Department of

Architectural Engineering


Isolation Rooms & Pressurization Control

Isolation systems can be classified in three basic categories :

  • Negative Pressure Isolation Rooms
  • Positive Pressure Isolation Rooms
  • Multi-level Biohazard Laboratories

Also, dual-purpose systems now exist that can be controlled to serve as either negative pressure or positive pressure isolation rooms. The isometric view shown below illustrates the basic design principle for pressure control of isolation rooms. It includes an ante room for separating the isolation room from the corridor of the facility. In this diagram, air is supplied to the isolation room and exhausted from both the isolation room and the ante room. The balance of airflow, or the difference between between supply and exhaust, will dictate whether the room experiences positive or negative pressure with respect to ambient.

In this diagram, air is supplied to the isolation room and exhausted from both the isolation room and the ante room.

In this diagram, air would flow between the isolation room and the ante room, mostly through the gaps in and around the door. For a positive pressure room the air would flow from the isolation room to the ante room, where it would be exhausted partly by the exhaust duct and partly by flowing out to the corridor. In a negative pressure room, air would flow from the ante room to the isolation room. Pressure control is maintained by modulating the main supply and exhaust dampers based on a signal from a pressure transducer located inside the isolation room. This is by no means the only possible design -- there are various configurations of supply and exhaust ductwork, dampers and control systems that will accomplish pressurization.

Negative Pressure Isolation Rooms

Negative Pressure Isolation Rooms maintain a flow of air into the room, thus keeping contaminants and pathogens from reaching surrounding areas. The most common application in the health industry today is for Tuberculosis (TB) Rooms. The infectivity of TB is extremely high and these rooms are essential to protect health workers and other patients.

The CDC recommends 6-12 air changes per hour (ACH) for TB Rooms. An ante room is always recommended, as this provides a barrier between the TB Room and hallways and limits the impact of opening doors and traffic. The exhaust air is normally filtered through a HEPA (High Efficiency Particulate Air) filter before being exhausted to the outside, where it is ultimately rendered harmless by natural elements. Air which is recirculated within the room is also normally filtered. Ultraviolet Germicidal Irradiation (UVGI), commonly known as UV light, may be used to augment HEPA filters, but cannot be used in place of HEPA filters, as their effectiveness on airstreams is limited.

The exact air pressure differential which is required to be maintained is nominal only, as it merely indicates the airflow direction. It is sometimes stated as 0.001"wg, but this is not a pressure which is practical to measure, and therefore other criteria are given such as maintaining an inward velocity of 100 fpm, or exhausting 10% of the airflow, or exhausting 50 cfm more than the supply. The exact criteria will always be dependent on both the size and the airtightness of the subject facility.

Positive Pressure Isolation Rooms

Positive Pressure Isolation Rooms maintain a flow of air out of the room, thus protecting the patient from possible contaminants and pathogens which might otherwise enter. The most common application today is HIV Rooms and rooms for patients with other types of immunodeficiency. For such patients it is critically important to prevent the ingress of any pathogens, including even common fungi and bacteria which may be harmless to healthy people.

Design criteria for HIV Rooms are similar to those for TB Rooms. Air supplied to or recirculated in HIV Rooms is normally filtered through HEPA filters, and UVGI systems are sometimes used in conjunction with these. Anterooms are recommended and the air pressure differential criteria as described for TB Rooms applies similarly.

Approximately 15% of AIDS patients also suffer from TB, and this presents a unique design problem. One solution is to house the positive pressure (HIV) room within a negative pressure (TB) room, or vice-versa, which would be similar to a pair of nested biohazard levels. A much less expensive alternative is to design an entire house or building as a positive pressure (HIV) room, and this makes the outdoor air play the part of the second pressure barrier as it will effectively sterilize any exiting pathogens. Exhaust HEPA filters are still recommended, however, to protect any passersby.

Pressurization Control in Buildings

The basic principle of pressurization for microbial contaminant control is to supply air to areas of least contamination (greatest cleanliness) and stage this air to areas of progressively greater contamination potential. It could be assumed that in non-biohazard facilities, the exhaust or exfiltration from the building could go directly to the outside. In medical facilities, like TB clinics, this air is often HEPA filtered and sometimes given UVGI exposure before exhausting to the outside, though the reasons for this are primarily because of litigation concerns and not based on any known realities.

An alternate perspective on the design principle of pressurization control is to exhaust air from those areas which have the greatest contamination potential, and allow air to be staged, or cascaded, from progressively cleaner areas, or the areas it is desired to protect. Systems which combine both negative pressurization in contaminated areas with positive pressurization in clean, or protected, areas will have the greatest degree of protection and control. Below is an illustration of the basic principle of cascading airflows from clean areas to areas of progressively greater microbial conatmination potential.

In the above diagram, a facility is depicted which has offices and isolation rooms, separated by corridors and other areas (storage rooms, labs). Air is supplied to the areas, usually offices, maintained at the greatest positive pressure (marked with a '++'), and exhausted from the areas maintained at the greatest negative pressure (marked with a '- -'). Transfer air (exfiltration/infiltration) is identified with purple arrows. This represents one possible arrangement, but facilities often differ markedly in layouts, and the presuurization scheme must be adapted individually for each facility. The unlabeled rooms in the diagram above could be laboratories, which usually have independently operating exhaust hoods or separate ventilation systems. If not, they would be generally be designed as double negative pressurization areas.

Biohazard Laboratories

Biohazard laboratories are merely isolation rooms with strict requirements defining their degree of airtightness, pressurization and associated equipment. There are four biohazard levels, in level 1 defines a simple isolated area, and in which level 4 defines a near perfectly airtight zone requiring breathing apparatus and airtight anterooms or staging areas. Specific information on laboratory design is widely available from various sources, including ANSI, ASHRAE and the CDC.


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