Nasal polyps form when chronic inflammation in the sinuses causes the nasal lining to swell, fill with fluid, and grow into soft, painless masses that hang into the nasal passages. About 0.65% of the global population has them. The exact reason some people develop polyps while others don’t remains unclear, but the process involves a specific type of runaway immune response combined with genetic susceptibility, associated health conditions, bacterial activity, and environmental exposures.
The Inflammatory Process Behind Polyp Growth
Nasal polyps aren’t tumors or abnormal growths in the traditional sense. They’re the result of a particular kind of inflammation called type 2 inflammation that, in healthy people, would resolve on its own but in polyp-prone individuals never fully shuts off. Three signaling molecules drive this process: IL-4, IL-5, and IL-13. These are chemical messengers produced by immune cells, and they remain persistently elevated in nasal polyp tissue compared to healthy sinus tissue.
Here’s what that looks like in practice. IL-4 and IL-13 recruit a type of white blood cell called eosinophils to the sinus lining and keep them there. IL-5 signals the bone marrow to produce even more eosinophils. Meanwhile, IL-13 stimulates mucus-producing cells to go into overdrive, flooding the sinuses with thick mucus. The immune system also produces large amounts of IgE, the same antibody involved in allergic reactions, which triggers mast cells to release still more inflammatory signals. The result is a self-reinforcing loop: inflammation damages the sinus lining, the damaged lining lets in more irritants and bacteria, and those irritants provoke more inflammation. Over time, the chronically swollen tissue balloons outward into a polyp.
People with nasal polyps have measurably higher levels of these inflammatory markers in both eosinophilic polyps (the most common type) and non-eosinophilic polyps. The tissue itself undergoes remodeling, meaning its structure physically changes in ways that make the barrier between the inside of your nose and the outside world less effective at keeping irritants out.
Asthma, Allergies, and Related Conditions
Nasal polyps rarely appear in isolation. They cluster with other inflammatory conditions, especially asthma. About 26 to 48% of people with nasal polyps also have asthma, and polyp patients are 7.5 times more likely to have asthma than people with chronic sinus inflammation who don’t develop polyps. Roughly 88% of asthmatics show at least some radiographic evidence of sinus inflammation, though only about 7% develop full polyps.
Allergic rhinitis (hay fever) also shows a strong connection, though it works differently than you might expect. The percentage of allergy sufferers who develop polyps is about the same as the general population, around 0.5 to 4.5%. But when researchers look at it from the other direction, 51 to 86% of people who already have polyps test positive for at least one airborne allergen. No single allergen has been identified as the trigger, though sinus symptoms can worsen during allergy season. Other conditions that appear more frequently in polyp patients include acid reflux and sleep apnea.
Men develop nasal polyps more often than women, though the reasons for this remain unknown.
Aspirin Sensitivity and Samter’s Triad
One of the most severe forms of nasal polyps occurs in a condition sometimes called Samter’s triad: the combination of nasal polyps, asthma, and a reaction to aspirin or similar anti-inflammatory drugs. The underlying problem is a malfunction in how the body processes a fatty acid called arachidonic acid. Normally, this molecule gets broken down through multiple pathways. In people with Samter’s triad, the pathway that aspirin blocks is already underperforming, so taking aspirin forces even more arachidonic acid through an alternative route that produces potent inflammatory compounds called leukotrienes. These leukotrienes drive intense, persistent inflammation that fuels aggressive polyp growth and makes the polyps more likely to return after surgery.
The Role of Bacteria
Bacteria living in the sinuses, particularly Staphylococcus aureus, play an important role in keeping nasal polyps inflamed and growing. These bacteria produce toxins that act as “superantigens,” meaning they activate the immune system far more broadly than a normal infection would. One well-studied toxin damages the sinus lining in several ways at once: it breaks down the tight junctions between cells that normally form a protective barrier, triggers the production of inflammatory signals, and generates reactive oxygen species that cause further tissue damage.
