Background First, some basics. Because of the affect of gravity on the atmosphere, air pressure falls as you ascend above sea level. The result is a decline in the absolute amount of oxygen available, which results in a decline in the amount of oxygen available to your tissues. Without sufficient oxygen, your tissues can't convert nutrients into energy. Tolerance to high altitude varies tremendously between individuals, and can vary from trip to trip for the same individual.
Cold weather seems to worsen the affects of altitude.
Those with heart and lung conditions such as asthma, emphysema, angina or a history of heart attacks need to be particularly wary of altitude and may experience symptoms at lower altitudes. Even a childhood history of lung disease, for example, might put a patient at increased risk of a problem at high altitude. The altitude functions almost as a 'stress test,' which can reveal a variety of underlying medical conditions. Everyone who visits a high altitude destination will breath more frequently and experience increased heart rate and pulse. Dryness of the skin and mucous membranes occurs, as does minor headache.