The big news today, of course, is that the biotoxin ricin appears to have been found in the office of Bill Frist, the Senate's majority leader. An envelope filled with the powder was discovered in Frist's mail room. And now, three Senate office buildings have been shut down while the mail is checked there, too.

There's something odd about the discovery, however. As near as I can tell (and I may have this wrong) people don't get hurt from inhaling ricin -- at least, not a little bit of it. Eating the stuff is what's so deadly. That's why previous terror plots using ricin have involved poisoning the food or water supply.

"When you talk about anthrax versus this ricin -- the same amount of anthrax, about one kilogram -- you would need four metric tons of ricin to produce the same effect. So a little bit of it really isn't going to create much of a problem," CNN's Dr. Sanjay Gupta said.

"To the best of my knowledge, in a human being, inhaling it has never hurt anybody," Frist tells the New York Times.

THERE'S MORE: "Ricin may actually indicate a fair bit of sophistication: it suggests an attacker familiar enough with it to know it would survive the irradiation of senate email unharmed, unlike other living biological agents," writes reader TH.

Barbara Hatch Rosenberg, who heads the Federation of American Scientists' chemical and biological weapons efforts, disagrees.

"Ricin is easy to make," she writes. "ANYONE could have done it. I suspect that this is routine nut stuff."

What's more, Rosenberg adds, "if it was not weaponized to form an aerosol, it would have to be eaten to be toxic (eg, eating with contaminated hands--unlikely to deliver much of a dose). The lack of precautions being taken at Dirksen suggests they know already it is not aerosolizable."

AND MORE: Gary Ackerman, with the Center for Nonproliferation Studies (CNS), warns that ricin can be ultra-deadly in an aerosol form. "About 3 milligrams will kill the average adult," he notes.

Making ricin aerosolizable is easier than doing it to, say, anthrax -- a living organism that can be killed at it's crushed into a fine powder. But, according to a CNS report from last year, ricin "aerosolization by means of a dispersal device... would require extensive prior research, development, operational planning, and testing, and [is] thus probably beyond the means of most terrorists."

"This is not a weapon of mass casualties," Ackerman says, referring to the Frist letter. "This is intended to get media attention."

AND MORE: The White House was also sent a ricin letter -- way back in November, ABC News is reporting. Another ricin package was left in a South Carolina post office last October.