An air conditioning assembly for an environmental chamber includes an air handling unit for supplying air to the environmental chamber. A plurality of ducts in fluid communication with the air handing unit and in fluid communication with the environmental chamber are configured to draw air from one side of the environmental chamber through at least one outlet vent, and to blow air into an opposite side of the environmental chamber through at least one equal but opposite inlet vent. At least one air conditioning means in fluid communication with the at least one air handling unit is configured to condition the air flow passing through the air handling unit. The at least one air conditioning means includes an altitude simulator for changing the oxygen content of the air flow. In use, the air conditioning assembly is configured to create substantially laminar air flow throughout the environmental chamber.