Bioaerosol Contamination of Ambient Air as the Result of Opening Envelopes Containing Microbial Materials

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Abstract
Mailing envelopes containing pathogenic spores of bacillus anthraxes, which have recently been used by terrorists to infect humans, calls for a new investigation to identify a level of possible contamination of ambient air as a result of the opening of such envelopes. Here we show that opening an envelope and unfolding a letter aerosolize microbial particles located inside and create their cloud with the diameter equivalent to the length of the letter side along which it was folded. With no motion of an envelope recipient (first case study presented in this paper), the front of the cloud moves due to forced convection caused by the impulse at opening and reaches a human face (approximately 50 cm from the opening zone) in about 6 sec. The concentration of particles at that distance is about three times lower compared to the concentration in the source. Further spread of the cloud brings its front to the distances of 1 and 1.5 meters within 25 and 55 seconds with the corresponding concentrations of around 10% and 5% compared to the source respectively. The second case study presents the results for a more realistic scenario when an envelope recipient, after observing a dust cloud appearing as the result of the opening of the envelope, recoils in fright creating additional air flows significantly disturbing the aerosol propagation described in the former study. It was found theoretically and verified by experiments that the amount of particles captured by the letter recipient varies significantly depending on the geometrical characteristics of the human, distance to the opening zone, reaction time, and recoil velocity.