The toxin also pushes the immune system toward the same type 2 inflammatory pattern that characterizes polyps, promoting eosinophil activity and IgE production. Studies have shown that exposure to this toxin increases the permeability of polyp tissue in a dose-dependent manner, meaning more toxin equals more barrier breakdown. IgE antibodies specifically targeting these bacterial toxins have been found in polyp patients, suggesting that an allergic-type reaction to the bacteria themselves helps sustain the chronic inflammation. This creates another vicious cycle: polyps provide a sheltered environment where bacteria can form colonies, and those bacterial colonies produce toxins that make the polyps worse.
Cystic Fibrosis
Cystic fibrosis is one of the strongest individual risk factors for nasal polyps. The incidence ranges from 6 to 48% across different studies, with one finding polyps in 39.1% of patients. The mechanism is distinct from typical polyps. Cystic fibrosis causes abnormally thick mucus throughout the body, and in the sinuses this thick mucus slows the tiny hair-like structures (cilia) that normally sweep debris and pathogens out of the nasal passages. The resulting blockage creates a low-oxygen, high-carbon-dioxide environment that promotes infection and chronic inflammation. Polyps in cystic fibrosis patients tend to appear after age six and are sometimes the symptom that leads to a cystic fibrosis diagnosis, though this happens in fewer than 1% of cases.
Genetic Susceptibility
No single gene causes nasal polyps, but genetics clearly influence who develops them. Research on asthmatic patients has identified specific variations in a gene called HLA-DRA, part of the immune system’s machinery for recognizing foreign substances, that are significantly associated with polyp development. Four genetic variations and one combination of variations showed a strong link to polyps in asthmatic patients. Interestingly, these genetic markers appeared more relevant in aspirin-tolerant asthmatics than in those with aspirin sensitivity, suggesting that different subtypes of nasal polyps may have different genetic underpinnings.
The broader picture is that nasal polyps likely involve multiple genes interacting with environmental factors. No specific genetic or environmental factor has been definitively established as the sole cause, which is consistent with a disease driven by a complex interplay between inherited immune tendencies and external triggers.
Air Pollution and Environmental Triggers
Airborne particulate matter is increasingly recognized as a significant environmental risk factor. Fine particles smaller than 10 micrometers (and especially those smaller than 2.5 micrometers) penetrate deep into the nasal passages and trigger inflammatory responses in the airway lining. Epidemiological data from South Korea has linked higher particulate exposure to increased rates of chronic sinus disease.
What makes particulate matter particularly relevant to polyps is how it interacts with allergens. When the sinuses are exposed to both particulate matter and an allergen like dust mites simultaneously, the type 2 immune response is amplified beyond what either would cause alone. Animal studies confirm this: co-exposure to particulate matter and dust mites produced greater airway reactivity and higher levels of the same inflammatory signals found in nasal polyps. Diesel exhaust particles and dust particles both trigger the production of inflammatory compounds in nasal tissue through specific cellular signaling pathways. For people already predisposed to polyps, living or working in areas with high air pollution likely increases the frequency and severity of flare-ups.
Why the Lining Breaks Down
A common thread across all these causes is damage to the epithelial barrier, the thin layer of cells lining the inside of your sinuses. In healthy sinuses, this barrier keeps irritants, allergens, and bacteria on the outside. In polyp-prone individuals, three things converge to compromise it: the barrier itself has structural defects (possibly genetic), bacteria and their toxins actively break it down, and the immune system’s inflammatory response causes collateral damage to the very tissue it’s trying to protect. Once the barrier is weakened, substances that would normally be kept out penetrate into deeper tissue, activating more immune cells, producing more inflammation, and driving the cycle that ultimately produces polyps.
This is why nasal polyps are so prone to recurrence even after surgical removal. Surgery removes the physical polyp, but the underlying barrier dysfunction, immune dysregulation, and environmental exposures that created it persist. Effective long-term management typically requires addressing the inflammatory process itself rather than just the polyps it produces.