The higher the altitude, the less dense is the air, and also the colder is the temperature. People climbing Mount Everest have to wear clothing that keeps them well insulated from the intense cold. Though, Mount Everest has been climbed by a few climbers without oxygen cylinders (I have not researched how many), most climbers carry oxygen cylinders with them. Also, cooking at high altitudes take longer and uses much more fuel than cooking the same meal at sea level. Then there is altitude sickness, that can affect certain climbers enough to force them to return to lower altitudes.