Above 2,500 meters, the concentration of oxygen in the air starts to drop, which makes it harder for you to get sufficient oxygen. Also decreasing air pressure has the effect of causing liquid to leak from capillaries into the lungs and brain, which can be fatal. The human body has the ability to adjust, but this is a gradual process. You can avoid this threat by limiting your rate of ascent. If you have symptoms, descend immediately. Make sure you take enough rest and drink plenty. The guides are highly trained and will keep an eye on you. They know what to do in case of an emergency.