Is air duster (canned air) dangerous?

It can be if used improperly. Aerosol dusters contain pressurized refrigerant, not breathable air as the common names (e.g. “canned air”, “compressed air”) for it suggests. Take care to use in a ventilated area, avoid contact with any liquid refrigerant that is expelled, keep the can away from high heat and flames, and avoid puncturing the can. Since you are potentially blowing around particles, eye protection is also a good idea. See the SDS (safety data sheet) for more specifics.