Abstract: An altitude simulation system is disclosed for simulating an altitude within an enclosure or mask, wherein various improvements are provided, including: (a) a more effective use hypoxpic air generated by the system via recirculating techniques and improvements in air leakage, (b) improvements in determining when a simulated altitude is reached, (c) improvements in controlling hypoxic air generators so that peak electrical power loads are reduced and there are enhanced failsafe features for protecting the health of users, (d) using a pulse oximetry device for measuring oxygen saturation in a user's blood to thereby vary the oxygen content in the air provided to the user.