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.

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.

Nasal congestion is the blockage of the nasal passages usually due to membranes lining the nose becoming swollen from inflamed blood vessels.

Nasal decongestants target the discomfort directly. These come as nasal sprays, inhalers, and as oral pills.

Nasal congestion has many causes and can range from a mild annoyance to a life-threatening condition. Most people prefer to breathe through the nose (historically referred to as "obligate nasal breathers"). Nasal congestion in an infant in the first few months of life can interfere with breastfeeding and cause life-threatening respiratory distress; in older children and adolescents it is often just an annoyance but can cause other difficulties.

Nasal congestion can interfere with the hearing and speech. Significant congestion may interfere with sleep, cause snoring, and can be associated with sleep apnea. In children, nasal congestion from enlarged adenoids has caused chronic sleep apnea with insufficient oxygen levels and hypoxia, as well as right-sided heart failure. The problem usually resolves after surgery to remove the adenoids and tonsils, however the problem often relapses later in life due to craniofacial alterations from chronic nasal congestion.

Nasal congestion can also cause mild facial and head pain, and a degree of discomfort, often from allergies or the common cold.

Nasal obstruction characterized by insufficient airflow through the nose can be a subjective sensation or the result of objective pathology. It is difficult to quantify by subjective complaints or clinical examinations alone, hence both clinicians and researchers depend both on concurrent subjective assessment and on objective measurement of the nasal airway. Often a doctor's assessment of a perfectly patent nasal airway might differ with a patient's complaint of an obstructed nose.

Common issues that lead to overuse of topical decongestants:

- Deviated septum

- Upper respiratory tract infection

- Vasomotor rhinitis

- Cocaine use and other stimulant abuse

- Pregnancy (these products are not considered safe for pregnancy)

- Chronic rhinosinusitis

- Hypertrophy of the inferior turbinates

The characteristic presentation of RM involves nasal congestion without rhinorrhea, postnasal drip, or sneezing following several days of decongestant use. This condition typically occurs after 5–7 days of use of topical decongestants. Patients often try increasing both the dose and the frequency of nasal sprays upon the onset of RM, worsening the condition. The swelling of the nasal passages caused by rebound congestion may eventually result in permanent turbinate hyperplasia, which may block nasal breathing until surgically removed.

First-generation antihistamine has been suggested as first-line therapy to treat post-nasal drip.

PND is suggested to be a cause of extra-oral halitosis, especially when a sinus infection is also present. Acid reflux or heartburn is believed to aggravate and in some cases cause post-nasal drip. Post-nasal drip can be a cause of laryngeal inflammation and hyperresponsiveness, leading to symptoms of vocal cord dysfunction (VCD).

Nasal mucosa has rich blood supply and has venous sinusoids or "lakes" surrounded by smooth muscle fibers. These smooth muscle fibers act as sphincters and control the filling and emptying of sinusoids. Sympathetic stimulation causes vasoconstriction and shrinkage of mucosa, which leads to decongestion of nose. Parasympathetic stimulation causes not only excessive secretion from the nasal gland but also vasodilatation and engorgement, which lead to rhinorrhoea and congestion of nose. The autonomic nervous system, which supplies the nasal mucosa, is under the control of the hypothalamus. Therefore, emotions play significant role in nonallergic rhinitis.

Nonallergic rhinitis cases may subsequently develop polyps, turbinate hypertrophy and sinusitis.

Causes can be remembered by mnemonic HERNIA:

- Hereditary factors: the disease runs in families

- Endocrine imbalance: the disease tends to start at puberty and mostly involves females

- Racial factors: whites are more susceptible than natives of equatorial Africa

- Nutritional deficiency: vitamins A or D, or iron

- Infection: "Klebsiella ozaenae", diphtheroids, "Proteus vulgaris", "E. coli", etc.

- Autoimmune factors: viral infection or some other unidentified insult may trigger antigenicity of the nasal mucosa.

Specific infections, such as syphilis, lupus, leprosy and rhinoscleroma, may cause destruction of the nasal structures leading to atrophic changes. Atrophic rhinitis can also result from long-standing purulent sinusitis or radiotherapy of the nose, or as a complication of surgery of the turbinates. The United Kingdom National Health Service has stated that "Most cases of atrophic rhinitis in the UK occur when the turbinates are damaged or removed during surgery". Some authors refer to as Atrophic rhinitis secondary to sinus surgery as the empty nose syndrome.

Sinusitis is a common condition, with between 24 and 31 million cases occurring in the United States annually. Chronic sinusitis affects approximately 12.5% of people.

Nasal polyps resulting from chronic rhinosinusitis affect approximately 4.3% of the population. Nasal polyps occur more frequently in men than women and are more common as people get older, increasing drastically after the age of 40.

Of people with chronic rhinosinusitis, 10% to 54% also have allergies. An estimated 40% to 80% of people with sensitivity to aspirin will develop nasal polyposis. In people with cystic fibrosis, nasal polyps are noted in 37% to 48%.

