How good is the human sense of smell?
Humans are more sensitive to the smell of rain than sharks are to the scent of blood.
Humans can detect geosmin, the chemical that creates the 'earthy' smell of rain, at just 5 parts per trillion. This makes our sense of smell for this specific compound 200,000 times more sensitive than a shark's ability to track blood in the water.
Nerd Mode
The distinct scent of rain on dry earth is caused by a chemical compound called geosmin. This organic compound is produced by Streptomyces, a genus of Actinobacteria found in soil. When raindrops hit the ground, they trap tiny air bubbles that then burst, releasing these molecules into the air as aerosols. This process is scientifically known as petrichor.Human sensitivity to geosmin is extraordinary. Research published in journals like Nature indicates that the human nose can detect geosmin at concentrations as low as 5 parts per trillion. To visualize this, it is equivalent to detecting a single teaspoon of a substance diluted into 200 Olympic-sized swimming pools. In contrast, a shark's highly praised sense of smell allows it to detect blood at roughly one part per million.Evolutionary biologists believe this hyper-sensitivity developed as a survival mechanism. For early hominids living in arid environments, the ability to smell rain from miles away was crucial for finding water and tracking seasonal migrations of game. Furthermore, because geosmin is produced by healthy soil bacteria, it likely served as a biological indicator for fertile land and clean water sources. This deep-rooted connection explains why the smell of rain remains one of the most universally pleasant scents to humans today.
Verified Fact
FP-0004394 · Feb 19, 2026