When air is injected in to the veins, it leads to an air embolism which is a condition where there is the formation of a bubble in the bloodstream.
The problems due to an embolism can range from pain, heart failure, damage to other vital organs and death. This happens as the bubble does not allow the free flow of blood and organs are starved of oxygen.
Usually, a few small bubbles of air while taking an injection do not affect the body. Air embolism is the result of trauma or bigger medical accidents where a large amount of air or other gases enters the vascular system.
It will stop the countinues flow of blood to the parts of body.As a result the part die by the lack of blood(oxygen).Finaly this causes death.So it is very dangerous to inject air into a person's vein