Slide 010 Epidemiologic Basis of Tuberculosis Control
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Droplet size and satuartion of air, thus the extent that evaporation can take palce, will determine whether a droplet will ever reach the ground from 2 meters or whether evaporation to droplet nuclei is fast enough that they (if containing tubercle bacilli) will never reach the ground but remain suspended in the air.

The figure shows that below the size of about 100 micrometers, droplets will evaporate before they can reach the ground, and, if containing particles like tubercle bacilli, will remain suspended in the air as infectious droplet nuclei.  Tubercle bacilli are much smaller and measure only 1 to 3 micrometers in diameter.  Thus, even if several are clumped together, their size will be sufficiently small to remain airborne.

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Last update: September 10, 2010