|Classification and external resources|
Labeled cross section of the nasal cavities
Rhinorrhea or rhinorrhoea is a condition where the nasal cavity is filled with a significant amount of mucous fluid. The condition, commonly known as "runny nose", occurs relatively frequently. Rhinorrhea is a common symptom of allergies or certain diseases, such as the common cold or hay fever. It can be a side effect of crying, exposure to cold temperatures, cocaine abuse  or withdrawal, such as from opioids like methadone. Treatment for rhinorrhea is not usually necessary, but there are a number of medical treatments and preventative techniques available.
Signs and symptoms 
Rhinorrhea is characterized by an excess amount of mucus produced by the mucous membranes that line the nasal cavities. The membranes create mucus faster than it can be processed, causing a backup of mucus in the nasal cavities. As the cavity fills up, it blocks off the air passageway, causing difficulty breathing through the nose. Air caught in nasal cavities, namely the sinus cavities, cannot be released and the resulting pressure may cause a headache or facial pain. If the sinus passage remains blocked, there is a chance that sinusitis may result. If the mucus backs up through the Eustachian tube, it may result in ear pain or an ear infection. Excess mucus accumulating in the throat or back of the nose may cause a post-nasal drip, resulting in a sore throat or coughing. Additional symptoms include sneezing, nosebleeds, and nasal discharge.
Cold temperature 
Rhinorrhea is especially common during winter months. Cold-induced rhinorrhea occurs due to a combination of thermodynamics and the body's natural reactions to cold weather stimuli. One of the purposes of nasal mucus is to warm inhaled air to body temperature as it enters the body. In order for this to happen, the nasal cavities must be constantly coated with liquid mucus. During cold, dry seasons, the mucus lining nasal passages tends to dry out, meaning that mucous membranes must work harder, producing more mucus to keep the cavity lined. As a result, the nasal cavity can fill up with mucus. At the same time, when air is exhaled, water vapor in breath condenses as the warm air meets the colder outside temperature near the nostrils. This causes an excess amount of water to build up inside nasal cavities. In these cases, the excess fluid usually spills out externally through the nostrils.
Rhinorrhea can be a symptom of other diseases, such as the common cold or influenza. During these infections, the nasal mucous membranes produce excess mucus, filling the nasal cavities. This is to prevent infection from spreading to the lungs and respiratory tract, where it could cause far worse damage. Rhinorrhea caused by these infections usually occur on circadian rhythms. Over the course of a viral infection, sinusitis (the inflammation of the nasal tissue) may occur, causing the mucous membranes to release more mucus. Acute sinusitis consists of the nasal passages swelling during a viral infection. Chronic sinusitis occurs when one or more nasal polyps appear. This can be caused by a deviated septum as well as a viral infection.
Rhinorrhea can also occur when individuals with allergies to certain substances, such as pollen, dust, latex, soy, shellfish, or animal dander, are exposed to these allergens. In people with sensitized immune systems, the inhalation of one of these substances triggers the production of the antibody immunoglobulin E (IgE), which binds to mast cells and basophils. IgE bound to mast cells are stimulated by pollen and dust, causing the release of inflammatory mediators such as histamine. In turn, this causes, among other things, inflammation and swelling of the tissue of the nasal cavities as well as increased mucus production. Particulate matter in polluted air and chemicals such as chlorine and detergents, which can normally be tolerated, can make the condition considerably worse.
Rhinorrhea is also associated with shedding tears, whether from emotional events or from eye irritation. When an excess amount of tears are produced, the liquid drains through the inner corner of the eyelids, through the nasolacrimal duct, and into the nasal cavities. As more tears are shed, more liquid flows into the nasal cavities. The buildup of fluid is usually resolved via mucus expulsion through the nostrils.
Head trauma 
If caused by a head injury, rhinorrhea can be a much more serious condition. A basilar skull fracture can result in a rupture of the barrier between the sinonasal cavity and the anterior cranial fossae or the middle cranial fossae. This rupture can cause the nasal cavity to fill with cerebrospinal fluid. This condition, known as cerebrospinal fluid rhinorrhoea or CSF rhinorrhea, can lead to a number of serious complications and possibly death if not addressed properly.
Other causes 
Rhinorrhea can occur as a symptom of opioid withdrawal accompanied by lachrymation. Other causes include cystic fibrosis, whooping cough, nasal tumors, hormonal changes, and cluster headaches. Rhinorrhea can also be the side effect of several genetic disorders, such as primary ciliary dyskinesia.
In most cases treatment for rhinorrhea is not necessary and will clear up on its own - especially if it is the symptom of an infection. For general cases, blowing your nose can get rid of the mucus buildup. Alternatively, saline nasal sprays and Vasoconstrictor nasal sprays may also be used, but may become counterproductive after several days of use, causing Rhinitis medicamentosa.
In recurring cases, such as those due to allergies, there are medicinal treatments available. For cases caused by histamine buildup, several types of antihistamines can be obtained relatively cheaply from drugstores.
People who prefer to keep clear nasal passages, such as singers, who need a clear nasal passage to perform, may use a technique called "nasal irrigation" to prevent rhinorrhea. Nasal irrigation involves rinsing the nasal cavity regularly with salty water or store bought saline solutions.
- "Palatal necrosis due to cocaine abuse". US National Library of Medicine. Retrieved 2012-09-21.
- Eileen Trigoboff; Kneisl, Carol Ren; Wilson, Holly Skodol (2004). Contemporary psychiatric-mental health nursing. Upper Saddle River, N.J: Pearson/Prentice Hall. p. 274. ISBN 0-13-041582-0.
- "Rhinorrhea". Online Etymology Dictionary. Retrieved 2011-09-21.
- "Nasal discharge". Medline Plus. US NLM/NIH. Retrieved 2007-11-01.
- "Rhinorrhea Overview". FreeMd. Retrieved 2011-09-21.
- "Why Does Cold Weather Cause Runny Noses?". NPR. Retrieved 2011-09-22.
- "Why Does My Nose Run?". Kids Health. Retrieved 2011-09-22.
- Smolensky MH, Reinberg A, Labrecque G (May 1995). "Twenty-four hour pattern in symptom intensity of viral and allergic rhinitis: treatment implications". The Journal of Allergy and Clinical Immunology 95 (5 Pt 2): 1084–96. PMID 7751526.
- "Rhinorrhea – Definition, Symptoms, Causes, Diagnosis and Treatment". Prime Health Channel. 2011-08-30. Retrieved 2011-09-24.
- Dipiro, J.T.; Talbert, R.L.; Yee, G.C. (2008). Pharmacotherapy: A Pathophysiologic Approach (7th ed.). New York, NY: The McGraw-Hill Companies, Inc. pp. 1565–1575. ISBN 978-0-07-147899-1.
- Welch et al. (2011-07-22). "CSF Rhinorrhea". Medscape. Retrieved 2011-09-22.
- "Opioid Withdrawal Protocol". Mental Health and Addiction Services. Retrieved 2011-09-24.
- Aubrey, Allison (2007-02-22). "Got a Runny Nose? Flush it Out!". NPR. Retrieved 2011-09-21.
- Runny Nose: A Guide for Parents at the Pennsylvania Medical Society
- Cold and flu advice (NHS Direct)
- How to Wipe Your Nose
- How to Wipe Your Nose on Your Hands