I presume the question is specifically asked about human beings other than about all living things. A healthy person requires to have the oxygen saturation (SpO2) of arterial blood at a minimum level of 92 % to breathe comfortably. If the oxygen saturation drops down to a level lower than 92%, the person will suffer short breath or breathing difficulty when he moves or works. However, if the person remains stationary or inactive, he or she may not feel any significant discomfort at all even when the oxygen saturation level drops down to below 80% although he or she may feel tired and sleepy.