Effects of high altitude on humans

The human body can perform best at sea level, where the atmospheric pressure is 101,325 Pa or 1013.25 millibars (or 1 atm, by definition). The concentration of oxygen (O2) in sea-level air is 20.9%, so the partial pressure of O2 (pO2) is . In healthy individuals, this saturates hemoglobin, the oxygen-binding red pigment in red blood cells. When pO2 drops, the body responds with altitude acclimatization. The International Society for Mountain Medicine recognizes three altitude regions which reflect the lowered amount of oxygen in the atmosphere: • High altitude = • Very high altitude = • Extreme altitude = above Travel to each of these altitude regions can lead to medical problems, from the mild symptoms of acute mountain sickness to the potentially fatal high-altitude pulmonary edema (HAPE) and high-altitude cerebral edema (HACE). The higher the altitude, the greater the risk. Expedition doctors commonly stock a supply of dexamethasone, to treat these conditions on site. Research also indicates elevated risk of permanent brain damage in people climbing to above . Humans have survived for two years at , 475 millibars of atmospheric pressure), which is the highest recorded permanently tolerable altitude; the highest permanent settlement known, La Rinconada, is at . At altitudes above , 383 millibars of atmospheric pressure), sleeping becomes very difficult, digesting food is near-impossible, and the risk of HAPE or HACE increases greatly. Death zone is in the death zone, as are the summits of all eight-thousanders. The death zone in mountaineering (originally the lethal zone) was first conceived in 1953 by Edouard Wyss-Dunant, a Swiss physician and alpinist. It refers to altitudes above a certain point where the amount of oxygen is insufficient to sustain human life for an extended time span. This point is generally tagged as , less than 356 millibars of atmospheric pressure). , the second highest mountain on Earth, is in the death zone. At an altitude of , the atmospheric pressure is sufficiently low that water boils at the normal temperature of the human body. This altitude is known as the Armstrong limit. Exposure to pressure below this limit results in a rapid loss of consciousness, followed by a series of changes to cardiovascular and neurological functions, and eventually death, unless pressure is restored within 60–90 seconds. Even below the Armstrong limit, an abrupt decrease in atmospheric pressure can cause venous gas bubbles and decompression sickness. A sudden change from sea-level pressure to pressures as low as those at can cause altitude-induced decompression sickness. ==Acclimatization==

The human body can adapt to high altitude through both immediate and long-term acclimatization. At high altitude, in the short term, the lack of oxygen is sensed by the carotid bodies, which causes an increase in the breathing depth and rate (hyperpnea). However, hyperpnea also causes the adverse effect of respiratory alkalosis, inhibiting the respiratory center from enhancing the respiratory rate as much as would be required. Inability to increase the breathing rate can be caused by inadequate carotid body response or pulmonary or renal disease. and non-essential bodily functions are suppressed, resulting in a decline in food digestion efficiency (as the body suppresses the digestive system in favor of increasing its cardiopulmonary reserves). Full acclimatization requires days or even weeks. Gradually, the body compensates for the respiratory alkalosis by renal excretion of bicarbonate, allowing adequate respiration to provide oxygen without risking alkalosis. It takes about four days at any given altitude and can be enhanced by drugs such as acetazolamide. Eventually, the body undergoes physiological changes such as lower lactate production (because reduced glucose breakdown decreases the amount of lactate formed), decreased plasma volume, increased hematocrit (polycythemia), increased RBC mass, a higher concentration of capillaries in skeletal muscle tissue, increased myoglobin, increased mitochondria, increased aerobic enzyme concentration, increase in 2,3-BPG, hypoxic pulmonary vasoconstriction, and right ventricular hypertrophy. Pulmonary artery pressure increases in an effort to oxygenate more blood. Full hematological adaptation to high altitude is achieved when the increase of red blood cells reaches a plateau and stops. The length of full hematological adaptation can be approximated by multiplying the altitude in kilometres by 11.4 days. For example, to adapt to of altitude would require 45.6 days. The upper altitude limit of this linear relationship has not been fully established. High altitude causes decreased blood flow to the placenta, even in acclimatized women, which interferes with fetal growth. ==Adaptation==

