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 laying 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 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 its also been shown to stop some bacteria from producing ammonia which might eliminate some of the stuff which is 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 is ingested it can destroy red blood cells and you need those Plus, according to the US Department of Health and Human Services, Naphthalene is “Reasonably anticipated to be a human carcinogen” So, peeing on the stuff every once in a while is probably okay but you wouldn’t wanna get any closer to it than that. Many urinal cakes these days are made of a compound called a molecule that looks a little bit like a throwing star and is often used as a naphthalene replacement in moth balls. 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, 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. Thanks for asking and thanks, especially, to all of our patrons on Patreon who keep these answers coming. If you’d like to submit a question to be answered, or get some videos a few days early, go to and don’t forget to go to youtube.com/scishow and subscribe.