[Table of Contents]

Volume: 22S1 • April 1996

Guidelines for Preventing the Transmission of Tuberculosis in Canadian Health Care Facilities and Other Institutional Settings

IV. TB MANAGEMENT PROGRAM

E. Engineering Controls to Minimize TB Transmission

Engineering control measures comprise one of the components of the TB management program and are used in conjunction with other measures to minimize the transmission of M. tuberculosis within health care facilities. This section provides information about ventilation, high-efficiency particulate air (HEPA) filtration and ultraviolet germicidal irradiation (UVGI). It also discusses the cleaning of rooms and equipment.

The concentration of infectious particles suspended in the air can be reduced through a variety of engineering control measures. The most common method is to remove or dilute the concentration of infectious particles by adding uncontaminated air to the room and forcing contaminated air out of the room. Infectious particles may also be reduced by being trapped in a filter system or killed by exposure to ultraviolet radiation.

Canadian and American studies have revealed that many health care facilities do not have isolation rooms that meet national standards. The Canadian TB Hospital Readiness Study reported that 17.8% of health care facilities had at least one isolation room with a minimum of six air changes per hour, air exhausted outside the building, and negative pressure to the corridor⁽²⁷⁾. A 1992 U.S. survey of 729 health care facilities showed that 26% of isolation rooms and 89% of emergency rooms did not meet the minimal recommendations, (e.g., six air changes per hour, negative air pressure, and air exhausted to the outside of the building)⁽⁷⁾.

The Special Requirements for Heating, Ventilation, and Air Conditioning (HVAC) Systems in Health Care Facilities: A National Standard of Canada (the Canadian Standards Association - CSA)⁽⁵⁹⁾ outlines ventilation requirements for various rooms or areas, including patient rooms, operating rooms, intensive care units, emergency and other treatment rooms, and isolation rooms. The CSA document recommends that isolation rooms should have nine air changes per hour, ventilation to outside the building, and appropriate relative pressurization depending on the isolation technique⁽⁵⁹⁾. The requirements for isolation rooms are not specific to TB since isolation precautions are required for a variety of other infectious diseases (e.g., varicella zoster, rubeola). Patients with TB should be isolated in a room where the air pressure is negative to the corridor, resulting in inward directional airflow.

Health care facilities should determine the number of isolation rooms they require. If several isolation rooms are necessary, consideration should be given to locating these rooms in one area of the facility. Health care facilities should also liaise with regional and public health authorities to determine the number of isolation rooms required for the region (see Section IV.A). Table 4 outlines minimal requirements for isolation rooms in health care facilities.

Poor engineering control measures play some role in the transmission of TB. Individuals with active TB have been the source of five reported outbreaks in health care facilities with inadequate engineering controls^(4,7,60-64). However, inadequate engineering controls were not the only identified transmission risk factor. Each facility also reported long delays before the individuals were diagnosed as having active TB and provided treatment. Transmission of TB was reduced in all five centres after increased awareness of the risk of transmission resulted in earlier diagnosis of TB and after changes were made in the ventilation system. In two facilities, the greatest reduction in transmission was seen after institution of isolation precautions and therapy for those with suspected TB but before the improved ventilation measures were implemented^(60,63). In another study, the rate of reduction in the transmission of TB attributed to the earlier diagnosis of individuals with active TB was much greater than reduction attributed to improvements in the ventilation system⁽⁶⁵⁾.

There are no studies that estimate the cost benefit of implementing current Canadian or U.S. engineering control standards. It is difficult to estimate the cost of converting a room into an isolation room since each room and health care facility differin design and construction. Implementation of the national standards will have costs beyond re-fitting the room(s) since maintaining directional air flow and a higher rate of air changes per hour may substantially increase energy costs⁽⁶⁶⁾.

Construction of new isolation rooms should comply with the principles outlined in the Canadian ventilation standards⁽⁵⁹⁾. Isolation rooms currently in existence should be evaluated and monitored to ensure that they meet, at a minimum, the following recommendations: six air changes per hour, inward directional air flow and air exhausted outside the building. Control mechanisms that enable the alteration of room pressure are available on the market. Switch selectable pressurization controls permit the selection of negative (infectious), positive (protective) or neutral pressurization depending on the requirements of the patient in the room. This type of switch may decrease the cost of operating the room for non-infectious patients. Concerns have been expressed about the effectiveness and reliability of these switches. If such a mechanism is chosen, continual monitoring to ensure proper function and clear policies with respect to who has the authority to change settings must be in place.

