I know the idea of voids or decompression has been brought up in the past but I wanted to add my ideas specifically on this idea.
Movies like to make us think that people explode if put in a void, but this is not necessarily true. People don't necessarily explode, though it is certainly deadly.
Where the damage comes from the most is when the void decompresses.From Wikipedia:
Humans and animals exposed to vacuum will lose consciousness after a few seconds and die of hypoxia within minutes, but the symptoms are not nearly as graphic as commonly depicted in media and popular culture. The reduction in pressure lowers the temperature at which blood and other body fluids boil, but the elastic pressure of blood vessels ensures that this boiling point remains above the internal body temperature of 37 °C. Although the blood will not boil, the formation of gas bubbles in bodily fluids at reduced pressures, known as ebullism, is still a concern. The gas may bloat the body to twice its normal size and slow circulation, but tissues are elastic and porous enough to prevent rupture.
When activating the void channel an elemancer would put pull the air away from a small area near the target for a short period of time.Rapid decompression can be much more dangerous than vacuum exposure itself. Even if the victim does not hold his or her breath, venting through the windpipe may be too slow to prevent the fatal rupture of the delicate alveoli of the lungs. Eardrums and sinuses may be ruptured by rapid decompression, soft tissues may bruise and seep blood, and the stress of shock will accelerate oxygen consumption leading to hypoxia. Injuries caused by rapid decompression are called barotrauma. A pressure drop of 13 kPa (100 Torr), which produces no symptoms if it is gradual, may be fatal if it occurs suddenly.
*No longer cause a knock down effect.
*Do higher damage in the form of energy loss, causing more internal damage and energy loss than external damage.
*External damage comes in the form of bleeding, swelling and bruising and lack of oxygen.
*Possibly add effects based on where the void hits. Suffocation causing a damage over time effect. Bleeding from body parts.
*Possibly a chance to instantly kill for a master level spell.