Fresh urine does not actually have an unpleasant odor, but the ammoniacal odor of stale (old) urine is due to bacterial decomposition of the urea in the urine, as well as the bacterial excrement itself. Nature’s bacteria will continue to feed as long as there is a food source, and moisture. That is why the ammonia odor becomes stronger on humid days, or soon after the carpet has been wet cleaned. Nature’s bacteria are not efficient in totally consuming urine. They need the help of special, genetically engineered bacteria to accomplish total source removal.