(2 of 4)
According to the conventional wisdom about anthrax, it takes 8,000 to 10,000 spores to trigger a case of inhalational anthrax. And while the letter that arrived at Senator Tom Daschle's office probably contained billions of spores, they would have to be aerosolized first--puffed into an inhalable cloud. That's easy enough to do in an envelope if there is even a small opening and enough pressure, such as that generated by a mail-sorting machine. Any gaps in the tape that sealed the Daschle letter, or even the porosity of the envelope, therefore, could explain the inhalational-anthrax cases at the post offices the Daschle letter passed through.
But that doesn't explain how a postal worker in the State Department mail-processing center got the disease or how Nguyen contracted it. Anthrax puffed from an envelope could easily settle on mail-processing machines--where spores have been found--or on other surfaces. They could also have settled on other letters, in what's known as cross-contamination. Anyone touching a cross-contaminated letter, especially someone with an open cut, would be at risk for skin anthrax--and in fact, the New Jersey woman's mailbox tested positive late last week, suggesting that this might be what happened to her.
But in order to be inhaled, cross-contaminated spores would have to be re-aerosolized, and that is hard to imagine, says William Patrick, a longtime Army biological-weapons researcher. "There's an electrostatic bond between the spore and the envelope," he says. "It takes a lot of energy to break the bond. They're just not going to be re-aerosolized in large enough quantities to provide an inhalation case." That would suggest that more than the three known letters have passed through the system. And given the tens of thousands of pieces of mail still impounded in Washington and New Jersey, some of them could still be there.
But it's also possible that the conventional wisdom is wrong. The only hard data on how many spores it takes to cause inhalational anthrax come from studies the Army did on monkeys in the 1950s. When the dose was 8,000 to 10,000 spores per animal, about half the monkeys died. But that doesn't prove that a lot fewer spores won't cause an infection. Says Philip Brachman, a professor of public health at Emory University who investigated a naturally occurring 1957 outbreak in Manchester, N.H., among millworkers who handled infected animal hides: "We don't know for certain what dosage of the organisms causes inhalation anthrax."
In fact, says Harvard's Matthew Meselson, a Nobel-prizewinning biologist who did an in-depth study of an anthrax accident at a Soviet bioweapons plant in Sverdlovsk in 1979, "there is no theoretical or experimental basis to believe in any sort of minimum threshold." A dozen or even fewer spores could be sufficient to kill, he suspects, under the right circumstances.