NIOSH Recommended Guidelines for Personal Respiratory Protection of Workers in Health-Care Facilities Potentially Exposed to Tuberculosis/II. Mode of Airborne Transmission and Potential for Worker Exposure
II. Mode of Airborne Transmission and Potential for Worker Exposure
A. Airborne Transmission of Tubercle Bacilli—When a person with infectious pulmonary tuberculosis coughs, sneezes, or speaks, particles that can carry viable tubercle bacilli (i.e., infectious particles) can be expelled and then become aerosolized as droplets (15,16). Tuberculosis bacilli are rod-shaped and vary in width from 0.2 to 0.6 µm, and from 0.5 to 4.0 µm in length (17,18). Of the aerosolized particles containing tubercle bacilli that are routinely expelled by a patient with infectious tuberculosis, or produced by clinical or laboratory procedures, the largest particles (e.g., exceeding 100 μm) settle onto surfaces and the tuberculosis bacilli, if present, cannot be inhaled (19). However, droplets less than about 100 μm evaporate rapidly to form stable droplet nuclei in the 1- to 4-µm size range (19). This conversion of droplets to droplet nuclei and the relevant size range of the nuclei required for access to the deep pulmonary spaces have explained in detail by Riley and O'Grady (19). One study indicated that 30% of the droplet nuclei resulting from coughs were less than 3 µm (20).
Droplet nuclei can remain airborne for prolonged periods of time (hours, at least) (3), increasing the likelihood that they will be inhaled by another person. Anyone who breathes air that contains these droplet nuclei can become infected with TB (3). After inhalation, droplet nuclei are small enough to reach the alveoli deep in the lung, where tuberculous infection is initiated (17,18).
Harris and McClement, in the textbook Infectious Diseases, summarized the many complex issues that determine risk of tuberculosis infection as follows (21):
The risk of airborne transmission is influenced by many factors, such as the rate and the concentration of expelled organisms, the physical state of the airborne discharge, and the volume and the rate of exchange of the air in the physical space into which the bacilli are ejected. However, the most important risk factor is the length of time an individual shares a volume of air with an infectious case of tuberculosis. Thus, intimate, prolonged, or frequent contact, as in the home or work place, provides the greatest risk of transmission.
Thus risk of infection of a susceptible health-care-facility worker is a function of several factors including:
- The concentration of droplet nuclei in the workplace air (10,15,19,22,23,24,25,26,27,28,29,30). There appears to be no exposure threshold for tubercle bacilli in droplet nuclei required to produce infection in a susceptible individual (22,30). Thus, any concentration of aerosolized droplet nuclei containing tubercle bacilli is assumed to present some risk of infection.
- The cumulative time that air containing droplet nuclei is breathed (25,22,24,28,29,31).
- The worker’s pulmonary ventilation rate (28,29).
Of these factors, the first two—concentration and cumulative time—are by far the most important and amenable to intervention.
Persons who share the same air with an infectious person for long periods of time are at greatest risk of becoming infected (32). This includes persons living in the same household with the infectious person and those who travel in the same vehicle (32). Because tuberculosis is transmitted by the airborne route, persons who sleep, live, work, or who are otherwise in contact or share air with an infectious person through a common ventilation system for a prolonged time are "close contacts" at risk of acquiring tuberculosis infection (33,34). Recently, CDC noted that (35),
Any person who shared the air space with an MDR-TB patient for a relatively prolonged time (¢.g., household member, hospital roommate) is at higher risk for infection than those with a brief exposure to an MDR-TB patient, such as a one-time hospital visitor. Exposure of any length in a smail, enclosed, poorly ventilated area is more likely to result in transmission than exposure in a large, well-ventilated space. Exposure during cough-inducing procedures (¢.g., bronchoscopy, endotracheal intubation, sputum induction, administration of aerosol therapy), which may greatly enbance TB transmission, is also more likely to result in infection.
However, the terms "long periods of time" and "prolonged time" sharing the air exhaled by an infectious person are subjective . There is one report of a 150- minute intubation and bronchoscopy where 10 of 13 susceptible occupants of an intensive care unit became infected (36). In another case, 27 new infections resulted among 67 susceptible office workers who were exposed for 160 hours to the air exhaled by an infectious office worker in the same building (29). Additionally, Bloom and Murray noted that (30):
...epidemiological findings support the likelihood that the majority of patients infected with TB have acquired infection from nonintimate contacts.
B. Health-Care-Facility Workers' Potential for Exposure to Tuberculosis—Clinical procedures that can result in high concentrations of aerosolized droplet nuclei include bronchoscopy, administering aerosolized drug treatments, autopsy, and physical therapies to the chest that induce coughing (10). Rapid transmission (e.g., several hours) to health-care-facility workers has been linked to proximity with patients with infectious tuberculosis during use of aerosolized pentamidine (37), intubation and suctioning with mechanical ventilation (38), prolonged intubation (39), bronchoscopy followed by emergency intubation (40), openabscess irrigation (41), and autopsy (42,43). Other specific clinical or laboratory procedures that produce droplet nuclei include the manipulation of lesions or processing of tissue or secretions containing tuberculosis bacilli.
U.S. Public Health Service guidelines for biosafety in microbiological and biomedical laboratories state in part (44):
Mycobacterium tuberculosis and M. bovis infections are a proven hazard to laboratory personnel as well as to others who may be exposed to infectious aerosols in the laboratory...
Biosafety Level 3 practices, containment equipment, and facilities... are recommended for activities involving the propagation and manipulation of cultures of M. tuberculosis or M. bovis and for animal studies utilizing nonhuman primates experimentally or naturally infected with M. tuberculosis or M. bovis.
Health-care-facility workers may receive exposures to droplet nuclei from confirmed or potential tuberculosis transmitters in outpatient clinics, emergency rooms, and similar locations where patients first make contact with health- care facilities and their workers. In most cases, the status of these patients as potential tuberculosis transmitters at this initial point of contact is not known. Workers in correctional facilities, homeless shelters, and other facilities where tuberculosis outbreaks may occur also come into close contact with persons with infectious tuberculosis before their transmitter status is known. When persons suspected of having, or diagnosed with, infectious tuberculosis are isolated, a limited number of health-care-facility workers are required to enter AFB isolation rooms to administer patient care, perform tests and procedures, and engage in other tasks. Persons with infectious tuberculosis may be transported from one isolation room to another through nonisolated areas of the facility. In each of these situations, health-care-facility workers may be exposed to aerosolized droplet nuclei. Hutton and Polder noted (45):
Until recently TB was probably not often transmitted in hospitals; when it was transmitted, it may have gone unrecognized because transmission did not result in rapid development of large clusters of active (and infectious) TB cases among contacts (both patients and HCFWs] . The recent outbreaks suggest that there may have been more of a problem with occult transmission of tuberculosis infection than was appreciated, especially in hospitals in high-incidence areas where there was a lack of TB surveillance among employees.