Good question! In this book, we are talking about going from close to sea level to anywhere above 2,000m.
Our bodies are used to working at home. When we go higher they need to adapt.
Altitude starts to have an effect around 1,500 – 2,000m. The body starts to behave slightly differently as it tries to make up for the change in oxygen levels. Go up too fast to about 2,500m and altitude illnesses are common.
Given enough time to adapt, most people can adjust to altitudes between 5,000m (Everest base camp) and 5,500m. Above 5,500m few people can adjust any more. Health and ability then get worse.
So what’s different about travel at altitude? The main difference is that as you go higher the air pressure gets lower (the air gets ‘thinner’), and this means for any single breath that you take there will be less oxygen for your body. Oxygen is needed to give you the energy to move, but is also needed simply to keep your body alive – for your brain and digestion to work, for healing cuts, and all those normal things your body does without you knowing about it.
As your body gets less oxygen it adapts. You breathe faster and deeper. It makes more red cells to carry more oxygen in the blood. Changes take time to happen. If you go slowly you should stay healthy. Go up too fast and you risk suffering from altitude related illnesses, such as Acute Mountain Sickness (AMS).