In this work, insulating hygroscopic polymer thin films have been applied as the vertical-channel sensing layer of extremely sensitive ammonia sensors to detect 5-1000 ppb ammonia. Hygroscopic polymers such as poly(ethylene glycol) (PEG), polyvinyl alcohol (PVA), and poly(acrylic acid) (PAA) serve as good absorbents of ammonia gas. The high-density nano-porous structure covered by a hygroscopic thin film acted like a nano-porous sponge (Nano Sponge) to absorb water, which forms an ionic conduction path in the 300 nm vertical channels. The influences of the polymer functional groups, pH value, relative humidity (RH), and other interfering gas analytes on the sensing performances of theNano Spongesensors have also been investigated. After adding glycerol, the Glycerol/PEGNano Spongesensors exhibited a stable response. Breath detection in a healthy group and a chronic kidney group of patients was examined using the Glycerol/PEGNano Spongesensor in a sensing system with a NaOH tube to control the relative humidity and to absorb the interfering species such as CO and H2S.