For those of you worried about injecting air in your system through the syringe read this. Your most likely not injecting anything.
This is very simple to understand, first of all when you are aspirating like i hope you all are doing, it is not air you are seeing when you pull back on the plunger it is a vacuum aka empty space. Same thing when you are drawing from a vial, if you pull the syringe back too fast and bubbles show up the majority of them are not more than empty space either.
If you can logically think about where the bubbles came from you can distinguish whether they are actually air or just a vacuum. You cannot draw air out of a liquid or out of your body as neither contain air.