There are millions of microscopic particulates floating around the atmosphere and inside the air of your house. As soon as you expose a needle to the air, it is no longer sterile.
The longer it is exposed to any non-sterile environment, you run the risk of cross contamination.
Rule of thumb: If you don't think they would do it at the hospital, don't do it at home!
Discard used tips and pins after every injection.
