Have you ever wondered why, even after you thought you cleaned up a urine stain, it reappears? It’s because as urine dries, it leaves behind a crystalline residue. Even the slightest increase in moisture or humidity reactivate the stain and odor. This process is a major reason that working with a carpet company that specializes in pet odor and stain removal is so important.Over time, as the urine appears to dry, the naturally occurring salts of the urine leave behind a crystalline type residue in the carpet and the padding beneath the surface. The slightest bit of moisture, even from humidity, can react with the crystallized material, instantly reactivating the odor and stain. This is when the stains begin to reappear, why the odor is so difficult to remove, and why topical treatments are usually ineffective.