The science behind altitude sickness is fairly simple to follow. Throughout the troposphere (ie from sea level to an altitude of approximately 10km), the air composition is in fact always the same, namely 20% oxygen and nearly 80% nitrogen.
So when you read that there is a lack of oxygen at the summit, that’s not strictly true – oxygen still makes up 20% of the air. So the problem is not lack of oxygen – but the lack of air pressure.
To put it in more precise terms: atmospheric pressure drops by about tenth for every 1000m of altitude. Thus the air pressure at the top of Kilimanjaro is approximately 40% of that found at sea level.
In other words, and to put it in layman’s terms, though each breath inhaled at the summit is 20% oxygen, just as it is at sea level, it becomes much harder to fill your lungs since the atmosphere is not pushing so much air into them. As a result, every time you breathe on Kibo you take in only about half as much air, and thus oxygen, as you would if you took the same breath in Dar es Salaam.
Oxygen and us: the truth
This can, of course, be seriously detrimental to your health; oxygen is, after all, pretty essential to your physical well-being. All of your vital organs need it, as do your muscles. They receive their oxygen via red blood cells, which are loaded with oxygen by your lungs and then pumped around your body by your heart, delivering oxygen as they go. Problems arise at altitude when that most vital of organs, the brain, isn’t getting enough oxygen and malfunctions as a result; because as the body’s central control room, if the brain malfunctions, so does the rest of you, often with fatal consequences.
So how does a lack of oxygen lead to altitude sickness?
Fortunately, your body is an adaptable piece of machinery and can adjust to the lower levels of oxygen that you breathe in at altitude. Unconsciously you will start to breathe deeper and faster, your blood will thicken as your body produces more red blood cells, and your heart will beat faster. As a result, your essential organs will receive the same level of oxygen as they always did.
But your body needs time before it can effect all these changes. Though the deeper, faster breathing and heart-quickening happen almost as soon as your body realizes that there is less oxygen available, it takes a few days for the blood to thicken. And with Kilimanjaro, of course, a few days is usually all you have on the mountain, and the changes may simply not happen in time. The result, is AMS.
AMS, or acute mountain sickness (also known as altitude sickness), is what happens when the body fails to adapt in time to the lack of air pressure at altitude. There are three levels of AMS: mild altitude sickness, moderate altitude sickness and severe altitude sickness. On Kilimanjaro, it’s fair to say that most people will get some symptoms of the illness and will fall into the mild-to-moderate categories.
Having symptoms of mild AMS is not necessarily a sign that the sufferer should give up climbing Kili and descend immediately. Indeed, most or all of the symptoms suffered by those with mild AMS will disappear if the person rests and ascends no further, and assuming the recovery is complete, the assault on the summit can continue.
The same goes for moderate AMS too, though here the poor individual and his or her symptoms should be monitored far more closely to ensure that they are not getting any worse and developing into severe AMS. This is a lot more serious and sufferers with severe AMS should always descend immediately, even if it means going down by torchlight in the middle of the night.