It is estimated that 81.6 million people live at elevations above . Genetic changes have been detected in high-altitude population groups in Tibet in Asia, the Andes of the Americas, and the Amhara of Ethiopia. This adaptation means irreversible, long-term physiological responses to high-altitude environments, associated with heritable behavioural and genetic changes. The indigenous inhabitants of these regions thrive well in the highest parts of the world. These humans have undergone extensive physiological and genetic changes, particularly in the regulatory systems of oxygen respiration and blood circulation, when compared to the general lowland population. Compared with acclimatized newcomers, native Amhara, Andean and Himalayan populations have better oxygenation at birth, enlarged lung volumes throughout life, and a higher capacity for exercise. and less susceptibility to chronic mountain sickness (CMS). Andeans possess a similar suite of adaptations but exhibit elevated hemoglobin concentration and a normal resting ventilation. These adaptations may reflect the longer history of high altitude habitation in these regions. A 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. ==Mitigation==

Mitigation may be by supplementary oxygen, pressurisation of the habitat or environmental protection suit, or a combination of both. In all cases the critical effect is the raising of oxygen partial pressure in the breathing gas. ==Other hazards==

The ambient air temperature is predictably affected by altitude, and this also has physiological effects on people exposed to high altitudes. The temperature effects and their mitigation are not inherently different from temperature effects from other causes, but the effects of temperature and pressure are cumulative. The temperature of the atmosphere decreases by a lapse rate, mostly caused by convection and the adiabatic expansion of air with decreasing pressure. At the peak of Mount Everest, the average summer temperature is and the average winter temperature is . At such low temperatures, frostbite and hypothermia become risks to humans. Frostbite is a skin injury that occurs when exposed to extreme low temperatures, causing the freezing of the skin or other tissues, commonly affecting the fingers, toes, nose, ears, cheeks and chin areas. Hypothermia is defined as a body core temperature below in humans. to hallucinations and cardiac arrest. In addition to cold injuries, breathing cold air can cause dehydration, because the air is warmed to body temperature and humidified from body moisture. Symptoms of sunburn include red or reddish skin that is hot to the touch or painful, general fatigue, and mild dizziness. Other symptoms include blistering, peeling skin, swelling, itching, and nausea. ==Athletic performance==

For athletes, high altitude produces two contradictory effects on performance. For explosive events (sprints up to 400 metres, long jump, triple jump) the reduction in atmospheric pressure means there is less resistance from the atmosphere and the athlete's performance will generally be better at high altitude. For endurance events (races of 800 metres or more), the predominant effect is the reduction in oxygen, which generally reduces the athlete's performance at high altitude. One way to gauge this reduction is by monitoring VO2max, a measurement of the maximum capacity of an individual to utilize O2 during strenuous exercise. For an unacclimated individual, VO2max begins to decrease significantly at moderate elevation, starting at 1,500 metres and dropping 8 to 11 percent for every additional 1000 metres. Explosive events Sports organizations acknowledge the effects of altitude on performance: for example, the governing body for the sport of athletics, World Athletics, has ruled that performances achieved at an altitude greater than 1,000 metres will be approved for world record purposes, but carry the notation of "A" to denote they were set at altitude. The 1968 Summer Olympics were held at altitude in Mexico City. The world records in most short sprint and jump records were broken there. Other records were also set at altitude in anticipation of those Olympics. Bob Beamon's record in the long jump held for almost 23 years and has only been beaten once without altitude or wind assistance. Many of the other records set at Mexico City were later surpassed by marks set at altitude. An elite athletics meeting was held annually in Sestriere, Italy, from 1988 to 1996, and again in 2004. The advantage of its high altitude in sprinting and jumping events held out hope of world records, with sponsor Ferrari offering a car as a bonus. One record was set, in the men's pole vault by Sergey Bubka in 1994; Endurance events , Switzerland (elevation 1,856 m or 6,089 ft). Athletes can also take advantage of altitude acclimatization to increase their performance. Another study from 2006 has shown performance gains from merely performing some exercising sessions at high altitude, yet living at sea level. The performance-enhancing effect of altitude training could be due to increased red blood cell count, more efficient training, or changes in muscle physiology. In 2007, FIFA issued a short-lived moratorium on international football matches held at more than 2,500 metres above sea level, effectively barring select stadiums in Bolivia, Colombia, and Ecuador from hosting World Cup qualifiers, including their capital cities. In their ruling, FIFA's executive committee specifically cited what they believed to be an unfair advantage possessed by home teams acclimated to the elevation. The ban was reversed in 2008. ==See also==