Nasal polyps form when chronic inflammation causes the lining of your sinuses to swell, fill with fluid, and eventually droop into your nasal passages as soft, painless growths. The underlying cause isn’t a single event but a cycle: something triggers prolonged inflammation in the sinuses, the tissue swells repeatedly, and over time those swollen areas develop into polyps. What kicks off that cycle varies from person to person, but the inflammation itself follows recognizable patterns.
Chronic Inflammation Is the Core Driver
The tissue lining your sinuses has a built-in defense system. Tiny hair-like structures called cilia sweep away irritants and mucus, while tight junctions between cells act as a physical barrier against bacteria, allergens, and pollutants. In people who develop nasal polyps, both of these defenses break down. The tight junctions between cells become leaky, cilia can’t clear debris efficiently, and the tissue produces fewer natural antimicrobial compounds. The result is that irritants linger in the sinuses far longer than they should, keeping the immune response stuck in “on” mode.
Once that happens, immune cells flood the area. In Western populations, the dominant pattern is called type 2 inflammation, driven by specific immune signals that attract eosinophils, a type of white blood cell associated with allergic reactions. Eosinophils release toxic proteins that damage the sinus lining further, which triggers more inflammation, which attracts more eosinophils. This self-reinforcing loop is what turns temporary swelling into permanent polyp tissue. Mast cells and basophils also accumulate in the area, contributing to excess mucus production and further tissue swelling.
Not all nasal polyps follow this pattern, though. A second type, called non-eosinophilic polyps, involves a different set of immune pathways more associated with bacterial infection than allergic response. This distinction matters practically: eosinophilic polyps tend to respond well to steroid treatment but have high recurrence rates after surgery, while non-eosinophilic polyps are more easily controlled with surgery and respond poorly to steroids.
Conditions That Raise Your Risk
Several health conditions dramatically increase the likelihood of developing nasal polyps, and in many cases polyps are considered a feature of the disease rather than a coincidence.
- Asthma: The same type 2 inflammatory process that narrows airways in asthma drives polyp growth in the sinuses. People with asthma, particularly severe or poorly controlled asthma, develop polyps at much higher rates.
- Allergic rhinitis: Chronic allergic inflammation in the nasal passages creates the prolonged immune activation that leads to tissue remodeling and polyp formation.
- Aspirin-exacerbated respiratory disease (AERD): This condition, sometimes called Samter’s triad, involves nasal polyps, asthma, and reactions to aspirin or similar pain relievers. It affects roughly 10% of people with nasal polyps and tends to cause especially aggressive, fast-recurring growths.
- Cystic fibrosis: Thick, sticky mucus overwhelms the sinus lining’s ability to clear itself, creating a perfect environment for chronic infection and polyp development. Polyps in children are uncommon enough that their presence can prompt testing for cystic fibrosis.
- Eosinophilic granulomatosis with polyangiitis (formerly Churg-Strauss syndrome): This rare blood vessel disease causes widespread eosinophilic inflammation, including in the sinuses.
Nasal polyps have also been linked to obstructive sleep apnea, likely because large polyps physically obstruct airflow during sleep.
How Aspirin Sensitivity Fuels Polyp Growth
AERD deserves its own explanation because the mechanism is distinct and the polyps it produces are notably severe. In people with this condition, the body’s metabolism of a fatty acid called arachidonic acid is fundamentally skewed. Normally, this molecule gets processed into a mix of pro-inflammatory and anti-inflammatory compounds that balance each other out.
In AERD, taking aspirin or ibuprofen blocks one processing pathway, which shunts the raw materials into another pathway that produces powerful inflammatory molecules called leukotrienes. At the same time, levels of a key anti-inflammatory compound (prostaglandin E2) drop. The combined effect is a surge in eosinophil activity, blood vessel dilation, excess mucus secretion, and bronchoconstriction. Over time, this creates intense, persistent sinus inflammation and polyps that grow back aggressively even after surgical removal.
The Role of Bacteria and Biofilms
Staphylococcus aureus, a common bacterium that lives in many people’s noses without causing problems, plays a surprisingly active role in polyp formation. In people with chronic sinusitis, staph bacteria organize into biofilms: structured colonies that embed themselves in the sinus lining and resist both the immune system and antibiotics. These biofilms are found in 42 to 80% of chronic sinusitis cases, with an even higher prevalence when polyps are present.
What makes staph particularly damaging is its production of superantigens, toxins that essentially hijack the immune system. Normal immune responses are targeted and proportional. Superantigens bypass the usual checks and trigger a massive, uncoordinated activation of immune cells. This causes an explosion of the same inflammatory signals (IL-4, IL-5, IL-13) that drive eosinophilic polyp growth, along with a flood of histamine and antibodies. The result is severe, self-perpetuating inflammation that ordinary treatments struggle to quiet down.
Staph bacteria also directly interact with receptors on sinus tissue cells, stimulating them to produce inflammatory signals even without releasing toxins. And certain fungi, particularly Alternaria species, may work alongside staph to break down the sinus lining’s physical barrier, making it easier for bacteria to invade deeper tissue.
Genetics and Family History
Nasal polyps tend to cluster in families, suggesting a genetic component to susceptibility. That said, most people with polyps don’t have a single identifiable gene mutation causing the problem. The genetic influence is more likely a collection of inherited traits: a tendency toward leaky sinus barriers, an immune system predisposed to eosinophilic responses, or subtle differences in mucus production.
The clear exceptions are cystic fibrosis and primary ciliary dyskinesia, both of which involve well-characterized genetic mutations. Cystic fibrosis results from mutations in the CFTR gene on chromosome 7, which controls fluid balance across cell membranes. Primary ciliary dyskinesia involves defective cilia that can’t clear mucus properly. In both cases, the genetic defect creates the conditions for chronic sinus inflammation from an early age. Beyond these specific conditions, researchers have identified differences in how certain genes are activated or silenced in polyp tissue, pointing toward inherited susceptibility rather than a single “polyp gene.”
Environmental and Lifestyle Triggers
Air pollution is an emerging risk factor for both developing nasal polyps and making existing ones worse. A study examining five types of air pollution found that short-term exposure to particulate matter, sulfur dioxide, nitrogen dioxide, carbon monoxide, and ozone all correlated with increased disease severity in people with polyps. Fine particulate matter (PM2.5) stood out in particular: at high concentrations, each unit increase in PM2.5 was associated with a roughly 5% increased risk of developing the eosinophilic type of polyps, the kind most prone to recurrence.
Smoking is more commonly associated with eosinophilic polyps than non-eosinophilic ones, fitting the pattern of an irritant that promotes the type 2 inflammatory response. Occupational exposures to dust, chemicals, or fumes likely follow a similar logic, though the evidence is less well quantified. Chronic exposure to airborne allergens, whether from dust mites, mold, pet dander, or pollen, keeps the sinus lining in a state of ongoing immune activation, particularly in people whose immune systems are already wired for allergic responses.
Why Polyps Keep Coming Back
One of the most frustrating aspects of nasal polyps is their recurrence rate. This makes more sense when you consider that removing a polyp surgically doesn’t address the underlying inflammatory environment that created it. If the immune system continues producing eosinophilic inflammation, if staph biofilms remain embedded in the sinus lining, or if the epithelial barrier stays compromised, new polyps will form in the same tissue. This is why treatment increasingly focuses on controlling the inflammatory process itself rather than just removing the growths, and why identifying which type of inflammation is driving your polyps matters for choosing an approach that actually keeps them from returning.