Mouth breathing, medically known as chronic oral ventilation, is long-term breathing through the mouth. It often is caused by an obstruction to breathing through the nose, the innate breathing organ in the human body. Chronic mouth breathing may be associated with illness. The term "mouth-breather" has developed a pejorative slang meaning. Etymology In the early 20th century, "mouth-breather" was a technical term used by doctors to describe children who were breathing through their mouths due to an underlying medical condition. English lexicographer Jonathon Green notes that by 1915, the phrase "mouth-breather" had developed a pejorative connotation within English slang, defined as a "stupid person." Currently, the Macmillan Dictionary defines the term "mouth breather" as a pejorative noun that is used to mean "a stupid person." Cause of inability for nasal breathing Jason Turowski, MD of the Cleveland Clinic states that "we are designed to breathe through our noses from birth — it's the way humans have evolved." Infants for example in the first six to twelve months of postnatal can only use their noses to breathe unless crying is involved. Thus, the impact of chronic mouth breathing on health is a research area within orthodontics (and the related field of myofunctional therapy) and anthropology. It is classified into three types: obstructive, habitual, and anatomic. There is a noted order of cause and effect leading to airway dysfuntion related to mouth breathing. This first starts with an inflammatory reaction then leading to tissue growth in the area which leads to airway obstruction and mouth breathing and then finally an altered face structure. Nasal breathing produces nitric oxide within the body, while mouth breathing does not. In addition, the Boston Medical Center notes that the nose filters out particles that enter the body, humidifies the air we breathe and warms it to body temperature. In contrast, however, mouth breathing "pulls all pollution and germs directly into the lungs; dry cold air in the lungs makes the secretions thick, slows the cleaning cilia, and slows down the passage of oxygen into the bloodstream." As a result, chronic mouth breathing may lead to illness. In about 85% of cases, it is an adaptation to nasal congestion, and frequently occurs during sleep. More specialized causes include: antrochoanal polyps; a short upper lip which prevents the lips from meeting at rest (lip incompetence); and pregnancy rhinitis which tends to occur in the third trimester of pregnancy.