Why does only one nostril get blocked?
― Your left and right nose take turns resting every few hours
You're not catching a cold, yet you notice one nostril is blocked. A while later, it's the other side that's blocked. This isn't a malfunction. Right now, inside the noses of many people, this "left-right shift work" is happening. And remarkably, just a slight narrowing of the passage cuts airflow dramatically.
At night, you lie down on your side under the covers. After a while, the nostril on the lower side starts to feel blocked.
You roll over to face the other way, and the side that was blocked clears right up — while the other side now starts to block.
Even during the day, if you press one nostril shut and breathe, you'll often notice a clear difference in how easily air passes through each side. What on earth is happening inside your nose?
There are only two reasons
The inner wall of the nose has a mucous membrane that fills with blood and swells like a sponge. The body alternates which side swells and which side shrinks.
The amount of air flowing through a tube is proportional to the "fourth power" of its thickness (radius). Just a 20% narrowing in radius drops the flow to about 40%. That's why it feels "blocked."
In other words, when one nostril feels blocked, the passage isn't completely closed. Even a slight narrowing feels like "barely any air gets through," because of the law governing flow. Let's look at this step by step.
Inside the nose, there's a "cushion" that swells
Deep inside each nostril is a long, narrow passage. From its outer wall, a fold-like ridge projects inward. Beneath the mucous membrane on the surface of this ridge lies a network of fine blood vessels that can fill with blood. When blood fills in, the membrane swells; when it drains, the membrane shrinks.
How much blood fills in is controlled by nerves you can't consciously move (the autonomic nervous system). In most people, while one side is swollen, the other side is shrunk. After a few hours, the roles switch. This is called the "nasal cycle." It's reported to occur in about 70% of adults, and the cycle length varies considerably between people, from roughly 2 to 7 hours.
Look at Figure 1. How easily air passes through each side rises and falls like a seesaw. Yet the combined airflow through both sides together is said to stay roughly constant. While one side rests, the other works a bit harder. That's why, normally, the nose as a whole rarely feels stuffy.
Why does "a little swelling" block so much?
Here's where the behavior of air or water flowing through a narrow tube comes in. Near the tube wall, air is held back by the wall and barely moves. Only the middle part, away from the wall, can flow fast.
As the tube narrows, two effects combine: the "passage area shrinks" and the "fraction held back by the wall grows." As a result, the flow drops in proportion to the fourth power of the radius. If the radius becomes 0.8 times as large, the flow is about 0.8 multiplied by itself four times — roughly 0.41 times. At half the thickness, only 1/16th of the air flows.
Try moving the slider in Figure 2. Narrowing the blocked side's passage just slightly sends most of the air over to the other side. Because the nasal passage is only a few millimeters wide to begin with, even a slight swelling of the membrane produces this large effect.
It's reported that lying on one side can block the lower nostril within minutes to around ten-odd minutes. The nerves are thought to sense the pressure on that side of the body, causing the lower membrane to swell and the upper one to shrink. That's why the sides switch every time you roll over.
Alternating blockage between sides is normal. On the other hand, if the same side stays blocked for weeks on end, or if blood or discolored discharge keeps coming from only one side, it could be due to the shape of the nasal passage or a growth. In young children, it may even be a toy or other object stuck inside. Also, over-the-counter decongestant nasal sprays that constrict blood vessels can, with prolonged use, actually make congestion worse. Follow the usage period stated in the instructions.
Summary
Having just one nostril blocked comes from the "nasal cycle," in which the mucous membrane deep in the nose swells on alternating sides — a normal mechanism occurring daily in many people. The combined airflow of both sides stays roughly constant. And because airflow is proportional to the fourth power of the passage's thickness, even a slight swelling of the membrane is enough for that side to feel "blocked."
The nose works by alternating left and right.
A 20% drop in thickness cuts airflow by 60%.
The behavior of things flowing through narrow tubes matters elsewhere in the body too. The mechanism that returns blood from your legs to your heart is explained in "Why do your legs swell when you stand for too long?", and what determines how long you can hold your breath is covered in "Why can't you hold your breath for even a minute?"
- Hold a cold hand mirror, or the back of a metal spoon, horizontally just below your nostrils.
- Close your mouth and breathe out normally through your nose. Two fog patches will form on the mirror, one for each side. Compare their size — the larger one is the side flowing more easily right now.
- Repeat every 2–3 hours and note which side was larger each time. Try it again after lying on your side for about 10 minutes, and the fog patch on the side you were lying on should be smaller.
Comparing airflow with a fogged mirror is a simple check that's long been used in clinical exams, too. Some people's cycles aren't very distinct, but that too falls within normal individual variation.
For those who want to know more ― terms, formulas, and textbook connectionsWe clearly mark which level each part belongs to, from junior high science to university specialist subjects
- JHSCovered in junior high school science
- HSCovered in high school "Physics" or "Biology"
- HS+High school enrichment content, or textbook sidebar material
- Univ.Not covered in high school — university-level specialist content (fluid dynamics, physiology, otorhinolaryngology)
- ResearchNot even taught as settled fact at university — something researchers are still actively investigating
JHSTerminology: this phenomenon has a name
- Nasal cycle: the phenomenon in which the mucous membranes of the left and right nostrils alternately swell and shrink every few hours.
- Nasal turbinate (conchae): a fold-like ridge projecting from the outer wall of the nasal passage. Beneath its surface mucous membrane lies a network of blood vessels that fills and swells.
- Autonomic nervous system: nerves that regulate bodily functions independent of conscious will. In the nose, it separately controls the amount of blood filling the membrane on each side.
