Opponents of altitude training argue that an athlete's red blood cell concentration returns to normal levels within days of returning to sea level and that it is impossible to train at the same intensity that one could at sea level, reducing the training effect and wasting training time due to altitude sickness. Altitude training can produce slow recovery due to the stress of hypoxia. Exposure to extreme hypoxia at altitudes above can lead to considerable deterioration of skeletal muscle tissue. Five weeks at this altitude leads to a loss of muscle volume of the order of 10–15%.