You’ve undoubtedly smelled indole. Perfumers add it to flowery fragrances, but it’s also added to chocolates, coffees, and fruity-flavored sweets. That doesn’t sound bad—until you learn that concentrated indole smells like poop. Because it’s actually found in poop.
Indole is molecule that looks like two mostly-carbon rings glued together, little spikes of hydrogen one each joint of the ring, and one nitrogen added in for spice. Although it can be solid, it usually released into the atmosphere in individual pieces, and get picked up by your nose. What do they smell like? That depends on who you ask.
In large concentrations, most people agree that it smells like poop. It does so because your most frequent contact with indole is when it comes out in your poop. (The concentration is especially high if you’ve had foods like turkey or milk, which contain tryptophan. Tryptophan gets turned into indole and indoxyl sulfate in your digestive system.)
However in small concentrations, indole will smell very sweet and flowery. Your second most frequent interaction with indole probably comes when you walk by common jasmine. These flowers produce indole naturally, and indole is present in high concentrations in jasmine oil. It’s also usually in almost any kind of flowery perfume. No matter how many other scents are in there, you’ll smell a little indole.
Why does a lot of indole smell like feces, but a little bit smells like a flower? We don’t know for sure—but it seems that when scent hits your nose in large concentrations, it binds to a wider range of receptors than it does when only a trace of it wafts into a nostril. This may be one of the reasons why a ton of perfume not only smells overwhelming, but actually smells bad. Too much scent will activate the receptors of your nose associated with bad scents, while a small amount will stick to a few more acceptable receptors.
But that may not be all. According to some, while too much indole smells bad, it doesn’t really smell like feces, and while a little smells good, it doesn’t really smell flowery. Instead, both smell fresh, a little sweet, and have a sort of a lush, animal odor. This might be why it gets put in a surprising amount of foods like chocolate or coffee. It has an indefinable scent. We think of it as fresh and living, and a bit intimate—the way we think that certain odors smell “green.” It’s both dirty and pleasant.