Urine is made up of waste products including urea, uric acid, sodium chloride (salt), excess water and other wastes. The immediate smell of urine comes from the bacteria that feed on it. Uric acid crystals (crystallized urine residues) often remain on permeable surfaces and eventually emit an "ammonia" odor. If ambient conditions are humid or the surface with urine gets wet, the odor can be reactivated.