A runny nose, medically known as rhinorrhea, is the discharge of thin, watery mucus from the nose. This common symptom results from various biological responses within the nasal passages. While often a minor inconvenience, it signals the body’s attempts to protect itself or react to environmental changes.
The Body’s Nasal Defense System
The nasal passages play a role in conditioning the air before it reaches the lungs. This involves warming, humidifying, and filtering out foreign particles. Nasal mucus, a sticky fluid produced by glands within the nose and sinuses, forms a protective barrier. It consists primarily of water, along with proteins, antibodies, enzymes, and glycoproteins, trapping dust, allergens, bacteria, and viruses, preventing them from entering the lungs.
Microscopic, hair-like structures called cilia line the nasal passages, moving this mucus and trapped debris towards the back of the throat. This “mucociliary escalator” system allows the mucus to be swallowed, clearing foreign substances from the respiratory tract. Continuous mucus production and movement help maintain clear airways and protect the respiratory system.
Common Causes: Infections and Allergies
Viral infections, such as the common cold or flu, frequently cause a runny nose. When viruses enter the nasal passages, the immune system releases chemical messengers called cytokines. These signal nasal glands to produce more mucus, and blood vessels in the nose expand, increasing fluid leakage. This increased mucus production helps trap and flush out the virus, preventing further spread into the lungs.
Allergies are another common cause of increased nasal discharge. When exposed to allergens like pollen, dust mites, or pet dander, the immune system mistakenly identifies these substances as threats. This triggers histamine release from mast cells. Histamine causes inflammation, increases blood flow, and prompts nasal membranes to produce more mucus, often resulting in watery discharge, sneezing, and itching.
Beyond the Usual Suspects: Other Triggers
Environmental factors and bodily reactions can also lead to a runny nose. Irritants such as tobacco smoke, air pollution, or strong odors stimulate the nasal lining, causing excess mucus production as a protective mechanism. Exposure to cold air also triggers a runny nose, often termed “cold-induced rhinitis” or “skier’s nose.” The nose warms and humidifies cold, dry air before it reaches the lungs; in response, nasal glands increase fluid production.
Eating spicy foods can induce a runny nose, a phenomenon known as gustatory rhinitis. Capsaicin, a chemical in spicy foods, stimulates nerve endings in the nose, triggering increased mucus production. Non-allergic rhinitis, or vasomotor rhinitis, causes allergy-like symptoms without an allergic cause. This condition involves the expansion of blood vessels in the nose, often triggered by changes in temperature or humidity, strong odors, or even stress.
Excess tears, from crying or eye irritation, can drain into the nasal cavity. Tears pass from tear ducts into nasal passages, mixing with nasal mucus and causing the nose to run. Some medications also cause a runny nose as a side effect, including certain blood pressure medications, oral contraceptives, and nonsteroidal anti-inflammatory drugs (NSAIDs). Overuse of decongestant nasal sprays can also lead to rebound congestion and a runny nose.
When to Consult a Doctor
While a runny nose is often temporary and resolves on its own, certain signs indicate a need for medical evaluation. Consult a healthcare provider if symptoms persist for more than 10 days or worsen. A high fever, or thick, yellow, or green nasal discharge with facial pain, can suggest a bacterial infection requiring treatment.
Medical attention is also warranted if discharge is bloody, especially after a head injury. For infants, a runny nose causing difficulty with nursing or breathing should prompt a doctor’s visit. These situations may point to underlying conditions needing professional diagnosis and management.