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.
