Ever been in a kid’s room that wets the bed or an alley where the homeless and drunks have used it as a urinal and there is that awful smell of pee…well since the pee evaporates and is gone what exactly does urine leave behind that makes that smell? I wonder about this especially when it comes to alleys because unlike a kids mattress there is no fabric for the smell to hide in, when it comes to both cases, kids beds or homeless alleys I do not know what pee leaves behind to create that strong odor that is a little worse than poop in my opinion.
I know of no hard research on urine smell. Among mammal species, we’re really unusual, in that we train our children to ignore urine smells. Almost all the mammals use urine for marking territory, but we pretend not to. We tell each other that the pee on the floor of the men’s room is from some sloppy urinator, not from us. Dogs know, from one whiff, whether a male or female peed there and often which individual. All we claim to identify is, “Ewww, urine.”
Maybe I just worked too long as a janitor, but I think we have a lot more to learn about human scents.
I apologize for posting such an undocumented answer in General Questions.
The main odor is ammonia, which forms from the urea in the urine.
QtM, MD
AskNott, your post made me very curious. Are you of the opinion that humans can distinguish the urine of male, female, children… different types of human beings, in other words? I find this idea fascinating. In a sort of related vein, I read about a study where the scientists had women smell armpit odor on t-shirts from different men. The study found that the smells the women preferred were extremely likely to come from men whose immune systems were different than theirs (not a biologist, so I don’t know the exact mechanics of it). The upshot of this is the idea that human females, through smell, are attracted to those with whom genetics would be more likely to produce hardier offspring.
I think you’re right, the human sense of smell is disregarded often to our disadvantage. I think it plays into our behavior more than we know.
The steroids androstenone and androsta-4,16-dien-3-one contribute to the odor of aged urine.
ggurl, I’m not hip to the different immune system/sweat project you referred to. It sounds very interesting, though.
Yes, I do believe that some humans can distinguish between adult male and adult female urine. I know I can, and I believe anyone could learn to. As I said, we’re trained not to. It’s not hard to tell dog pee from cat pee. Whether I could tell one woman’s urine from another, well, maybe that would take some work. We all flush it away. I know I can pick out a certain woman’s cologne from a crowd. “Caesar’s Woman?®” I often wished I was Caesar. 
I remember reading about the study ggurl mentioned but I’m having some trouble Googling a cite for it.
But to continue the pitstink hijack:
male pitsmell relaxes women, changes periods
Well, that might explain the time-honored pattern of women swiping men’s shirt for their own casual wear. 
I saw the broadcast you are talking about. It was on the program Nature.
The following is from Natures website.
“In 1995, the famous ‘sweaty T-shirt’ study showed that women prefer the smell of sweat from men whose MHC genes are dissimilar to their own, hinting that the odour warns women not to mate with close relatives.”
Here is the URL if you are interested in reading more. http://www.nature.com/nsu/011115/011115-4.html
FWIW
Jim