The present invention relates generally to an environmental control unit for use in air handling systems that provides highly effective filtration of noxious gases (such as ammonia). Such a filtration system utilizes novel undoped precipitated silica or silicon-based gels to temporarily trap and remove such undesirable gases from an enclosed environment and permit recycling of such gases subsequently thereto. Such precipitated silicas need to exhibit specific pore sizes, pore volumes, pore diameters, and particles sizes to be effective for such a purpose. The combination of these particular properties permits highly effective noxious gas filtration such that excellent uptake and breakthrough results are attained, particularly in comparison with prior media filtration products. Methods of using and specific filter apparatuses are also encompassed within this invention.