The hyperbaric environmental control assembly (HECA) monitors and controls temperature, humidity, and CO2 levels in the Space Station Freedom airlock when the airlock is used for EVA prebreathing campouts and as a hyperbaric treatment facility. Prebreathing is required prior to extravehicular activity due to the differential between the station nominal pressure and the EVA suit pressure. Hyperbaric treatment is required in the event of decompression sickness.The HECA consists of an atmosphere recirculation circuit which provides air circulation and temperature control, and a separate CO2 and humidity control circuit. Temperature is controlled by transferring heat from the airlock to the station thermal control system through a compact heat exchanger. CO2 and humidity are removed using a dual-bed, regenerable, molecular sieve system. While one bed is adsorbing, the other bed is being desorbed by venting to space vacuum. Air is circulated by redundant foil-bearing fans in both circuits. The system includes sensors to monitor CO2 and water vapor partial pressures and temperature, and provides an interface to the airlock multiplexer demultiplexer (MDM) microprocessors for control of these parameters within prescribed ranges.CO2 and latent water production rates have been calculated from established metabolic profiles for both the campout and hyperbaric protocols. An analytical model has been used to predict carbon dioxide and humidity levels as functions of initial crewlock conditions and the specified loads. This model has demonstrated the suitability and robustness of the dual-bed molecular sieve system for the HECA.