Strokes from air bubbles
There is a generalised belief amongst injectors and the general population that injecting air is ‘not a good thing.’ Whilst this is true, it tends to be somewhat overstressed in terms of importance when priorities for injecting drug users are being considered. It is possible to observe some injectors taking little or no care about hygiene or cross-infection risks whilst injecting, but exhibiting infinite patience when expelling the minutest of air bubbles from a syringe.
Compared to the size of an air bubble, it takes a gigantic volume of air to cause circulatory problems (the blood would froth in the chambers of the heart). Although it is desirable not to introduce air into the veins, even a few 1 ml syringes completely full of air would be unlikely to cause any problems.
Carefully removing tiny air bubbles from a syringe can be seen as evidence that injectors are concerned about their health and are prepared to act to preserve it. Some injectors simply need more information about more important priorities such as hygiene.