Congestion isn’t caused by too much mucus blocking your nose, even though that’s what it feels like. The primary cause is swollen blood vessels inside the nasal lining. When something triggers inflammation, whether a cold virus, allergens, or even a shift in weather, the tissue lining your nasal passages fills with blood and fluid, narrowing the space air can pass through. Mucus overproduction plays a supporting role, but the swelling itself is what makes breathing feel so difficult.
The Structures That Swell
Inside each nostril sit curved bony shelves called turbinates, lined with a soft, moist tissue called mucosa. You have three sets on each side, but the lower and middle turbinates do the most work. They warm, humidify, and filter the air you breathe before it reaches your lungs. Most of your airflow passes between these two structures, which means even a small amount of swelling in this area has an outsized effect on how easily you can breathe.
The mucosa covering your turbinates is rich with blood vessels, and these vessels are designed to change size. When they expand, the tissue swells and partially blocks the airway. When they constrict, the passage opens back up. This flexibility is a feature, not a flaw. It becomes a problem only when inflammation keeps those vessels dilated far longer or wider than normal.
What Triggers the Swelling
Your nervous system controls the baseline size of nasal blood vessels through two competing signals. One branch (the sympathetic system) keeps vessels constricted and airways open. The other (the parasympathetic system) relaxes vessels and allows them to expand. Congestion can result from either a weakened constricting signal or an overactive relaxing signal, both of which let blood pool in the nasal tissue.
On top of that nervous system control, your body releases chemical messengers that directly widen blood vessels and make them leak fluid into surrounding tissue. Histamine is the most familiar of these, but it’s far from the only one. Other signaling molecules, including prostaglandins and bradykinin, act on the nasal lining to produce swelling, fluid leakage, and irritation. Bradykinin is particularly potent: even when sprayed onto the nasal lining of a healthy person, it triggers congestion, a runny nose, and a sore throat within minutes.
During an allergic reaction, levels of these chemical mediators spike sharply in nasal secretions. During a viral infection like a cold, the immune response floods the area with inflammatory signals that accomplish much the same thing. The trigger is different, but the downstream mechanism, swollen vessels and leaky tissue, is remarkably similar.
Where Mucus Fits In
While swelling is the main driver of that blocked feeling, mucus production genuinely does ramp up during illness and allergy. The cells responsible are goblet cells, scattered throughout the airway lining. Normally, they produce a thin layer of mucus that traps particles and keeps tissue moist. But inflammatory signals, particularly from the immune system’s allergy pathway, can reprogram these cells into overdrive. Immune molecules push goblet cells to multiply, grow larger, and produce far more mucus than usual.
This extra mucus combines with fluid that leaks from swollen blood vessels, creating the thick, heavy sensation in your sinuses. When you blow your nose and get temporary relief, you’re clearing this fluid, but the underlying tissue swelling remains. That’s why the stuffiness returns so quickly.
The Natural Nasal Cycle
Even when you’re perfectly healthy, your body cycles congestion between your two nostrils throughout the day. One side gently swells while the other opens up, then they switch. This is called the nasal cycle, and it’s driven by alternating dilation and constriction of blood vessels in the turbinates and septum. Most people never notice it because total airflow stays roughly the same.
During a cold or sinus infection, this cycle becomes much more dramatic. The amplitude and frequency both increase, meaning the congested side gets significantly more blocked than it normally would. Inflammation raises the baseline pressure in the blood vessels, so when the cycle shifts to swell one nostril, it swells far more than usual. This is why lying down with a cold often makes one side feel completely sealed shut, especially whichever side is lower. Rolling over typically shifts the congestion to the other nostril within a few minutes, which is the nasal cycle at work under amplified conditions.
Environmental Triggers
You don’t need an infection or allergy to get congested. Changes in temperature or humidity can trigger the nasal lining to swell on their own, a condition broadly called nonallergic rhinitis. Walking into cold air, sitting near a heater, or moving between a dry indoor environment and humid outdoor air can all provoke a stuffy or runny nose. The nasal lining reacts to these shifts as part of its job regulating the temperature and moisture of incoming air, but in some people, the response overshoots.
Strong odors, smoke, air pollution, and even spicy food can produce the same effect. The nervous system pathways involved are similar to those triggered by infection or allergy. The parasympathetic branch fires in response to the irritant, blood vessels dilate, and the mucosa swells. For people who experience this frequently without any identifiable allergen, the underlying issue is often an overly sensitive nervous system response rather than a structural problem in the nose.
Why Some Decongestants Don’t Work
Because congestion is fundamentally about blood vessel dilation, effective decongestants work by constricting those vessels. Nasal spray decongestants deliver medication directly to the swollen tissue, which is why they tend to provide fast, noticeable relief (though using them for more than a few days can cause rebound congestion that’s worse than what you started with).
Oral decongestants are a different story. In 2023, an FDA advisory committee unanimously concluded that oral phenylephrine, the active ingredient in many popular over-the-counter cold medications, does not work as a nasal decongestant at its standard dose. The FDA has since proposed removing it from shelves. The issue isn’t safety but efficacy: the drug is broken down so extensively during digestion that too little reaches the nasal blood vessels to produce a meaningful effect. If you’ve ever taken a cold pill and felt no improvement in your breathing, this may be why. The FDA’s action applies only to the oral form; phenylephrine in nasal sprays, which bypasses digestion entirely, is a separate matter.
Pseudoephedrine, another oral decongestant sold behind the pharmacy counter in many states, does constrict nasal blood vessels effectively when taken by mouth. For allergic congestion, antihistamines address the problem from a different angle by blocking histamine before it can trigger vessel dilation in the first place. Steroid nasal sprays reduce the underlying inflammation over days to weeks, making them particularly useful for chronic or seasonal congestion rather than a single bad cold.