Everest is an extremely inhospitable place. Temperatures at the top are typically around 36 degrees C below zero in the winter and can drop as low as 60 degrees C below. Temperatures only rise to an average of 18 below during the warmest part of the summer and monsoon storms make Everest insurmountable during this period. The jet stream buffets the top of Everest with hurricane force winds for much of the year. During April and May, the jet stream shifts somewhat, offering relatively calm weather and this is when most climbs occur.
Avalanches are a constant threat and they have claimed many lives. Fierce storms may blow up unexpectedly, trapping or blinding climbers. Shifting glaciers can open suddenly, creating deep crevasses, often obscured by snow.
Lack of oxygen is one of the major challenges posed by Everest. The oxygen levels at the top are only a third of what they are at sea level. Humans cannot survive for any length of time at elevation above 26,000 feet (8000 m), which on Everest is known as the "death zone." At this altitude, the human body is unable to acclimate to the low oxygen and begins to deteriorate. Most climbers must use oxygen and will have difficulty sleeping.
Even at moderate elevations, many people experience headaches and shortness of breath. However, if they stay at that elevation, the body will compensate by producing more red blood cells and all functions will return to normal. At higher elevations, these symptoms are extreme and may also include loss of appetite, nausea, vomiting, dizziness, irritability and insomnia.
When oxygen is severely limited, the body will compensate by increasing blood flow to the brain. At extremely high elevations, the brain can actually swell and blood vessels begin to leak, resulting in High Altitude Cerebral Edema, or HACE. When this happens, the climber may experience disorientation, hallucinations and even loss of consciousness. Similarly, High Altitude Pulmonary Edema, or HAPE, occurs when fluid accumulates in the lungs. This produces shortness of breath and chest tightness as well as coughing and bloody sputum.
Both HACE and HAPE are potentially life-threatening conditions. Descent is the best treatment and may require helicopter evacuation, as many patients are unable to descend on their own at that point. If descent is not possible, high altitude sickness is sometimes treated with Diamox, a drug that signals the brain to breathe more, or with dexamethasone, a steroid that may temporarily reduce swelling. If available, the patient can be placed in a Gamow bag, which is a portable high-pressure bag that increases oxygen tension and may stabilize the patient.
Or course, it is much better to avoid altitude sickness than to have to treat it. This is why Everest climbers typically make several trips up and down the mountain to increasingly high elevation camps to acclimate their bodies to high altitude conditions.
Other risks to Everest climbers include frostbite and hypothermia from the extreme temperatures, thrombosis or embolisms caused by thickening of blood in response to high altitude, extreme sunburn, and broken bones from falls. Often, a combination of natural forces and human physiology produces lethal consequences for Everest mountaineers.