Because telephone switching systems must operate continuously over decades, it is imperative that we understand the central switching office environments in which they operate. Quartz crystal microbalances (QCM) with electrodes of copper, silver, gold, or nickel were exposed in the Burbank, California, USA switching office environment and provided in situ, dynamic measurements of weight gain. After five months of winter exposure, crystals exposed by the return air duct had a significantly greater weight gain than the crystals exposed by a supply air duct. Crystal electrode surface composition was evaluated by Auger electron spectroscopy. We propose three reasons to account for the greater weight gain of crystals by the return air duct. These are surface interaction with chlorine compounds, adsorption of organic compounds, and surface interactions with NO2/O-3.