Eighty one subjects were examined for the relationship between BMI, Body fat, arterial oxygen saturation and arterial PO2 with the subjects at rest and after 5 minutes of walking on a treadmill ergometer at 3 mph at a 3% grade. They had BMIs between 19 and 50. All subjects were free of cardiovascular disease and had normal blood pressure making it safe for them to participate in mild exercise. They were all able to walk for at least 10 minutes without cardiovascular complications and were not taking any medications that altered the autonomic nervous system. The age was between 22 and 68. The results showed that above a BMI of about 30, there was an inverse relationship between BMI and oxygen saturation in fingertip blood (correlation -0.81. p<0.001). The reduction in O2 saturation with higher BMI was made much worse when walking at 3 mph at a 3% grade on a treadmill (p<0.01). Upon walking, saturation dropped for the first 5 minutes. Oxygen saturation was reduced by almost 5% after 5 minutes of exercise. The results indicate that caution should be used in people with higher BMIs on using therapeutic exercise and a pulse oximeter should be used for safety.