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The FBI claims that mail-sorting equipment crushed the killer anthrax in the letters to Senators Daschle and Leahy down to a fine powder.Is that possible?
Well, the anthrax spores in the Daschle letter were 1.5 to 3 microns, according to the Washington Post (and see this).
There are 25,400 microns in an inch.
Mail-sorting equipment is generally built to handle letters at least 1/4 inch thick. Correspondingly, U.S. Postal Service guidelines allow letters to be up to 1/4 of an inch thick.
The following U.S. Postal Service chart shows the standard size and thickness of letters that can be handled by mail-sorting machines:
| Dimension | Minimum | Maximum |
|---|---|---|
| Height | 3-1/2 inches | 6-1/8 inches |
| Length | 5 inches | 11-1/2 inches |
| Thickness | 0.007 inch | 1/4 inch |
Here is one of the U.S. Postal Service's mail sorting machines which actually processed an anthrax letter in 2001 (although not the one which processed the Leahy and Daschle letters): 
What does this all mean?
1/4 of an inch equals 6,350 microns. So the FBI is trying to say that a mail-sorting machine which is designed to process letters 6,350 microns thick crushed something down to 3 microns . . . 2,116 to 4,232 times smaller than the type of envelope sorting machines are designed to handle (the smaller number is compared to 3 micron thick anthrax powder and the larger is compared to 1.5 micron powder) .
I don't know about you, but my mail isn't crushed into oblivion when I get it.
On the other hand, the LA Times hints at a more likely explanation:
"Since the early 1990s, U.S. Army scientists at Dugway Proving Ground in Utah have made small quantities of weapons-grade anthrax that is virtually identical to the powdery spores used in the bioterrorist attacks that have killed five people, government sources say."
"Dugway's production of weapons-grade anthrax, which has never before been publicly revealed, is apparently the first by the U.S. government since President Nixon ordered the U.S. offensive biowarfare program closed in 1969. Scientists familiar with the anthrax program at Dugway described it to the Baltimore Sun on the condition that they not be named."
"Dugway's weapons-grade anthrax has been milled to achieve a concentration similar to that sent in a letter to Senate Majority Leader Tom Daschle, according to a source. The strain found in those letters is indistinguishable from that used most often by Dugway."