Other engineering control measures (filtration and ultraviolet lighting) or a combination of ventilation, filtration and ultraviolet lighting may produce the same reduction in infectious particles as those outlined in the Canadian ventilation standards⁽⁵⁹⁾. However, experience with alternative engineering control combinations is currently limited. Concerns have been raised about the effectiveness, reliability and ease of monitoring of these alternative environmental control measures. Research in this area is needed.

Existing engineering control systems should receive constant monitoring and maintenance⁽⁶⁶⁾. Even when appropriately designed and constructed, ventilation systems may deteriorate and fail to function as intended within 5 years⁽⁶⁷⁾. A survey of isolation rooms in six American health care facilities found that the ventilation and air flow patterns in the majority did not meet recommended standards, although originally designed to do so⁽⁶⁷⁾.

Two power sources (regular and emergency) should be connected to the engineering control system of isolation rooms or areas, if possible. This will provide for emergency power in the event of an electrical failure.

Ventilation

Recent ventilation recommendations^(36,68) are based on sound theory⁽⁶⁹⁾, although supportive evidence is limited. The only epidemiologic evidence that special ventilation practices may decrease nosocomial transmission of TB is from outbreak reports where recirculation^(50,70) or inadequate ventilation^{(2,3,46,47,49,71-73)} were factors that contributed to the outbreaks. Increased ventilation is effective under different conditions of exposure^{(49,69,74,75,76)}. However, HCWs may still experience exposure unless other methods of protection are used (e.g., masks).

Ventilation has three major components: rate of air change, direction of air flow, and location of air exhaust.

Rate of air change

There is no consensus about the recommended rate of air changes in isolation rooms^(36,59,68). A minimum rate of nine air changes per hour has been suggested to provide adequate ventilation in isolation rooms⁽⁵⁹⁾. It takes 46 minutes to reduce contaminant concentrations by 99.9% in a room with nine changes per hour. Increasing ventilation beyond nine changes per hour will decrease the time required to achieve a 99.9% reduction of contaminant concentration (see Appendix F). However, at high rates of air change, further increases in the number of air changes fail to provide a meaningful reduction in infectious particles. In addition, the cost of operating the ventilation system at higher ventilation rates progressively increases⁽⁷⁴⁾.

Until further information becomes available, it is recommended that newly constructed isolation rooms or areas have a minimum of nine air changes per hour and that those in existing facilities have at least six air changes per hour.

Ventilation rates should be monitored. Monitoring devices should be placed downstream of HEPA filters. Monitors should be carefully checked to ensure that they are working correctly because some types of monitors sample air though a small opening, which may easily become obstructed by lint particles.

Direction of air flow

Airborne infectious particles must not circulate from TB isolation rooms or areas into other areas of the health care facility. Thus, TB isolation rooms or areas should have air that flows inward to the patient's room and away from the corridor. Air flow within TB isolation rooms or areas should be from the area of least contamination (the doorway) to the area of greatest contamination (the patient) to provide the most protection to HCWs and visitors.

The easiest way to create this inward directional airflow is by ensuring that the volume of air exhausted from the isolation room or area is at least 10% greater than the volume of ducted air supplied (e.g., volumetric offset air flow control) provided the minimum rate of air flow is 50 cubic feet per minute (cfm). Anterooms may assist in maintaining inward directional air flow within the isolation rooms. The location of the bed, chairs or occupants influences the air flow pattern and should be taken into account when determining the location of the supply and exhaust grills. Supply and exhaust grills should be located in a manner that ensures that all parts of the room are adequately ventilated.

The inward directional air flow must be verified regularly. Two methods of monitoring have been suggested. Electronic monitoring can provide continuous or intermittent information about the efficacy of the inward directional air flow system and rate of air change. An alarm will sound in the continuous monitoring system whenever the air flow in the room is not inwardly directed or adequately ventilated. Alternatively, directional air flow may be monitored intermittently with smoke tube (pencil) tests. It is not known what the optimal monitoring frequency should be. A reasonable frequency pending further information would be to monitor at least every 6 months when the isolation area is not in use and weekly when in use.

Air exhaust (outside the building or recycled)

Ideally, the air from the isolation room or area should be safely exhausted outside the building⁽⁵⁹⁾. Air potentially contaminated with M. tuberculosis may be discharged to the environment if it is discharged away from public places in accordance with municipal by-laws. Exhaust air should be discharged vertically outside of the recirculation range of the building. Care should be exercised to minimize the re-entrainment of exhaust air into the fresh air intake of the health care facility or adjoining buildings. Stack height should be a minimum of 10 feet above the adjoining roof line and the air exit velocity should be a minimum of 1.5 times the wind velocity (usually 2,500-3,000 feet per minute)⁽⁷⁷⁾.

