The vast majority of high altitude trekkers will feel some effects of altitude above 3000m. There’s a lot of information out there and some of it sounds pretty scary. I read about altitude before trekking in the Himalayas and thought I would shared what I found.
Altitude sickness is caused by reduced atmospheric pressure at high altitudes. There are various symptoms and some people are effected more than others. It has nothing to do with fitness, but rather the speed at which your body acclimatises. I have listed the most common symptoms below.