How do I erase everything off my 2011 MacBook Air?
How do I erase all data from my laptop?
MacBook Air 11″, OS X 10.10
Question marked as
Top-ranking reply
Here is What to do before you sell, give away, or trade in your Mac - Apple Support
...which includes re-installation and other steps.
If your data was encrypted (as is typical now), then deleting the keys (as happens in a wipe and re-install) renders the data inaccessible.