need to be avoided.

Stagnation occurs when part of the room does not benefit from the clean supply air. Infectious particles in a stagnant spot are not being diluted or removed.

Short-circuiting occurs when the exhaust is located too close to the supply; the clean air is removed from the room before it can effectively mix with and dilute contaminants in the room air.

MAS offers consultation services on what equipment and designs may be needed for a particular application. In other words, we offer solutions. Neither one nor two pieces of equipment can meet the air purification requirements for the many and various settings in today’s healthcare environment. That is why we carry a full complement of products and services to meet the specific needs of our customers.

The air purification products we carry for negative and positive pressure rooms are quiet and all have the capability to incorporate ultraviolet germicidal irradiation (UVGI) within the units and we always recommend that they be purchased with this option. UVGI is an effective second line of defense should the HEPA develop micro-tears or holes allowing pathogenic material to pass through the filter.