Slide 005 Epidemiologic Basis of Tuberculosis Control
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Rather trivially, it is obvious that all people share the same airspace and as the transmission of M tuberculosis is by air, everybody is potentially exposed. To make a more tangible definition of exposure risk, a pragmatic definition seems appropriate. We will define, though a bit vaguely, "relevant" exposure as an exposure that entails a reasonably measurable risk of becoming infected with M tuberculosis given exposure.

Outdoors, the volume of air is infinite for any practical purpose, thus the probability of inhaling a tubercle bacillus is infinitesimally small. Only if an individual is rather closely exposed to the direct stream of air from a person shedding tubercle bacilli will the be any measurable risk of actually becoming infected. Pragmatically, we thus may define that outdoors relevant exposure requires that the exposed person is in talking distance with the exposing person.

Indoors, in contrast, the volume of air is more finite, particularly if ventilation is poor and bacilli thus may remain indoors for a prolonged period of time, and proximity between the exposing and the exposed becomes of much less importance. Indeed, a source of infection may leave a room and a person entering the same room subsequently may inhale tubercle bacilli that have remained airborne in that room.

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Last update: September 27, 2010