<P> Many deaths in high - altitude mountaineering have been caused by the effects of the death zone, either directly (loss of vital functions) or indirectly (wrong decisions made under stress, physical weakening leading to accidents). In the death zone, the human body cannot acclimatize . An extended stay in the zone without supplementary oxygen will result in deterioration of bodily functions, loss of consciousness, and, ultimately, death . </P> <P> Scientists at the High Altitude Pathology Institute in Bolivia dispute the existence of a death zone, based on observation of extreme tolerance to hypoxia in patients with chronic mountain sickness and normal fetuses in - utero, both of which present pO levels similar to those at the summit of Mount Everest . </P> <P> Studies have shown that the approximately 140 million people who live at elevations above 2,500 metres (8,200 ft) have adapted to the lower oxygen levels . These adaptations are especially pronounced in people living in the Andes and the Himalayas . Compared with acclimatized newcomers, native Andean and Himalayan populations have better oxygenation at birth, enlarged lung volumes throughout life, and a higher capacity for exercise . Tibetans demonstrate a sustained increase in cerebral blood flow, lower hemoglobin concentration, and less susceptibility to chronic mountain sickness (CMS). These adaptations may reflect the longer history of high altitude habitation in these regions . </P> <P> A significantly lower mortality rate from cardiovascular disease is observed for residents at higher altitudes . Similarly, a dose response relationship exists between increasing elevation and decreasing obesity prevalence in the United States . This is not explained by migration alone . On the other hand, people living at higher elevations also have a higher rate of suicide in the United States . The correlation between elevation and suicide risk was present even when the researchers control for known suicide risk factors, including age, gender, race, and income . Research has also indicated that oxygen levels are unlikely to be a factor, considering that there is no indication of increased mood disturbances at high altitude in those with sleep apnea or in heavy smokers at high altitude . The cause for the increased suicide risk is as yet unknown . </P>

Low concentration of oxygen in blood at high altitude