Altitude sickness occurs due to the lower levels of oxygen in the air at higher altitudes. Symptoms include headaches, loss of appetite, rapid breathing, feeling dizzy, and trouble breathing. 

It is a common condition and people usually adjust to the different oxygen levels given time. However, it can be dangerous, particularly for those going hiking, trekking or camping. It is not yet known why some people are more affected by altitude sickness than others, but it is not believed to be down to fitness or gender. 

Altitude sickness can be treated with extra oxygen, moving to a lower altitude where there is more oxygen in the air, or with medication. Drugs sometimes used to treat altitude sickness include carbonic anhydrase inhibitors and ophthalmic carbonic anhydrase inhibitors.