(Intro)

If you happen to frequent urinals odds are you've seen one that has a little block at the bottom of it. They're often called urinal cakes, and a lot of urinals have them, but here's a pro tip for you, they're not really cakes. So why are they there? Well, the key is in the more proper sounding name for these things, they're technically called urinal deodorizer blocks. Because that's what they do, they make urinals smell a little less awful, even though it doesn't always seem to make much of a difference.

Urine isn't  generally supposed to have a strong smell, but it can depending on certain factors. Some foods like coffee, or asparagus will often give pee a particular odor, and medical conditions like bladder infections or uncontrolled diabetes can also cause urine to smell. Then, there's the fact that urine contains ammonia, the higher the concentration the worse it smells. That generally happens when you're dehydrated. But when you leave urine lying around for a while bacteria can start digesting some of the other main components of urine. Mainly urea and uric acid, producing even more ammonia. So it isn't too surprising that a urinal would smell not great. There's a lot of pee going into that thing, and not all of it necessarily ends up in the drain, even if the urinal flushes. 

Enter the deodorizer block. In the early days of urinal cakes, probably around the turn of the 20th century, the little slabs were mostly made up of naphthalene, aka olden day moth balls. The main thing naphthalene does is vaporize making the air smell very strongly like mothballs, and therefore a lot less like urine. It's also been shown to stop some bacteria from producing ammonia, which might help eliminate some of the stuff that's causing the odor in the first place. 

Today, you might still encounter urinal cakes with this stuff in it but it's a lot less common, probably because too much naphthalene isn't very good for humans. When it's ingested, it can destroy red blood cells, and you need those. Plus according to the U.S. Department of Health and Human Services naphthalene is reasonably anticipated to be a human carcinogen. So peeing on the stuff once in a while probably okay, but you wouldn't want to get any closer to it than that.

Many urinal cakes these days are made of a compound called paradichlorobenzene, a molecule that looks a little bit like a throwing star, is often used as an naphthalene replacement in mothballs. That's because like naphthalene, paradichlorobenzene has a very strong smell, and it can interfere with some of the bacteria producing the extra smelly ammonia. However, paradichlorobenzene is also listed as a compound that's thought to be a human carcinogen, though there haven't been any major human studies. So I wouldn't advise eating a paradichlorobenzene urinal cake either, no matter how delicious that might sound. 

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