Altitude sickness (also known as acute mountain sickness) commonly occurs in climbers that reach a high altitude too quickly without time to adjust to the new environment. As altitude increases the atmospheric pressure decreases, meaning there is decreasing levels of oxygen in the air you breathe. This decrease in the amount of oxygen you are breathing in causes the symptoms of altitude sickness including headache, nausea, dizziness and exhaustion.
Altitude sickness is common amongst climbers and skiers who spend time at 2,500m (8,000 feet) above sea level. Symptoms tend to get more severe at altitudes of 3,600m (12,000 feet) or more. There is no specific demographic that is more likely to develop altitude sickness than others; it is not affected by age, sex, fitness or weight. Some people are more susceptible to getting altitude sickness than others, and it does not mean you will experience it in the future because you have had it previously.