Manned Space Systems are usually designed to support the crew atmospheric conditions equivalent to those at sea level. In phases with frequent Extra Vehicular Activities (EVA), a reduced pressure environment is preferable to facilitate the EVA suit prebreathing procedures. The Columbus Attached Pressurised Module (APM) will face both pressure scenarios during its life.Operation at different pressure levels primarily affects the performance of the Environmental Control System (ECS) of the pressurised elements. A lower air density results in reduced heat exchange, adversely affecting both the crew comfort and the electronics air cooling.This paper reports the results of a study performed to identify the constraints and the numerous potential problem areas related to APM operations at reduced pressure.Effects of the reduced pressure on the environmental parameters have been investigated. Atmosphere compositions suitable to both pressure scenarios are identifed in terms of oxygen, carbon dioxide and humidity contents. Particular attention is paid to the oxygen percentage and its relationship to the flammability of materials.The physiological parameters affected by the reduced pressure are evaluated and proper crew comfort requirements are proposed; impacts on the design and performances of the APM Temperature and Humidity Control (THC) are identified and problem areas are also highlighted.Moreover, degradation of electronics air cooling is analysed; proper countermeasures and limitations to provide minimum nominal performances with the APM Avionics Air Loops are presented. Finally, impacts on major component performance such as heat exchangers and fans are reported.