Counter-current parallel flow membrane contactors were studied to estimate likely formaldehyde removal rates from air occurring during membrane air humidification and evaporative cooling applications. A mathematical model and experimental measurements of formaldehyde removal are presented to assess the impact of possible operating conditions and contactor configurations on expected formaldehyde removal rates. Removal efficiencies were predominately controlled by shell side mass transfer coefficients. However, removals decrease when water velocity within the fiber lumens is low and water supplied to the modules approach module evaporative losses. While removal rates were found to be adequate to control formaldehyde emissions in most space air-conditioning applications, large volumes of water usage appear to be necessary. (C) 2010 Elsevier B.V. All rights reserved.