Performing a breath hold after an exhalation lowers the oxygen saturation of the blood to simulate the effects of high-altitude training. I have monitored the blood oxygen saturation of thousands of individuals as they practice breath holds, and by far the greatest change to oxygen saturation occurs after an exhalation. For most people, after four or five days of practice, a drop of oxygen saturation below 94 percent can be observed—a level comparative to the effects of living at an altitude of 2,500 to 4,000 meters.
