Great read in the NY Times today. May have to get the book.

http://www.nytimes.com/2010/06/09/di...tml?ref=dining

(snippet)
Onions, shallots, scallions and leeks share a special stockpiled
chemical and a second defensive enzyme. They produce a sulfur molecule
that’s small and light enough to launch itself from the damaged
tissue, fly through the air and attack our eyes and nasal passages.
This long-distance weapon is called the lachrymatory factor because it
makes people’s eyes water.

“The lachrymatory factor is extremely potent,” Dr. Block said. “Only
tiny amounts get anywhere near your face when you cut onions, but it’s
still enough to make you tear up. When I smelled the pure compound it
was overwhelmingly painful, like being punched in the eye socket