What causes urine smells

Chemicals and gases (principally ammonia and mercaptans) emitted by urine (and feces) are what cause the awful smell. Soon after a pet urinates (or eliminates), the urine begins a process of decomposition. During the first part of the process the urea turns into ammonia and then produces mercaptans and other gases. These mercaptans are what give skunk spray and rotting cabbage their foul smelling odors.

Even if you thoroughly clean surfaces that have been dirtied by urine, if you don't deal with the smell and get rid of it completely it will only become more pungent over time. The urine odor also acts as a trigger or homing beacon to your pet calling them back to urinate (or defecate) in the same spot again and again, only further worsening the problem. Bacteria and molds (which further add to the presence of allergens) can also thrive in the moist areas left by the urine.

How to remove with trouble spots: The single most important step you can take towards getting rid of potential allergy triggering urine (or feces) dirtied areas is to find them and then to completely clean and deodorize them.
Canines and felines are very different in the way they react to "accidents" in the house.

While dogs are much more blatant offenders and will often urinate or defecate right in front of you cats are much more sneaking, hiding their accidents behind furniture, under beds or in secret places that only they know of.
If in a room of your house there is the unmistakable smell of urine in the air but you can't see any visible stains, you can try using a black light to locate the source of the odor. Black lights are available from hardware stores and are simple to use. First turn off or block out all light from the room and then shine the black light around the room. Any urine on the walls, floors, furniture will be highly visible as florescent green spots or blotches.

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