NIOSH Recommended Guidelines for Personal Respiratory Protection of Workers in Health-Care Facilities Potentially Exposed to Tuberculosis/III. Methods for Worker Protection—Controlling Airborne Transmission of Tuberculosis
III. Methods for Worker Protection—Controlling Airborne Transmission of Tuberculosis
A. Previous Recommendations for Personal Respiratory Protection—The existing CDC guidelines include extensive recommendations regarding the use of respirators in certain higher-risk areas for preventing the transmission of tuberculosis in health-care settings (10). These recommendations include in part:
[Section II.D.2.c.] For persons exposed to tuberculosis patients. Appropriate masks, when worn by health-care providers or other persons who must share air space with a patient who has infectious tuberculosis, may provide additional protection against tuberculosis transmission. Standard surgical masks may not be effective in preventing inhalation of droplet nuclei (46), because some are not designed to provide a tight face seal and to filter out particulates in the droplet nucleus size range (1-5 microns). A better alternative is the disposable PR [NIOSH-certified, particulate respirator]. PRs were originally developed for industrial use to protect workers. Although the appearance and comfort of PRS may be similar to that of cup-shaped surgical masks, they provide a better facial fit and better filtration capability. However, the efficacy of PRS in protecting susceptible persons from infection with tuberculosis has not been demonstrated.
A reexamination by NIOSH of the role of personal respiratory protection, especially the particulate respirator, in protecting health-care-facility workers against tuberculosis infection transmitted in health-care settings is presented in the next section.
B. The "Hierarchy of Controls"—Prudent occupational health practice calls for application of a hierarchy of controls to any occupational health hazard (47,48,49,50). The control hierarchy is long-standing and has wide-spread acceptance in the occupational-health community because it is based on broad practical experience, and scientific and technical logic (51).
NIOSH has supported the necessity of an ordered approach to evaluating a series or combination of effective control strategies to protect workers (47). The Institute has recommended the following essential characteristics of specific control solutions (47): - The levels of protection afforded workers must be reliable, consistent, and adequate.
- The efficacy of the protection for each individual worker must be determinable during use throughout the life span of the system.
- The solution must minimize dependence on human intervention for its efficacy so as to increase its reliability.
- The solution must consider all routes of entry into worker's bodies and should not exacerbate existing health or safety problems or create additional problems of its own.
The fundamental strength of the control hierarchy is that it minimizes the likelihood that prevention will "break down" to the extent that results in a hazardous exposure to workers. The control hierarchy for a recognized hazardous source proceeds as follows:
1. Under ordinary circumstances, the most effective and reliable control method is substitution of a less hazardous substance or source of exposure for the more hazardous one. Obviously, when the source of a hazardous exposure is a person with infectious tuberculosis, "substitution" as a potential control method is not possible.
2. The next most effective approach is to prevent or contain hazardous emissions at their source. In the health-care setting, this is best implemented through administrative controls (e.g., rapid identification, early treatment, and isolation of potential tuberculosis transmitters; limiting worker access to acid-fast bacilli (AFB) isolation rooms; other isolation precautions). Other administrative controls might include providing necessary services and procedures (e.g., portable X-ray units) in the room of a confirmed or potential tuberculosis transmitter rather than moving the infectious person to the service. Additionally, engineering controls should be used (e.g., negative-pressure ventilation for AFB isolation rooms to contain any airborne hazard to these rooms; booths, hoods, tents, or other devices for containing droplet nuclei at the source–i.e., a person with infectious pulmonary tuberculosis).
As a type of source control, it has been recommended that persons with infectious tuberculosis cover their noses or mouths when sneezing or coughing and wear surgical masks (15,52). As stated in 1990, both techniques are intended to serve as methods to control the infectious-source (10):
A simple but important source-control technique is for infectious patients to cover all coughs and sneezes with a tissue, thus containing most liquid drops and droplets before evaporation can occur [53]. A patient's use of a properly fitted surgical mask or disposable, valveless particulate respirator (PR) (see section II.D.2.c) also may reduce the spread of infectious particles. However, since the device would need to be worn constantly for the protection of others, it would be practical in only very limited circumstances (e.g., when a patient is being transported within a medical facility or between facilities).
Numerous potential limitations of these two techniques must be recognized. Neither the efficacy nor reliability of either technique has been adequately evaluated in clinical or laboratory studies.
Masking of patients is only partially effective as was noted in this caution given in 1982 (15):
Masking a coughing patient when someone enters his room may reduce the addition of bacilli to the air; this will not completely eliminate the hazard of transmission, however, since the room air would already be contaminated if the patient had been coughing without covering his mouth.
