Isolation Room Design Guidelines From CDC and ASHRAE
Consulting engineers are being called upon by healthcare clients to design isolation rooms that meet the urgent need of protecting their employees and patients from highly pernicious infectious diseases. ASHRAE and the CDC (Center for Disease Control and Prevention) in Atlanta provide guides for the design of new and the renovation of existing isolation rooms. This issue of the Specifier’s Guide extracts key criteria from CDC and ASHRAE publications to provide HVAC designers with an overview. The references listed at the conclusion of this discussion should be consulted, along with applicable codes, when designing isolation rooms.
The following is from "Guidelines for Preventing the Transmission of Mycobacterium Tuberculosis in Healthcare Facilities", 1994, published by the Center for Disease Control and Prevention.
The Reason Behind the Need
Transmission of Mycobacterium Tuberculosis is a recognized risk to patients and healthcare workers in healthcare facilities. Transmission is most likely to occur from patients who have unrecognized pulmonary or laryngeal TB, are not on effective anti-TB therapy, and have not been placed in TB isolation. Several recent TB outbreaks in healthcare facilities, including outbreaks of multidrug-resistant TB, have heightened concern about nosocomial transmission. Patients who have multidrug-resistant TB can remain infectious for prolonged periods, which increases the risk for nosocomial and/or occupational transmission of Mycobacterium Tuberculosis. Increases in the incidence of TB have been observed in some geographical areas; these increases are related partially to the high risk for TB among immunosuppressed persons, particularly those infected with human immunodeficiency virus (HIV). Transmission of Mycobacterium Tuberculosis to HIV-infected persons is of particular concern because these persons are at high risk for developing active TB if they become infected with the bacteria. Thus, healthcare facilities should be particularly alert to the need for preventing transmission of Mycobacterium Tuberculosis in settings in which HIV-infected persons work or receive care. (4) CDC, page 1.
Recommendations for designing and operating ventilation systems have been published by ASHRAE (1), AIA (2), American Conference of Governmental Industrial Hygienists, Inc. (3), and the CDC (4). Ventilation systems for healthcare facilities should be designed and modified when necessary by ventilation engineers in collaboration with infection-control and occupational health staff. (4) CDC, page 69.
Mycobacterium Tuberculosis is carried in airborne particles, or droplet nuclei, that can be generated when persons who have pulmonary or laryngeal TB sneeze, cough, speak, or sing. The particles are an estimated 1-5 mm in size, and normal air currents can keep them airborne for prolonged time periods and spread them throughout a room or building. Infection occurs when a susceptible person inhales droplet nuclei containing Mycobacterium Tuberculosis, and these droplet nuclei transverse the mouth or nasal passages, upper respiratory tract, and bronchi to reach the alveoli of the lungs. (4) CDC, page 4.
Environmental factors that enhance the likelihood of transmission include: a) exposure in relatively small, enclosed spaces; b) inadequate local or general ventilation that results in insufficient dilution and/or removal of infectious droplet nuclei; and c) recirculation of air containing infectious droplet nuclei. (4) CDC, page 5.
Infection Control Measures
The TB infection control program should be based on a hierarchy of control measures. The first level of the hierarchy, and that which effects the largest number of persons, is using administrative measures intended primarily to reduce the risk for exposing uninfected persons to persons who have infectious TB. The second level of the hierarchy is the use of engineering controls to prevent the spread and reduce the concentration of infectious droplet nuclei. These controls include: a) direct source control using local exhaust ventilation; b) controlling direction of airflow to prevent contamination of air in areas adjacent to the infectious source; c) diluting and removing contaminated air via general ventilation; and d) air cleaning via air filtration or ultraviolet germicidal irradiation (UVGI). (4) CDC, page 6-7.
Research has demonstrated that UVGI is effective in killing or inactivating tubercle bacilli under experimental conditions and in reducing transmission of other infections in hospitals, military housing and classrooms. Because of the results of numerous studies and the experience of TB clinicians and mycobacteriologists during the past several decades, the use of UVGI has been recommended as a supplement to other TB infection-control measures in settings where the need for killing or inactivating tubercle bacilli is important. (4) CDC, page 88-89.
Air from TB isolation rooms and treatment rooms used to treat patients who have knows or suspected infectious TB should be exhausted to the outside in accordance with applicable federal, state and local regulations. The air should not be recirculated into the general ventilation. In some instances, recirculation of air into the general ventilation system from such rooms is unavoidable (i.e. in such existing facilities in which the ventilation system or facility configuration makes venting the exhaust to the outside impossible). In such cases, HEPA filters should be installed in the exhaust duct leading from the room to the general ventilation system to remove infectious organisms and particulates the size of droplet nuclei from the air before it is returned to the general ventilation systems. Air from TB isolation and treatment rooms in new or renovated facilities should not be recirculated into the general ventilation system. (4) CDC, page 29-30.
To prevent the escape of droplet nuclei, the TB isolation room should be maintained under negative pressure. Doors to isolation rooms should be kept closed, except when patients or personnel must enter or exit the room, so that negative pressure can be maintained. (4) CDC, page 29.