If it is not possible to exhaust the air to the outside, it must be filtered before being re-circulated. Air can be filtered to remove 99.9% of infectious particles 0.3 microns in size. This may involve significant maintenance costs (see HEPA filtration below).

High Efficiency Particulate Air (HEPA) Filtration

HEPA filters remove particulates and droplet nuclei from room exhaust air. Proper installation and testing, as well as maintenance, is critical to ensure proper functioning of HEPA filters. The efficacy of these filters may be affected by improper installation or maintenance, and, when used for the recirculation of air, by installation without appropriate prefilters. Maintenance and monitoring must be performed by adequately trained personnel using aerosol challenge techniques (Dioctyl phthalate; DOP test) at least annually. Filter housings must have provision for such testing, as well as isolation dampers for gaseous decontamination. If filters are not decontaminated prior to removal, the maintenance personnel should use personal respiratory protection (e.g., the masks used by other HCWs in the facility to prevent TB transmission) (see Section IV.F). The filters should be handled and disposed of as contaminated waste.

Portable HEPA filter devices are becoming available. To date, evaluation of their effectiveness is limited.

Ultraviolet Germicidal Irradiation (UVGI)

Microorganisms are inactivated by UV wavelengths within a range of 250 to 280 nanometres (nm). The maximum wavelength for microbicidal activity is 260 nm and modern mercury lamps emit about 95% of their radiation near that level (254.7 nm). Inactivation of microorganisms, including M. tuberculosis, is due to destruction of nucleic acid via induction of thymine dimers. UV radiation has several potential applications; however, its germicidal effectiveness and use are influenced by the following factors: type of microorganism and UV intensity, wavelength of light emitted, organic matter, presence of dirty tubes, type of suspension, temperature, and by distance from the UV lamp. Ultraviolet lamps have been used as adjuncts to other engineering control measures in areas such as air ducts and waiting rooms.

UV light has been used successfully to reduce nosocomial transmission in some high risk settings⁽⁷⁶⁾. Properly installed, UV light will have a germicidal efficacy equivalent to 20 air changes per hour⁽⁶⁹⁾. UV light has the advantage of being relatively inexpensive (lamps cost less than $500 and replacement tubes less than $100⁽⁷⁶⁾). However, regular monitoring and maintenance are required.

UV light may theoretically cause cataracts and skin cancer. However, light of 254 nm wavelength produced by currently manufactured lamps has greatly reduced the risk because it cannot penetrate the eye, and less than 5% will penetrate the skin. The risk of eye and skin irritation can be minimized by appropriately positioned and baffled lamps. Despite this, problems have occurred when UV lights have not been installed properly or have not been monitored and maintained correctly. One report documented an epidemic of ultraviolet-induced skin erythema and keratoconjunctivitis among patients and visitors in a health care facility with incorrectly installed ultraviolet lights⁽⁶⁶⁾.

No definitive recommendations can be made for, or against, the use of UVGI in the TB management program at this time although the American Thoracic Society and the American College of Chest Physicians are reviewing this issue and will likely be publishing new recommendations with respect to use of UVGI. Current data do not support the use of UVGI as the sole source of engineering controls. Although the exact role of UVGI is unclear, it may be considered a useful adjunct in ventilation ducts or in high-risk areas, such as bronchoscopy suites, autopsy suites, or other areas where patients with undiagnosed TB may be seen frequently.

Cleaning of Rooms and Equipment

TB is spread almost exclusively by airborne transmission and only rarely are equipment, environment, or fomites involved in transmission. Recommendations for decontamination, disinfection and sterilization are listed below.

• Equipment (critical or semi-critical) which may be contaminated with secretions containing M. tuberculosis should receive meticulous physical cleaning prior to a high-level disinfection or sterilization process, which is determined by the intended use and the composition of the item. Guidelines for cleaning, disinfecting and sterilizing critical and semi-critical equipment have been published⁽⁷⁸⁾.
• The environment, such as walls, floors and other surfaces, may be contaminated with M. tuberculosis but has not been associated with transmission of infection. Usual housekeeping procedures are adequate and require no special germicidal agents or extraordinary cleaning procedures. Guidelines for housekeeping have been published⁽⁷⁸⁾.
• Fomites, such as laundry, dishes, clothes, books and personal effects, require no specific precautions apart from general good housekeeping and hygiene.

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