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 hearing and speech. Significant congestion may interfere with sleep, cause snoring, and can be associated with sleep apnea or upper airway resistance syndrome. 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. Prevalence of kyphosis has been linked to nasal obstruction in a study.