Canned air is a wonderful and useful thing. But it can be dangerous if not used properly.
RIGHT
Holding the can of air upright, use short, gentle blasts to dislodge dust and debris from computer components and the like.
WRONG (but funny)
It's probably not the best idea in the world to hold the can of air upright and ask your friend to pantomime steering a car and make motor noises, then use one short, gentle blast to blow his hair around while yelling, "Convertible! Convertible!" and laughing immoderately. But still.
WRONG (and horrifying)
It's a bad idea to hold the can upside down while spraying it. This will cause the surface being sprayed to freeze immediately. Therefore, it's a very bad idea to hold the can upside down while spraying someone's skin.
(Because I am actually pretty squicked out by this myself, I put the photo illustration of the consequences of unsafe canned air handling after the jump.)
Ew. This is my friend's leg three days after getting frostbitten by canned air. The picture here is (approximately) actual size.
The perpetrator of this was not familiar with canned air safety, and he meant no harm. My friend--we'll call her "Dakota"--took it pretty much in stride and says it's healing, but no kidding, this is a gnarly burn.
Get well soon, Dakota! Everyone else, please, for the love of god, enjoy your canned air responsibly!