Why Do We Have Two Nostrils?

Humans have two nostrils not because smell needs “left vs. right” localization, but because the two sides of the nose can perform better at different times—improving how many odor molecules get to the olfactory receptors. Stanford University’s olfactory research project found that, across the day, one nostril often inhales air faster and more effectively than the other, and which side is dominant shifts over time. That matters because odorants are absorbed at different rates: some molecules are taken up quickly and must reach receptors before they’re absorbed earlier along the nasal passage, while others are absorbed more slowly and need time to travel deeper before they can be detected. Two nostrils therefore act like a built-in timing and throughput system, letting the body sample a wider range of odor chemistry rather than relying on a single “best” airflow path.

From there, the discussion stretches from everyday biology to the chemistry of space. Space itself is a vacuum, so there’s no air to carry smell; attempting to smell in a helmet-less scenario would lead to rapid expulsion of air from the nose, throat, and lungs, leaving little more than brief sensations from evaporation and bodily fluids. Yet when objects are taken into space and later returned to a habitable atmosphere—such as inside a space station—astronauts report unusual smells. One commonly described profile is a metallic, burn-like odor, sometimes compared to welding fumes, believed to come from high-energy particles that cling to suits and then react with air after re-entry. A candidate source involves polycyclic aromatic hydrocarbons, combustion-derived molecules found widely in interstellar space; evidence from the University of Toledo (2004) suggested they may exist in old nebulae, and they can also form during high-temperature cooking.

The chemistry becomes tangible through attempts to recreate “space smells” on Earth. NASA reached out to Steven Pearce of Omega Ingredients to develop a perfume-like replication of odors associated with space exposure. The approach relies on identifying the chemical makeup of extraterrestrial material and assembling those ingredients locally. Researchers have also reported ethyl formate in the dust cloud Sagittarius B2; while no one could safely “whiff” that region directly, ethyl formate has been described as smelling like raspberries and rum when brought into an Earth-like environment.

Closer to home, the nose’s internal experience is hard to pin down because constant exposure leads to desensitization. Neural adaptation explains why people notice restaurant smells at first and then stop noticing them mid-meal. Smell also ties strongly to memory: a single odor can instantly trigger where someone encountered it and who was present, potentially because olfactory signals route through the limbic system, which is central to emotion and memory. Clinical observations align with that link—patients with memory loss from brain damage often show impaired smell.

Finally, smell is framed as both protective and social. The Westermarck effect describes reduced attraction between individuals raised together early in life, and one mechanism involves olfaction, allowing humans to distinguish relatives or co-raised individuals from strangers. The transcript also introduces anosmia for inability to smell and ends with a wordplay-driven idea about “olfactory” meaning: smell isn’t an inherent property of molecules so much as a result of how those molecules interact with receptors—“no us, no smell.”