A deep dive into petrichor, the smell that follows rain

It’s that familiar smell that comes after a light rain, petrichor. And it turns out, there’s a good reason we can detect it.

The origin of the term “petrichor” comes from a 1964 journal on nature, where a group of Australian researchers coined the word.

“Some Australian scientists came up with this word petrichor,” said Dan Riskin, evolutionary biologist. “Petra, which means rock, and ichor, which means sort of like a perfume. So it’s the perfume of rocks, and it turns out not to be the rocks themselves that give that smell. But it’s a pretty good name.”

Riskin says the smell actually comes from the soil. More specifically, all the microorganisms in the soil.

“There are microbes, there are algae, there are fungi,” Riskin told CTV News. “And all of them are making these little chemicals that get spritzed up into the air like a perfume, so that when you take a whiff, you’re not smelling the rocks and you’re not smelling the rain. You’re smelling the life that lives in those soils.”

Humans don’t typically have a strong sense of smell compared to other animals, but surprisingly, they have a very high sensitivity to petrichor, at around 10 parts per trillion.

“The various papers that I came across said that it’s even more sensitivity than sharks have to a drop of blood in the ocean,” said Dr. Julia Boughner, professor in the department of Anatomy, physiology and Pharmacology at the University of Saskatchewan. “So that suggests that chemical is really important to humans.”

While humans can detect the smell and generally find it pleasant, Boughner says not all animals find it appealing.

“There's evidence that bees are attracted to the scent, but only when there's a light rain, not a heavy rain,” said Boughner. “And then apparently you can also smell it better with a light rain because that's what activates the chemicals. Fruit flies don’t like it, mosquitoes do, and then a tiny worm that we often use in science, they don’t like it.”

Riskin says one conclusion as to why humans like the smell of petrichor could be linked to evolution and the ability to find land that grew things in it.

“If you can imagine back to our ancestors living off the land and needing to know when the fruits are going to show up, or when those seeds that you planted are going to grow, that smell is of huge importance,” he said.

In addition to the possible adaptation to lush landscapes, Boughner says petrichor has noticeable effects on humans.

“Petrichor has interesting effects where it has a relaxing effect, kind of like forest bathing,” she said. “Those chemicals actually create a feeling of good health and can, in some cases, be helpful for humans. So just being around them in nature apparently is useful for us.”

Boughner says what she finds interesting is that so many different types of animals throughout history, even dating back to the Cambrian era with trilobites, have the ability to respond to petrichor.

“To me, that’s just amazing to think that maybe a Tyrannosaur had receptors for petrichor and that other animals throughout their past were living in a way that was picking up environmental cues that maybe we are also smelling today,” she said.

Riskin says it’s nice to know the science behind things that are beautiful, and knowing the deeper purpose of detecting petrichor is a great example.

“You know a flower smells good and looks good, but then you understand that it's actually evolved to attract animals and those smells have a purpose,” said Riskin.

“It deepens the sort of beauty of it. And I think with petrichor, it's just a nice smell that anybody can appreciate. But when you start to think about the fact that it's our body's sensing when the land is ready to receive seeds, to grow things for us, and that it just adds a lot to the whole story and makes it all the more beautiful.”