High-altitude sickness, or mountain sickness, affects people who travel to elevations they're not used to. The higher the elevation, the less oxygen is in the atmosphere. That means when you breathe in at high elevations, your lungs taken in less oxygen than they're accustomed to and your blood oxygen level drops, causing hypoxia. There are three main types of altitude sickness: acute mountain sickness, high-altitude pulmonary edema and high-altitude cerebral edema. While acute mountain sickness is the most common, it's also the least dangerous. High-altitude pulmonary edema, or HAPE, is rare, but it's the leading cause of altitude sickness-related death. High-altitude cerebral edema, or HACE, can be fatal, as well.
High-altitude pulmonary edema occurs when your heart and breathing speed up to try to get more oxygen flowing to your tissues. Sometimes, fluid leaks from your pulmonary blood vessels into your lungs, making it difficult to breathe and take in necessary oxygen. High-altitude pulmonary edema affects two to four percent of people who ascend higher than 14,000 feet (4,270 meters). High-altitude cerebral edema happens when fluid retention and low oxygen cause your brain to swell. Signs that might indicate HACE include confusion and disorientation when you ascend to high elevations. Though high-altitude cerebral edema is less common than high-altitude pulmonary edema, it's a more severe condition. It's more likely to affect someone who already has HAPE than someone else. Both conditions gradually worsen over a few days.
People with pre-existing heart or lung problems are more prone to experience a serious form of altitude sickness than healthy people. Another risk factor for a potentially fatal case of altitude sickness is ascending to high altitudes too quickly. If you have symptoms of acute mountain sickness at lower altitudes, you have to wait until your body adjusts before continuing to ascend.