Why we have two nostrils instead of one big hole
If you close one eye or put a finger to your ear, you immediately feel a sense of loss. Two eyes help us see the world while two ears help us locate sounds. But there isn’t the same dramatic sense of loss if you block one nostril. Unlike the eyes or ears, they are in almost exactly the same position on our face. So why not just have a big hole like the mouth? Why do we have two nostrils?
It turns out that each nostril behaves differently from the other nostril throughout the day. This is called the nasal cycle and it plays a vital role in our overall health. At a certain point, one nostril draws in air more quickly. Later, the dominant nostril shifts. Throughout the day, the dominant nostril keeps changing. This alternating airflow seems to help us breathe and smell more effectively.
How our nostrils take turns breathing
We are programmed to breathe through our noses. Mouth breathing is only really necessary when we need more air during exercise or respiratory distress, or when the nose is blocked. Unlike the mouth, the nose does more than just draw air in and out of the lungs. One of its main functions is to prepare air for the lungs, something the mouth cannot do. The nose filters dust and pollutants, warms the air to body temperature, and adds the right amount of humidity so that the air is 100% humidity before reaching the lungs. Without this process, the air would be colder and drier, which would irritate and constrict the airways and potentially lead to inflammation.
Having two nostrils helps the nose cope with this demanding task of preparing air for the lungs. “The fact that we have two nostrils is not unusual, because we have two eyes and two ears,” says Ronald Eccles, an emeritus professor at Cardiff University who founded the Common Cold Centre. “What’s unusual is that the nostrils alternate airflow from side to side. This can allow one side of the nose to rest.”
It is better for you to breathe through your nose than to breathe through your mouth. Video: Why you should breathe through your nose with James Nestor / BBC Maestro
Studies have shown that at no time do both nostrils take in the same amount of air. Every few hours, one side of the nose is opened more and handles most of the airflow while the other side handles less air, allowing it to collect moisture.
How each nostril smells differently
Smell is closely linked to breathing. When we breathe, odor molecules enter the nostrils, dissolve in the mucous membrane, and bind to neurons that send signals to the brain. Thanks to the nasal cycle, air circulates through the nostrils at different speeds and each nostril therefore handles odors differently.
When we breathe, one nostril is closed more than the other and therefore the airflow is slower. This slower airflow means that slow-absorbing chemicals have more time to dissolve into the mucosa. Experiments suggest that people smell chemicals absorbed slowly through a resting or more closed nostril more strongly.
However, the more closed nostril is not as effective at detecting quickly dissolving chemical odors. Meanwhile, the faster airflow from the more open nostril means the fast-dissolving chemicals can reach more of the odor-detecting tissue in your nose and send more signals to the brain. So basically each nostril smells slightly different.
“It’s not a dull smell that hits you,” says Thomas Hummel, director of the Interdisciplinary Center for Smell and Taste at the Technical University of Dresden.
“You perceive chemicals differently because they are absorbed differently.” This usually happens without us being aware of it. The alternating airflow ensures that each nostril provides different inputs to the brain. The brain then combines this information to get more information and a richer sense of smell.
Two nostrils improve our ability to locate odors
The distance between our two nostrils is not as great as the distance between the eyes or ears. But having two nostrils can still help us locate odors. “The brain is capable of using even small pieces of information,” says Matthew Grubb, professor of neuroscience at King’s College London, who focuses on the olfactory system. “There is pretty strong evidence that one of the things the nervous system can do is use information from both nostrils to determine where an odor is coming from.”
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In one experiment, scientists asked blindfolded participants to sniff out a 33-foot-tall trail of chocolate in the grass. Participants wore a device that fitted to the nose and mixed odors from the outside world so that there was no difference in the smell from each nostril. This made participants slower and less accurate in locating and tracking the chocolate odor than when they were not wearing the device.
Two nostrils could give us an edge against colds
Two nostrils could even provide benefits other than breathing and smell: they could help us fight viral infections. When you have a cold, one nostril is much more congested while the other takes care of most of the breathing. A severely blocked nostril leads to an increase in the temperature of the nasal passages. This can repel cold viruses, because viruses don’t reproduce well at high temperatures.
Having two nostrils is far from redundant. We may not notice the nasal cycle, but it remains a key part of how the nose works. The nostrils work together to improve the way we breathe and smell. So the next time you take a deep breath or smell a delicious pie, don’t take both nostrils for granted.
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