Cationic contamination is known to cause performance degradation and reduced lifetime in polymer electrolyte based electrochemical systems. Calcium is an important cationic impurity due to its prevalence in roadside particulates and as an airborne contaminant. The role of calcium ion (Ca2+) is investigated in-situ by injecting a solution of calcium sulfate (CaSO4) in deionized (DI) water into the cathode of a polymer electrolyte membrane (PEM) fuel cell through a nebulizer. Stability tests are conducted to determine the effects at various current densities with various Ca2+ concentrations. It is found that 5 parts per million (ppm, molar ratio) eq. Ca2+ in air is sufficient to lead to high cell performance loss at 1 A/cm(2) as well as severe membrane degradation. Precipitation of CaSO4 is found at the contact regions between the gas diffusion layers (GDL) and bipolar plates of the cathode at all test conditions. The amount of precipitation becomes sufficient to cause mass transport issues. (c) 2014 The Electrochemical Society. All rights reserved.