Now we have all the pieces to fully understand what happened to our two aeronauts. They experienced hypoxia. Specifically, our adventures experienced hypoxic hypoxia. Their lungs couldn't get enough oxygen into their blood because of the altitude. They had actually ascended into the dead zone, the altitude at which air pressure is less than one-third of the atmospheric pressure at sea level. Normal pressure at sea level is themed one standard atmosphere. Past the dead zone, however, human life becomes unsustainable for significant periods of time. Thanks to the quick thinking of Coxwell and his previous high altitude experiences, the two aeronauts could descend to a habitable altitude where the pressure of oxygen increased and the two could breathe again as they got to their balloon back down to earth. Now, Glacier and Coxwell's record-setting altitude of 29,000 feet may not seem that impressive. I mean, modern passenger planes regularly ascent to 40,000 feet. Really besides some discomfort in the ears and crying babies, there's rarely an issue. Certainly, no one on the plane is passing out from hypoxic hypoxia. The comfort that we experience on these passenger planes is thanks to what are called countermeasures, engineering solutions that mitigate the burden of traveling to high altitudes. For commercial airlines, air is captured from the outside of the plane and directed into the cabin at high pressures to maintain comfortable breathing. Cabin altitude is a concept that measures pressure by comparing the pressure inside the plane to a corresponding altitude. Passenger planes keep cabin altitudes below 8,000 feet and some even aim for as low as 4,500 feet, making it nearly impossible for any of the passengers to develop altitude sicknesses. Altitude sickness is the body's attempt to adjust to changing altitudes. Rinconada, Peru is the highest permanent human settlement at 16,700 feet. When visiting Rinconada, the altitude can trigger increased heart rate and respiration as your body attempts to compensate for the decreased ability to get oxygen. For a little while, you can even experience nausea as the body incorrectly attributes the diminished oxygen-carrying capacity to ingested toxins. Give yourself a couple of days, however, and your body can acclimatize to the effects of high altitude by producing more red blood cells and mitochondria to attract what little oxygen remains. Olympic athletes actually take advantage of this process and train in mountainous regions like Peru or Switzerland. It then gives them an increased respiration efficiency when they then perform at lower altitudes. Back inside the plane now, we're at a nice cruising altitude of 35,000 feet. The outside air pressure is only 25 percent of what it is at sea level. If the pressure rapidly decreases, masks will descend from the ceiling to provide oxygen. As we have seen, decreases in pressure can cause confusion, tunnel vision, and loss of dexterity very rapidly. This is why airline safety demonstrations will always tell you to put your mask on before helping others. Non-commercial pilots who will rapidly change altitudes also take advantage of this supplemental oxygen. If cabin altitudes ever exceed 14,000 feet, which can occur in the loss of cabin pressure, supplemental oxygen becomes a necessity. Finally, we looked at space where everything becomes more extreme. There is no atmospheric pressure once you leave Earth atmosphere. This extreme lack of pressure means that supplemental oxygen and artificial atmospheres become necessities. A spaceship at takeoff will have a pure oxygen atmosphere, but only one-third of a standard atmospheric pressure. Similarly, the iconic astronaut spacesuits used for extravehicular activities, EVAs support a pure oxygen environment at one-third of an atmosphere. These artificial atmospheres have an internal pressure of only one-third of an atmosphere because of the lack of external pressure. Like Glacier's hot air balloon, if the pressure inside is far greater than the pressure outside, the suits could burst. Keeping the internal pressure low reduces stress on the suit or ship. It may seem obvious that in space, all artificial atmospheres would have low pressures with high oxygen percentages. However, the International Space Station is designed to withstand higher pressures and support 20 percent oxygen at one full atmospheric pressure. This is actually to avoid oxygen toxicity. They say you can have too much of a good thing, and it turns out that oxygen is no different. Overall, the first challenge, avoiding hypoxia frames almost every engineering and medical decision in extreme medicine.
