Was only the other day that my mate pointed out to me that 451 is DEA if you transcribe it. And after all, there are all kinds of paper and they don't all catch fire at one given temperature, do they?
If I recall correctly, Mr. Bradbury called a firehouse and asked the guy there "what temperature does paper ignite at?" and they thought about it for a moment and looked up some value in a reference manual. So don't go reading too much into the number. Also, note that the (US) DEA was founded in 1973, while _451_ was published in 1953.
Also, casual Internet browsing suggests values closer to 450 celsius for the autoignition point of typical paper, so don't go relying too much on any one number you get from the title of a book (or from a hacker news post, for that matter)