Because both techniques are heavily dependent on patient behavior, the reliability of both methods and the efficacy of mouth-covering are likely to be highly variable.
With regard to the efficacy of patient masking, a patient's expired airflow takes the path of least resistance, resulting in marginal leakage outward past a mask's face seal. Such airflow patterns deflect at least some of the contagious expired air rather than filtering all of the expired air with its droplet-nuclei load (46,54).
With regard to face-seal leakage of particulate respirators (PRs), respirator specialists, manufacturers, and OSHA consider this class of respirators to permit up to 10% (55,56) to 20% (57,58) inward face-seal leakage even after passing a fit test performed by a qualified individual. Existing standard performance tests for surgical masks have not addressed either inward or outward face-seal leakage (59). The inward face-seal leakage for these masks is assumed to be higher than 10% to 20% if the masks are not properly fitted to the wearer's face, tested for an adequate fit by a qualified individual, and then fit checked by the wearer every time these masks are donned. It is reasonable to expect at least as much for outward leakage from a masked patient.
As discussed in section IV.G starting on page 27, surgical masks and NIOSH-certified PRS cannot be reliably fit checked by their wearer before every respirator use to assure a tight face seal. Thus the amount of reduction in droplet nuclei exhaled by a masked patient is unknown. In summary, some trapping of exhaled aerosols will occur in a mask covering a patient's nose and mouth, but the extent of trapping is unknown. Correspondingly, potentially hazardous leakage will inevitably occur past a patient's mask, but the amount of leakage is also unknown.
3. Next in the hierarchy of controls are engineering controls to interrupt the pathway of hazardous emissions from the source to the worker(s) (e.g., use of negative -pressure atmospheres and other special ventilation requirements for private isolation rooms to contain the droplet nuclei in the confines of these rooms). This is the rationale for isolation precautions in hospitals (52).
Under certain circumstances, engineering controls may be neither feasible, effective, reliable, or applicable. In such cases, changes in or implementation of work practices or schedules, hazard training programs, and other administrative modifications may reduce the risk of exposure (e.g., minimizing the time a worker is in a room occupied by a potential transmitter).
4. The last, and generally least reliable control measure is to establish barriers between the worker and the hazardous work environment (e.g., personal protection equipment such as appropriate respirators used by workers in conjunction with a comprehensive personal respiratory protection program). For some infectious diseases, the barrier of immunity can be erected through vaccination of susceptible persons. Vaccination against tuberculosis using BCG vaccine has not been recommended for health-care workers or other adults at high risk for acquiring tuberculosis infection (60). The AMA has reported that even should BCG vaccine be recommended for certain health-care workers, "the latter should be aware that the vaccine may not afford significant protection against tuberculosis" (4).
NIOSH strongly supports the concept of a hierarchy of controls, which is the foundation of current practice for preventing exposures to hazards in the workplace. Substitution, administrative controls and work practices, and engineering controls because of their greater reliability should receive the highest priority. However, when the effectiveness and reliability of other control measures are not known, cannot completely control the hazard, or cannot be assured under all conditions that can be reasonably anticipated, personal respiratory protection is an essential addition to the armamentarium of control. This is why surgical masks for patient-care personnel have been traditionally indicated for infectious diseases such as Lassa fever, Marburg virus disease, smallpox, and tuberculosis in combination with special-ventilation private rooms (52). The purpose of masks for patientcare personnel have been explained as follows (52):
In general, masks are recommended to prevent transmission of infectious agents through the air. Masks protect the wearer from inhaling 1) large-particle aerosols (droplets) that are transmitted by close contact and generally travel only short distances (about 3 feet) and 2) small-particle aerosols (droplet nuclei) that remain suspended in the air and thus travel longer distances . . . If the infection is transmitted over longer distances by air, we recommend masks for all persons entering the room.
At the present time, the exposure of workers to aerosolized TB droplet nuclei cannot be completely controlled at the infectious source nor is it plausible that exposures can be completely prevented by interrupting the pathway of contagious emissions between a person with infectious tuberculosis and workers nearby in the same room. Also at present, it appears impossible to determine the quantitative efficacy and reliability of each available control method. Hence it is impossible to assure that health-care-facility workers will not be exposed to some aerosolized droplet nuclei at certain locations and during certain procedures. If an infectious person is there, the risk of infection is assumed to exist.
Therefore, for a limited range of locations and procedures, the full hierarchy of controls is necessary. For tuberculosis, these measures include the use of effective and reliable personal respiratory protection in addition to the administrative and engineering controls. Respirators can never be considered an adequate substitute for administrative and engineering controls. These NIOSH guidelines for the selection and use of respirators assume that all indicated administrative and engineering isolation precautions have been rigorously implemented as a prerequisite.