Research Brief

Measuring and Improving the Performance of Airborne Infection Isolation Rooms in Health Care Facilities

Properly-functioning airborne infection isolation rooms (AIIRs) help to prevent the spread of infectious diseases in health care facilities.  By establishing ventilation that maintains the air in these rooms at a pressure lower than connected spaces, infectious particles generated by patients who are coughing and sneezing are not allowed to reach other patients, visitors, or staff members not involved in treating the patient.  Protective environments like those in bone marrow transplant units are intended to prevent susceptible patients from being exposed to infectious airborne particles generated by other patients, visitors, and staff members.  In contrast to AIIRs, protective environments are created by ventilating a room to establish an air pressure higher than in connected spaces.

Performance of Existing AIIRs

The need for effective AIIRs in hospitals has taken on greater urgency with increased awareness of the threats of bioterrorism and pandemic influenza.  With funding from the U.S. Department of Health and Human Services' Health Resources and Services Administration supplied through the Minnesota Department of Health, the performance of 678 AIIRs in 140 hospitals was evaluated and compared with construction design guidelines from the American Institute of Architects (AIA) and the Centers for Disease Control and Prevention (CDC).  Room performance versus some of these guidelines was self-reported by hospitals using a Web-based survey; operation relative to other parameters was evaluated during site visits to each facility. 

Measurements of the pressure in AIIRs indicated that only 32 percent achieved the recommended pressure of -2.5 Pascals (Pa) relative to the pressure in surrounding areas.  AIIRs with solid ceilings had an average pressure differential of -4.4 Pa, which was significantly higher than the average differential of -2.0 Pa for rooms with false ceilings.  This finding indicates that designing rooms with solid ceilings is a better approach than using false ceilings. 

Other problematic findings were that only 36 percent of AIIRs had self-closing doors and that only about half of the rooms were ventilated at the recommended rate of 12 air changes per hour.  More positive observations included that more than three-quarters of the rooms had permanently installed pressure monitors and that 90 percent of the rooms could not be converted from a negative pressure AIIR to a positive pressure protective environment. 

Filters are installed in air handling units to remove particles from the air supplied to building spaces.  The efficiency of these filters is reported by manufacturers and these values are relied upon by facility managers when making purchasing decisions.  Measurements of the filter efficiency for air supplied to the AIIRs in this survey indicated that the actual efficiency was less than the rated efficiency for 107 of 112 filter banks tested.  This finding suggests that filters in air handling units at hospitals may not be protecting patients, visitors, and staff from airborne infectious agents as well as anticipated.

Overall, the results indicate that hospitals are not all maintaining AIIRs to correspond with current guidelines. The findings also support the contention that having tightly sealed rooms helps maintain appropriate pressure differentials.  This research, which was led by University of Minnesota MPH student Stefan Saravia, was published in June 2007 in the American Journal of Infection Control.  The results are being used by the Minnesota Department of Health to improve the performance of AIIRs in the state. 

Developing Guidelines for Performance
of AIIRs and Protective Environments

Ongoing research led by PhD student Rolando Gonzalez and funded in part by the Association for Professionals in Infection Control and Epidemiology (APIC) is investigating the pressure differential needed to keep infectious agents inside an AIIR when the door to the room is repeatedly opened and closed.  To perform this study, office and lab spaces were modified to operate like isolation rooms.  Carbon dioxide was released into these rooms as a surrogate for airborne infectious agents and the fraction of carbon dioxide leaking from the room was measured and evaluated statistically as a function of pressure differential, ventilation flow, frequency of door opening, and the presence of an anteroom between the isolation room and the outer hallway.  The results of this research indicate that pressure differences larger than current recommendations may be required to keep agents inside the room when the door is opened and closed.  These results will be valuable as guidelines published by the AIA and CDC are reconsidered.

New research includes developing methods for commissioning newly built or remodeled AIIRs and protective environments and establishing locations for ventilation supply and return ducts in isolation rooms and protective environments to minimize the transfer of airborne infectious agents between a patient and the health care personnel who enter the room.

Research Brief is published by the University of Minnesota School of Public Health, 420 Delaware Street, S.E., Minneapolis, MN 55455. www.sph.umn.edu.