Based on recent theories on the role that fungus may play in the development of chronic sinusitis, antifungal treatments have been used, on a trial basis. These trials have had mixed results.

Patulous Eustachian tube is a physical disorder. The exact causes may vary depending on the person. Weight loss is a commonly cited cause of the disorder due to the nature of the Eustachian tube itself. Fatty tissues hold the tube closed most of the time in healthy individuals. When circumstances cause overall body fat to diminish, the tissue surrounding the Eustachian tube shrinks and this function is disrupted.

Activities and substances which dehydrate the body have the same effect and are also possible causes of patulous Eustachian tube. Examples are stimulants (including caffeine) and exercise. Exercise may have a more short-term effect than caffeine or weight loss in this regard.

Pregnancy can also be a cause of patulous Eustachian tube due to the effects of pregnancy hormones on surface tension and mucus in the respiratory system.

Granulomatosis with polyangiitis can also be a cause of this disorder. It is yet unknown why.

Adenoiditis occurs mainly in childhood, often associated with acute tonsillitis. Incidence decreases with age, with adenoiditis being rare in children over 15 years due to physiological atrophy of the adenoid tissue.

Rhinitis medicamentosa is a form of drug-induced nonallergic rhinitis which is associated with nasal congestion brought on by the use of certain oral medications (primarily sympathomimetic amine and 2-imidazoline derivatives) and topical decongestants (e.g., oxymetazoline, phenylephrine, xylometazoline, and naphazoline nasal sprays) that constrict the blood vessels in the lining of the nose.

Chronic rhinitis is a form of atrophy of the mucous membrane and glands of the nose.

The flow of blood normally stops when the blood clots, which may be encouraged by direct pressure applied by pinching the soft fleshy part of the nose. This applies pressure to Little's area (Kiesselbach's area), the source of the majority of nose bleeds, and promotes clotting. Pressure should be firm and be applied for at least five minutes and up to 20 minutes; tilting the head forward helps decrease the chance of nausea and airway obstruction. Swallowing excess blood can irritate the stomach and cause vomiting.

Viruses that may cause adenoiditis include adenovirus, rhinovirus and paramyxovirus. Bacterial causes include Streptococcus pyogenes, Streptococcus pneumoniae, Moraxella catarrhalis and various species of Staphylococcus including Staphylococcus aureus.

Upon examination of a suspected case of patulous Eustachian tube, a doctor can directly view the tympanic membrane with a light and observe that it vibrates with every breath taken by the patient. A tympanogram may also help with the diagnosis. Patulous Eustachian tube is likely if brisk inspiration causes a significant pressure shift.

Patulous Eustachian tube is frequently misdiagnosed as standard congestion due to the similarity in symptoms and rarity of the disorder. Audiologists are more likely to recognize the disorder, usually with tympanometry or nasally delivered masking noise during a hearing assessment, which is highly sensitive to this condition.

When misdiagnosis occurs, a decongestant medication is sometimes prescribed. This type of medication aggravates the condition, as the Eustachian tube relies on sticky fluids to keep closed and the drying effect of a decongestant would make it even more likely to remain open and cause symptoms. The misdiagnosed patient may also have tubes surgically inserted into the eardrum, which increases the risk of ear infection and will not alleviate patulous Eustachian tube. If these treatments are tried and failed, and the doctor is not aware of the actual condition, the symptoms may even be classified as psychological.

Incidentally, patients who instead suffer from the even rarer condition of superior canal dehiscence are at risk for misdiagnosis of patulous Eustachian tube due to the similar autophony in both conditions.

Children have 2-9 viral respiratory illnesses per year. In 2013 18.8 billion cases of upper respiratory infections were reported. As of 2014, upper respiratory infections caused about 3,000 deaths down from 4,000 in 1990. In the United States, URIs are the most common infectious illness in the general population. URIs are the leading reasons for people missing work and school.

There is low or very-low quality evidence that probiotics may be better than placebo in preventing acute URTIs. Vaccination against influenza viruses, adenoviruses, measles, rubella, "Streptococcus pneumoniae", "Haemophilus influenzae", diphtheria, "Bacillus anthracis", and "Bordetella pertussis" may prevent them from infecting the URT or reduce the severity of the infection.

The traditional theory is that a cold can be "caught" by prolonged exposure to cold weather such as rain or winter conditions, which is how the disease got its name. Some of the viruses that cause the common colds are seasonal, occurring more frequently during cold or wet weather. The reason for the seasonality has not been conclusively determined. Possible explanations may include cold temperature-induced changes in the respiratory system, decreased immune response, and low humidity causing an increase in viral transmission rates, perhaps due to dry air allowing small viral droplets to disperse farther and stay in the air longer.

The apparent seasonality may also be due to social factors, such as people spending more time indoors, near infected people, and specifically children at school. There is some controversy over the role of low body temperature as a risk factor for the common cold; the majority of the evidence suggests that it may result in greater susceptibility to infection.