We know that the main problem is the urine in the restrooms, but what causes urine to smell?

The urine smell in restrooms is typically generated by the urine that ends up in places other than in the bowl. This material — be it on toilet surfaces or on the floor and baseboards, then becomes a feeding ground for bacteria. The resulting combination, coupled with time, never smells good.
— Jarret Chirafisi, Product Manager – Air & Hand Care, Amrep, Inc., Marietta, Ga.

The majority of the odor is actually created by odor causing bacteria that use urine as a food source. The more bacteria that are allowed to grow the stronger the odor. The odor causing bacteria can penetrate porous surfaces, grout, and even etched porcelain causing odor issues in restrooms if not maintained properly.
— Nicole Livingston, R&D Technical Manager, Rochester Midland, Rochester, N.Y.

Urine is a waste product produced in the kidneys that filters out the harmful byproducts produced in the body. Since it is a waste product, it can contain various salts, trace amount of hormones, proteins, carbohydrates, fatty acids and a nitrogen containing component called urea. The components in urine have a natural odor to them because of the various chemical components in it. Fresh urine is mostly water. Urine odor problems in restrooms often come from spills on the floor. The water portion of the urine dries and the other components concentrate in it. There are other factors that will cause issues in a restroom, such as the normal bacteria found on the floor. These organisms can use the urea, protein and carbohydrates as a food source and produce other byproducts such as hydrogen sulfide and various ammonia compounds which our noses perceive as foul.
— Jason Welch, Microbiologist, Spartan Chemical Company, Inc., Maumee, Ohio

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Remove Odor, Don’t Mask It