- Hypoxia – At high altitude the air has expanded and is thinner.
 - The brain stops working correctly when exposed to the unpressurized air eventually leading to death
 - Recovery is rapid once oxygen taken.
 
Symptoms can be:
- Tunnel vision.
 - Euphoria.
 - Tingling.
 - Reduced cognitive function.
 - Purple nose and lips.
 - Reduced coordination.
 - Shaking.