There's a chemical mixed in with the air to prevent condensation. Also, most canned-air products are aerosol-like dispensers. So you get the high (for idiots who sniff these things...idiots) from the propellents, not the air...
You can find environmentally friendly canned-air products, too. They're harder to find, though, and have a habit of not lasting long and "spitting."
You can also get a re-chargable canister, which has a pump that's kinda like a BB-gun, where you pump it up for the pressure instead of using propellents. The problem here is that they're a pain, often big, and there's no prevention against spitting moisture at your components.