Why Humans Have Two Nostrils — The Nasal Cycle That Improves Breathing and Smell

Why We Have Two Nostrils

If you close one eye or plug one ear you immediately notice a loss: two eyes give depth to vision and two ears help locate sounds. Blocking one nostril, however, usually feels less dramatic because the nostrils sit so close together. But the paired design of the nose plays an important, active role in how we breathe and smell.

The Nasal Cycle

Our nostrils don’t share airflow equally at every moment. Instead, they alternate in dominance over a period of hours in a pattern called the nasal cycle. At any given time one nostril is more open and handles most of the airflow, while the other is relatively reduced and spends time restoring moisture and mucosal function. This swapping seems to optimize both air conditioning and olfaction.

How the Nose Prepares Air

We are designed primarily to breathe through the nose. Unlike the mouth, the nose conditions incoming air: it filters out dust and pollutants, warms the air to near body temperature, and humidifies it to protect delicate lung tissue. Without these processes, cold, dry air can irritate and constrict the airways and increase inflammation.

Two Nostrils, Two Olfactory Inputs

Smell and breathing are closely linked. Odor molecules enter the nostrils, dissolve in the mucus lining and bind to sensory neurons that send signals to the brain. Because each nostril experiences different airflow speeds during the nasal cycle, they sample odors differently.

When airflow is slower through a more closed nostril, slowly absorbed chemicals have more time to dissolve into mucus and stimulate receptors. Conversely, the more open nostril’s faster airflow favors quickly dissolving molecules and can deliver them across a larger area of olfactory tissue. Together, these complementary inputs give the brain richer information about odor identity and intensity.

"You perceive chemicals differently because they are absorbing differently," says Thomas Hummel, head of the Interdisciplinary Centre for Smell and Taste at Dresden University of Technology.

Smell Localization and Experiments

Although the distance between nostrils is small compared with eyes or ears, the brain can use subtle differences between the two inputs to help locate scents. In one experiment, blindfolded volunteers followed a 33-foot chocolate trail through grass. When a device mixed odors so both nostrils received identical input, participants became slower and less accurate at tracking the scent—evidence that the natural asymmetry between nostrils helps with odor localization.

Possible Defense Against Infection

Two nostrils may offer benefits beyond sensing and conditioning. During colds one nostril often becomes markedly congested while the other remains clearer. Severe blockage can raise the temperature inside the congested passage, and because many cold viruses reproduce less effectively at higher temperatures, alternating congestion might help limit viral replication.

Conclusion

Two nostrils are far from redundant. The nasal cycle and the complementary roles of each nostril improve how we condition air, detect different odor molecules, and even locate smells. Next time you take a deep breath or enjoy a pleasant scent, remember that two nostrils working in alternation make that experience richer and healthier.