For the past 20 years remarkable progress has been made in PEM fuel cell materials, component design, production, and system power density im-provements. However, there is still a lot to be done in the field of fuel cell system control, which makes it essential to understand the different physical phenomena within a working fuel cell and how they need to be controlled in order to improve efficiency, operating range and durability. This experimental study analyses the effects of ambient conditions, through the use of an environmental chamber, on a Horizon® H-100 20 cell stack, 22cm2 active area, open cathode, where the only active control mech-anism employed is a single fan that both, cools and provides the oxygen needed for the reaction. All the other control mechanisms are disconnected and a constant dry pure hydrogen flow rate of 1.8 SLPM is supplied to the stack. The objective is to isolate and determine the dif-ferent voltage losses with respect to the ambient conditions and currents.