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.