HEPA filters may be used in a number of ways to reduce or eliminate infectious droplet nuclei from room air or exhaust (supplement 3). These methods include placement of HEPA filters: a) in exhaust ducts discharging air from booths or enclosures into the surrounding room; b) in ducts or in ceiling or wall-mounted units, for recirculation of air within an individual room (fixed recirculation systems); c) in portable air cleaners; d) in exhaust ducts to remove droplet nuclei from air being discharged to the outside, either directly or through ventilation equipment; and e) in ducts discharging air from the TB isolation room into the general ventilation system. In any application, HEPA filters should be installed carefully and maintained meticulously to ensure adequate functioning. (4) CDC, page 32.
The following is from the 1995 ASHRAE Applications Handbook; Chapter 7, Healthcare Facilities.
Filtration, Ultraviolet Light, Chemical Sprays?
HEPA and/or ultra low penetration (ULPA) filters provide the greatest efficiency currently available. Attempts to deactivate viruses with ultraviolet light and chemical sprays have not proven reliable or effective enough to be recommended by most codes as a primary infection control measure. Therefore, isolation rooms and isolation anterooms with appropriate ventilation-pressure relationships are the primary means used to prevent the spread of airborne viruses in the hospital environment . (1) 1995 ASHRAE Applications Handbook, page 7.2.
The following are guidelines for filter installations from the 1995 ASHRAE Applications Handbook; page 7.2.
Filtration
- HEPA filters having DOP test efficiencies of 99.97% should be used on air supply systems serving rooms for clinical treatment of patients with a high susceptibility to infection due to leukemia, burns, bone marrow transplant, organ transplant, or acquired immunodeficiency syndrome (AIDS). HEPA filters should also be used on the discharge air from fume hoods or safety cabinets in which infectious or highly radioactive materials are processed. The filter system should be designed and equipped to permit safe removal, disposal, and replacement of contaminated filters.
- All filters should be installed to prevent leakage between the filter segments and between the filter bed and its supporting frame. A small leak that permits any contaminated air to escape through the filter can destroy the usefulness of the best air cleaner.
Air Management and Distribution
In general, outlets supplying air to sensitive ultraclean areas and highly contaminated areas should be located on the ceiling, with perimeter or several exhaust inlets near the floor. This arrangement provides a downward movement of clean air through the breathing and working zones to the contaminated floor area for exhaust. The bottoms of returns or exhaust openings should be at least 3 in. above the floor. (1) 1995 ASHRAE Applications Handbook, page 7.3.
Because of the cleaning difficulty and potential for buildup of contamination, recirculating room units must not be used in infectious isolation rooms. (1) 1995 ASHRAE Applications Handbook, page 7.5, table 3, note g.
To provide optimal airflow patterns, the air supply and exhaust should be located such that clean air first flows to parts of the room where HCWs are likely to work, and then flows across the infectious source and into the exhaust. (4) CDC, page 74.
The designers should work closely with healthcare planners and the code authority to determine the appropriate isolation room design. It may be desirable to provide more complete control, with a separate anteroom used as an airlock to minimize the potential that airborne particulates from the patients’ area reach adjacent areas. (1) 1995 ASHRAE Applications Handbook, page 7.7.
Room Use Flexibility
Some designers have provided isolation rooms that allow maximum space flexibility by using an approach that reverses the airflow direction by varying exhaust flow rate. This approach is useful only if appropriate adjustments can be ensured for different types of isolation procedures. (1) 1995 ASHRAE Applications Handbook, page 7.7.
Guidelines for the design of the infectious isolation room are plentiful. Less available are design guidelines for the protective isolation room. The HVAC design engineer can refer to ASHRAE (1) and AIA (2) documents for reference concerning Protective Isolation Rooms.
Summary
Approaches to the design of isolation rooms differ among recognized authorities. Local, state and federal agency codes vary. Consensus among the recognized authorities is not always possible, leaving the designer to evaluate project needs and available information.
This issue of the Enviro-Tec Specifier’s Guide should not be used in place of consultation with experts, who can assume responsibility for advising on ventilation system design and selection, installation, and maintenance of equipment.
References- American Society of Heating, Refrigeration and Air Conditioning Engineers. Chapter 7: Healthcare Facilities in the 1995 Applications Handbook. Atlanta: American Society of Heating, Refrigeration and Air Conditioning Engineers, Inc.
- American Institute of Architects, Committee on Architecture for Health. Chapter 7: General Hospital. In: Guidelines for construction and equipment of hospital and medical facilities. Washington, DC: The American Institute of Architects Press, 1987.
- American Conference of Governmental Industrial Hygienists. Industrial Ventilation: a manual of recommended practice. Cincinnati: American Conference of Governmental Industrial Hygienists, Inc., 1992.
- Centers for Disease Control and Prevention. Guidelines for preventing the transmission of Mycobacterium Tuberculosis in healthcare facilities, 1994. MMWR 1994; 43 (No. RR-13: [1-132]).
- Immunosuppressed: A condition in which the immune system is not functioning normally (i.e. severe cellular immunosuppression resulting from HIV infection or immunosuppressive therapy). Immunosuppressed persons are at greatly increased risk for developing active TB after they have been infected with M. Tuberculosis. No data is available regarding whether these persons are also at increased risk for infection with M. Tuberculosis after they have been exposed to the organism.
- Nosocomial: An occurrence, usually an infection, that is acquired in a hospital, or as a result of medical care.