The intent of this SAE Aerospace Information Report (AIR) is to describe the effects of the environmental changes on human physiology and the protection required to avoid negative consequences resulting from altitude exposure. A brief presentation of basic terms and considerations required to discuss the topic of human physiology at altitude is followed by an overview of the cardiovascular and respiratory systems. Issues specifically related to human exposure to altitude are discussed. Hypoxia, hyperventilation, barotrauma, and decompression sickness (DCS) are each addressed. One goal of this AIR is to demonstrate the necessity of oxygen use for prevention of physical and psychological problems, or loss of consciousness in an aircraft. This should provide a clear understanding as to why the use of supplemental oxygen is required for flight crew and healthy passengers at altitude greater than 10000 feet (3048 m).