System for thermally conditioning, humidifying and filtering reactant feed gases supplied to a stack of fuel cells using an evaporative element, a water spray mechanism and a heat exchanger. The evaporation element also functions as a filter. The evaporative element may take the form of a removable packing (filter media). Use of a controller to manage the rate of nebulized water addition to the feed gas stream is also described. Benefits in filter efficiency and extended filter service life along with volume, weight, and cost reduction in a fuel cell system are realized.