So don't even get me started on how "Flammable" isn't even a real word, and didn't exist until the DOT decided in the 1960's that Americans were too illiterate to understand that the word "Inflammable" meant stuff could burn, and mandated the signage on trucks to be changed to drop the "In" from the start of the word.
If you really want to make yourself crazy about the difference between inflammable, combustible, explosive, etc. (or make yourself nervous about what all those trucks are carrying), here's the DOT's hazardous materials list, which will tell you what all those numbers really mean:
http://hazmat.dot.gov/enforce/forms/ohmforms.htm#101-Sean
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