Establishing
Airborne infection isolation rooms (AIIR)
& Protective Environments (PE)

* From Table 6, full-text version, "Guidelines for Environmental Infection Control in Health-Care Facilities", U.S. Department of Health and Human Services, Centers for Disease Control and Prevention (CDC), Atlanta, GA 30333, Published: 2003.   See original document at above link, for references.

§ Pa is the abbreviation for Pascal, a metric unit of measurement for pressure based on air velocity; 250 Pa equals 1.0 inch water gauge.

¶ DOP is the abbreviation for dioctylphthalate particles of 0.3 μm diameter.

** If the patient requires both PE and AII, return air should be HEPA-filtered or otherwise exhausted to the outside.

^ HEPA filtration of exhaust air from AII rooms should not be required, providing that the exhaust is properly located to prevent re-entry into the building.

Achieving Negative Pressure in Hospital Isolation Rooms: Negative pressure is needed to control the direction of airflow between selected rooms in a health-care setting and their adjacent spaces to prevent contaminated air from escaping from the room into other areas (Figure 1). Control of a room's differential airflow and total leakage area is critical to achieving and maintaining negative pressure. Differential airflow, differential pressure, and leakage area are interrelated. This relation is illustrated (Figure 1) and is expressed in an empirical equation.

(Formula 1)

In the equation, A_E is the leakage area in square inches [for instance, air gap under the isolation room entry door]; DQ is the differential airflow rate in cubic feet per minute (CFM) [the difference between airflow exhausted from the negative pressure isolation room to an adjoining area through a HEPA filter, and, supply air entering the room from sources such as the HVAC system supply vent]; and DP is the differential pressure drop in inches of water gauge [the difference in pressure between the air in the adjoining area where the HEPA filtered isolation room air is exhausted to, and, the air within the negative pressure isolation room]. This empirical equation was used (Figure 1), which indicates that changing one parameter will influence one or both of the other parameters. For example, the control of differential pressure can frequently be improved by increasing the air tightness or seal of a room, HVAC system, and ensuring continuous monitoring. In a room that is already substantially tight (e.g., with 10 square inches of leakage), however, a small change in differential pressure will have a substantial affect on differential airflow. Similarly, a room with a more substantial leakage area (e.g., 300 square inches of leakage) requires a higher differential airflow rate to achieve a pressure differential of 0.01 inch of water gauge. Reducing the leakage in a room with 300 square inches of leakage can help achieve a pressure differential of 0.01 inch of water gauge (Figure 1). If the leakage area is reduced to approximately 40 square inches, a pressure differential of 0.01 inch of water gauge can be achieved by exhausting approximately 100 CFM more air from the room than is supplied to the room.

Room leakage can occur through cracks or spaces near doors, windows, ceiling, and utility connections. Steps should be taken to minimize these leaks. Changes in the performance of the HVAC system will affect the pressure differential in a room and can potentially cause a negative-pressure room to become positive-pressure. Therefore, each of these parameters requires close monitoring to ensure that minor changes in the performance of the HVAC system do not adversely affect the entire system. (Differential Room Pressure Monitors for Hospital Isolation Rooms)

Further information:

1. Guideline for Isolation Precautions: Preventing Transmission of Infectious Agents in Healthcare Settings, CDC, June 2007

2. CDC | TB | TB Guidelines - Infection Control & Prevention April 19, 2012

3. OSHA Safety and Health Topics, Healthcare Facilities
4. Guidelines & Recommendations for Ventilation, Construction, and Renovation of Hospitals
5. Guidelines for Design and Construction of Health Care Facilities, The Facility Guidelines Institute
6. Isolation Rooms & Pressurization Control pictorials, Penn State University Architectural Engineering Indoor Environment Center