JHSHSChecking with a formula: how much air flows through a nostril at 80% thickness?
The amount of air or water flowing slowly (smoothly) through a narrow tube is given by the following formula. Real airflow in the nose is actually more complex, but we use this as a guide to "how much thickness matters."
| In symbols | Q = π × r⁴ × ΔP ÷ ( 8 × μ × L ) |
| In words | Flow per second = pi × radius to the 4th power × pressure difference ÷ ( 8 × air viscosity × tube length ) |
| Where it comes from | It follows from the balance between the force of the pressure difference pushing air through, and the viscous force holding it back at the walls (the Hagen–Poiseuille equation). Since the left and right nasal passages meet at the back of the throat, the driving pressure difference is the same for both sides. So the ratio of left-to-right flow equals the fourth power of the ratio of their radii. |
| Q | Volume of air flowing per second (cubic meters per second) |
| r | Radius of the passage (meters) |
| ΔP | Pressure difference between the nostril entrance and the back of the throat (pascals) |
| μ | Viscosity of air (pascal-seconds) |
| L | Length of the passage (meters) |
| Radius of the open side (reference) | 1 |
| Radius of the blocked side (assumed) | 0.8 times the open side |
| Pressure difference, viscosity, length | same on both sides |
| Radius squared | 0.8 × 0.8 = 0.64 |
| Radius to the 4th power (= flow ratio) | 0.64 × 0.64 ≈ 0.41 |
| Combined flow (open side = 1) | 1 + 0.41 = 1.41 |
| Share flowing through the blocked side | 0.41 ÷ 1.41 ≈ 0.29 |
| For comparison: if the radius is halved, squared is | 0.5 × 0.5 = 0.25 |
| Its 4th power (flow ratio) | 0.25 × 0.25 ≈ 0.063 |
Just a 20% reduction in thickness drops that side's flow to about 40%, leaving only about 30% of the total flowing through it. At half the thickness, it's about 1/16th. The proportions shown by the slider in Figure 2 are drawn using this same formula.
HSHS+Viscous flow and bodily regulation
HSFluids have viscosity: the part touching the wall stops, while the center flows fastest. In high school biology, you learn that the autonomic nervous system, which works independent of will, has two branches: sympathetic and parasympathetic. The blood vessels in the nasal membrane are thought to constrict mainly when sympathetic activity increases, shrinking the membrane and improving airflow.
HS+In the Hagen–Poiseuille equation, flow is proportional to the fourth power of the radius because both the cross-sectional area (proportional to radius squared) and the center-line flow speed (also proportional to radius squared) depend on thickness. The same reason explains why a slight constriction of a blood vessel can greatly change blood flow.
Univ.Nasal airway resistance and flow regimes
Actual airflow inside the nose isn't clean laminar flow through a round pipe. The passage is a flat, slit-like gap, and depending on breath strength, the flow can start to turn turbulent. The Reynolds number is used as a guide to whether flow is laminar or turbulent. Clinically, rhinomanometry (measurement of nasal airway resistance) calculates resistance for each side from its pressure difference and flow rate. Nasal resistance is said to account for roughly half of the total resistance of the airway. The network of blood vessels beneath the membrane is called a capacitance vessel, and left-right differences in sympathetic tone are thought to produce the nasal cycle.
📖 For the derivation of the formula and further reading: Hagen–Poiseuille flow (Wikipedia, Japanese) ／ Nasal turbinate (Wikipedia, Japanese)
ResearchWhat's still not fully understood
- Why is alternation necessary at all? The leading theory is that the resting side's membrane recovers its ability to humidify air and clear away foreign particles, but this isn't settled.
- Does it broaden how we perceive smell? Experimental results suggest that, because flow speed differs between the two sides, odor molecules that dissolve easily and those that don't may each be sensed better by a different nostril.
- Where is the clock that sets the cycle? Which part of the brain commands the left-right switch, and how it relates to the cycle during sleep, are still being investigated.
In other words, even this article describes things "as currently understood." The nasal cycle has been known for over a century, yet its role is still debated.
Connections to the curriculum (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| JHS | Science, Year 2, "Animal body structure and function (respiration)" | The airway from nose to lungs |
| HS | Biology, "The autonomic nervous system" | How blood filling of the membrane is controlled separately on each side |
| HS+ | Physics (enrichment), "Viscosity and flow" | Why flow is proportional to the fourth power of the radius |
| Univ. | Fluid dynamics, physiology, otorhinolaryngology | The Hagen–Poiseuille equation, Reynolds number, rhinomanometry |
| Research | Olfaction and autonomic nervous system research | The meaning of alternation, the mechanism setting the cycle |
| ― | Everyday connections | Rolling over and nasal congestion, overusing nasal sprays, when to see a doctor |
- Kayser, R. (1895) Die exakte Messung der Luftdurchgängigkeit der Nase. Archiv für Laryngologie und Rhinologie, 3. (Regarded as the first recorded description of the nasal cycle.)
- Hasegawa, M., Kern, E. B. (1977) The human nasal cycle. Mayo Clinic Proceedings, 52.
- Eccles, R. (1996) A role for the nasal cycle in respiratory defence. European Respiratory Journal, 9.
- Sobel, N. et al. (1999) The world smells different to each nostril. Nature, 402.
- English Wikipedia, "Nasal cycle"
※This article is a general-audience science explainer. The figures given are approximations intended to aid understanding of the underlying mechanism. If nasal congestion persists for a long time, or if symptoms affect only one side persistently, please consult a doctor (an ear, nose, and throat specialist).