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." Overview
Jason Turowski, MD of the Cleveland Clinic states that "we are designed to breathe through our noses from birth — its the way humans have evolved." 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. : 281 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.
It produces particularly high rates of breast cancer among younger women with mutated genes, and despite being rare, 4% of women with breast cancer under age 30 have a mutation in this gene. PTEN:  Mutations cause Cowden syndrome, which produces hamartomas (benign polyps) in the colon, skin growths, and other clinical signs, as well as an increased risk for many cancers. CDH1:  Mutations are associated with lobular breast cancer and gastric cancer. STK11:  Mutations produce Peutz–Jeghers syndrome. It is extremely rare, and creates a predisposition to breast cancer, intestinal cancer, and pancreatic cancer. CHEK2:  Approximately one out of 40 northern Europeans have a mutation in this gene, making it a common mutation. It is also one of the most frequently mutated genes after BRCA among Hispanics in the United States. Considered a moderate-risk mutation, it may double or triple the carriers lifetime risk of breast cancer, and also increase the risk of colon cancer and prostate cancer. ATM:  Mutations cause ataxia telangectasia; female carriers have approximately double the normal risk of developing breast cancer. PALB2:  Studies vary in their estimate of the risk from mutations in this gene and the frequency of mutations in this gene may be different among different populations. It may be moderate risk, or as high as BRCA2.Approximately 45% of HBOC cases involve unidentified genes, or multiple genes. Prevention
People with BRCA1 and BRCA2 mutations are recommended to have a transvaginal ultrasound 1-2 times per year. Screening with CA-125 is also recommended.