Abstract
Among the 22 confirmed or suspected cases of anthrax during the bioterrorism incidents in 2001, all but two seemed traceable to spores from threat letters. Although no anthrax spores were found in the environments frequented by two females who died of the disease, a suspicion persists that the deaths were somehow related to the mail. This study assesses the spread and persistence of a mock biological agent from a source-letter to other mail and to a receiving mailbox. Successive placement and removal of letters were found to reduce the number of residual bio-agent particles in a mailbox. This suggests that a sweeping action is taking place that can affect the quantity of bio-agent remaining. The exercise supports the possibility that the two females were exposed to spores on mail that had been in their mailboxes although no trace of spores could later be found in their boxes.