Think of it this way, the tip is traveling 200-400 mph, take a guess what a piece of plastic/wood at that speed can do! Also, it takes exactly 0.000083 seconds for a prop to find your finger and slice-n-dice it once it is the arc!

(great article, titled "Propeller Tip Speed or Why did that prop rip up my finger so much?")
http://www.lcrc.org/stu22.htm