When the term altitude is used it usually refers to being at higher than sea level, to a point where the body cannot function as efficiently as at sea level.
The human body becomes accustomed to a certain altitude, then has difficulty if that altitude increases. The reason this happens is basically because the atmospheric pressure (measured in millibars) decreases the higher one goes. It does not mean that there is less oxygen, but it does mean that it is more spread out and therefore the body has trouble absorbing it into the blood stream. There are less oxygen molecules to bind to the hemoglobin in the blood. As a result, the tissues of the body literally suffocate. This is referred to as hypoxia.
Effects On The Body
Accute mountain sickness usually happens around 2500m above sea level. Warning signs include headache, exhaustion, shortness of breath, and perhaps vomiting.
High altitude cerebral edema is when the brain swells, and can happen at around 3030m or higher. Warning signs can include vomiting, bad headache, trouble talking, poor muscle control, and loss of mental function. In severe cases the person may lapse into a coma. At this level the body tries to compensate by sending more blood to the brain, hence the cerebral edema.
High altitude pulmonary edema is when the body diverts more blood to the lungs, again, to try to compensate. This can damage the lungs and lead to fluid build up. Warning signs include severe trouble breathing, high breathing rate, high heart rate, severe coughing, and wet sounds coming from the lungs. Obviously this is a life-threatening condition.
If work must be done at altitude, acclimatization is a must. This is simply involved one full rest day when climbing, for every 1000 meters climbed. Some people may require more rest time than others. And some people may simply not be able to acclimatize at all. Nearing 7000 meters acclimatization is simply not possible.