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

(positive & negative pressure isolation rooms for hospitals & medical facilities)

Airborne infection isolation room (AIIR). Formerly, negative pressure isolation room, an AIIR is a single-occupancy patient-care room used to isolate persons with a suspected or confirmed airborne infectious disease. Environmental factors are controlled in AIIRs to minimize the transmission of infectious agents that are usually transmitted from person to person by droplet nuclei associated with coughing or aerosolization of contaminated fluids. AIIRs should provide negative pressure in the room (so that air flows under the door gap into the room); and an air flow rate of 6-12 ACH ( 6 ACH for existing structures, 12 ACH for new construction or renovation); and direct exhaust of air from the room to the outside of the building or recirculation of air through a HEPA filter before returning to circulation (MMWR 2005; 54 [RR-17]).

Protective Environment (PE). A specialized patient-care area, usually in a hospital, that has a positive air flow relative to the corridor (i.e., air flows from the room to the outside adjacent space). The combination of high-efficiency particulate air (HEPA) filtration, high numbers (>12) of air changes per hour (ACH), and minimal leakage of air into the room creates an environment that can safely accommodate patients with a severely compromised immune system (e.g., those who have received allogeneic hemopoietic stem-cell transplant [HSCT]) and decrease the risk of exposure to spores produced by environmental fungi. Other components include use of scrubbable surfaces instead of materials such as upholstery or carpeting, cleaning to prevent dust accumulation, and prohibition of fresh flowers or potted plants.

Table 1: Engineered specifications for positive and negative pressure isolation rooms*

  Positive pressure areas (e.g., protective environments [PE]) Negative pressure areas (e.g., airborne infection isolation [AII])
Pressure differentials > +2.5 Pa§ (0.01″ water gauge) > -2.5 Pa (0.01″ water gauge)
Air changes per hour (ACH) >12 >12 (for renovation or new construction)
Filtration efficiency

Supply: 99.97% @ 0.3 μm DOP¶
Return: none required**

Supply: 90% (dust spot test)
Return: 99.97% @ 0.3 μm DOP¶ ^

Room airflow direction Out to the adjacent area In to the room
Clean-to-dirty airflow in room

Away from the patient (high-risk patient, immunosuppressed patient)

Towards the patient (airborne disease patient)

Ideal pressure differential > + 8 Pa > - 2.5 Pa

* 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.

Negative Pressure Isolation Room Leakage as function of differential pressure and differential airflow

(Formula 1)

In the equation, AE 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)

Figure 1: Empirical relation between differential airflow,
differential pressure, and leakage areas*

Empirical relation between differential airflow, differential pressure, and negative pressure isolation room leakage areas

Section "Achieving Negative Pressure in Hospital Isolation Rooms" adapted from "Guidelines for Preventing the Transmission of Mycobacterium tuberculosis in Health-Care Settings, 2005"; December 30, 2005 / 54(RR17);1-141, US Centers for Disease Control and Prevention.

Original source documents for Formula 1 & Figure 1:
Hayden II CS, Fischbach TJ, Johnston OE, Hughes RT, Jensen PA. A model for calculating leakage areas into negative pressure isolation rooms. Cincinnati, OH: US Department of Health and Human Services, CDC; 1996

Table 2: Required Differential Airflow vs.  Negative Pressure Isolation Room Leakage at Isolation Room Differential Pressure of 0.01 inches wc




(CFM) (Inches wc)
17.70 0.12 50.00 0.01
39.82 0.28 100.00 0.01
64.00 0.44 150.00 0.01
89.61 0.62 200.00 0.01
116.34 0.81 250.00 0.01
144.00 1.00 300.00 0.01
172.47 1.20 350.00 0.01
201.63 1.40 400.00 0.01
231.42 1.61 450.00 0.01
261.78 1.82 500.00 0.01
292.66 2.03 550.00 0.01
324.03 2.25 600.00 0.01
355.84 2.47 650.00 0.01
388.07 2.69 700.00 0.01
420.69 2.92 750.00 0.01
453.69 3.15 800.00 0.01
487.04 3.38 850.00 0.01
520.72 3.62 900.00 0.01
554.73 3.85 950.00 0.01
589.04 4.09 1,000.00 0.01
946.61 6.57 1,500.00 0.01
1,325.40 9.20 2,000.00 0.01
1,720.81 11.95 2,500.00 0.01
2,129.97 14.79 3,000.00 0.01
2,550.95 17.71 3,500.00 0.01
2,982.31 20.71 4,000.00 0.01

TABLE 3: Air changes per hour (ACH) and time required for removal efficiencies of 99% and 99.9% of airborne contaminants*

**Table 3 from Table 1 of "Guidelines for Preventing the Transmission of Mycobacterium tuberculosis in Health-Care Settings, 2005"; December 30, 2005 / 54(RR17);1-141, US Centers for Disease Control and Prevention.

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

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