What kind of damage can happen if sprayed in the eye with a can of air?

Depends. Depends on the distance from the eye, the pressure in the can and any contaminants in the can. If very powerful, fully loaded and close up, one might do siginficant damage. A single casual blast of canned air perhaps might do no harm and be of no consequence. I would caution to avoid this activity or risk.
Corneal injury. While regular air can do little more than dry out the eye, the propellant chemicals mixed with the air in spray cans (for cleaning computer keyboards, for example) can potentially cause a chemical burn on the cornea. Additionally, any particles or debris can hit the eye at a high rate of speed, leading to a corneal foreign body.