all types of parachuting techniques are dangerous, but halo/haho carry special risks. at high altitudes (greater than 22,000 feet, or 6,700 m), the partial pressure of oxygen in the earth's atmosphere is low. oxygen is required for human respiration and lack of pressure can lead to hypoxia. also, rapid ascent in the jump aircraft without all nitrogen flushed from the bloodstream can lead to decompression sickness, also known as caisson disease or "the bends".