at what altitude do humans need oxygen support?
965765
It differs a bit as those who live at high altitudes have built up a tolerance.  It also depends on how long you will be above a certain altitude.  However, prolongned exposure above 17,000 feet can result in hypoxia which causes fluid build up so that the person actually drowns from the fluid accumulation in their lungs.
498469
1 to 2 minutes is good, however 'freedivers' can hold theirs for 5 minutes or more, to do this they hyperventilate to a degree (sufficient so that they still have traces of CO2 in their bloodstream to enable the hypoxic drive - so that they don't pass out through lack of oxygen) - DO NOT try this without medical or instructor supervision.
834076
Air is about 20% elemental O2.\n\nhttp://www.mountainflying.com/oxygen.htm This site says 21%\nAnd this site:http://www.pubquizhelp.34sp.com/sci/air.html has a nice chart on it.
1067371
May 29, 2000 -- Unfortunately, there's not much you can do to prevent this problem -- though there are ways you can ease its effects. It's a fact: Being at a high elevation is stressful to your body. The concentration of oxygen is 21% at both sea level and at higher altitudes, but the decreasing air pressure at higher elevations means you get significantly fewer oxygen molecules per breath once you go over about 8,000 feet. \n\nBefore your trip, make an effort to hit the gym more often, or do a few extra laps at the track. This can minimize the dizziness, fatigue, shortness of breath, nausea, and headaches that signal altitude sickness. Fitness at sea level doesn't guarantee an easier time when you're 2 miles high. But being in good shape makes it more likely that your lungs will be able to cope with the challenges of the high life. \n\nThere are also ways to ease the body's transition to higher altitudes once your trip begins. If you're going higher than 8,000 feet, take it slow. When you arrive, try to let your body acclimate for a day or two. When hiking up to 15,000 feet or more, spread out your ascent over a period of two or three days to give your body more time to adapt. And once on your trip, watch what you eat and drink. You'll want your respiratory system operating at full capacity, so avoid alcohol and any drugs that might interfere with your breathing, such as sleeping medications. \n\nIf your symptoms are severe, a prescription drug called Diamox may help. It works by stimulating your breathing, so that you take faster and deeper breaths. It can be taken on a plane several hours before arrival or at the airport before departure.
737154
Oxygen gets you high. In a catastrophic emergency, you're taking giant panicked breaths. Suddenly you become euphoric, docile. You accept your fate. It's all right here. Emergency water landing - 600 miles an hour. Blank faces, calm as Hindu cows.
1002178
Carbon dioxide acts as a greenhouse gas at all altitudes. You are correct that it is heavier than air, and therefore is less prevalent at higher altitudes.
280386
Well, there are alot of factors in this room to be considered.    People require more than just oxygen to breath.  Though roughly 20% of atmospheric air is oxygen, the remainder of the  gasses (and especially the ratio of these) also contribute to how the lung works.\n\nSo, if this room was previously full of air at normal atmospheric conditions, then only the oxygen removed, and the addition of oxygen from the "cylinder" brought it back to atmospheric conditions, then yes, people would be able to breath.
814224
we are carrying our body upwards which require more burning of energy, which require more air, which will create respiratory problems
390672
having room for more oxygen, and other things for transport
845802
Just like how they say on an airplane: "apply the oxygen mask first to yourself and then to your child"... I believe we should make sure we are strong and healthy before trying to strengthen the rest of the world.  There is a strong point that strengthening the rest of the world will help us, too, but if we begin falling apart we won't be able to help ANYONE.
908874
Firstly, scientifically, the terms space and outer space are the same thing. The problem is that the Earth's atmosphere doesn't stop suddenly. \n\nIt actually seems to depend on your nationality. \n\nThe Federation Aeronautique Internationale has established an altitude of 100 km (62 miles) as a working definition for the boundary between atmosphere and space. This is used because, above an altitude of roughly 100 km (62 miles), a vehicle would have to travel faster than orbital velocity in order to derive sufficient aerodynamic lift from the atmosphere to support itself. \n\nThe United States currently designates people who travel above an altitude of 50 miles (80 km) as astronauts. BTW, this includes 8 pilots of the X-15 rocket plane, one of whom also met the FAI criterion.
1026062
you would probably suffocate from the high altitute b-cuz there isnt much oxygen up there.
974252
It's only a gradual transition, but earth's atmosphere is pretty thin: at 15,000 feet half of the atmosphere is below you and it's noticeably harder to breathe and easier to get sunburned.
198755
lack of oxygen causes this
386642
After eating, astronauts settle down to the assigned tasks of the day, either supervising experiments or performing routine maintenance on station equipment. It takes a complex array of machinery to keep people alive and well in orbit. Daily, each human breathes the equivalent of 0.9 kg. of liquid oxygen - enough air to fill a 3.5 cubic metre room - and drinks a total of 2.7 kg of water. To minimize on resupply needs, the ISS life-support systems are designed to recycle as much as possible. Waste water from urine and moisture condensed from the air is either purified and reused direct, or broken down by electrolysis to provide fresh oxygen. Carbon dioxide 'scrubbers' chemically remove that toxic gas from the air.
97270
you need alot of starting veleocity to gain altitude and distance.  you need the proper trajectory (probably 45 degs) and strong legs.
718206
breathing would not be as deep because the lungs cannot expand this far at such high altitudes\n\nevntually the lungs would get used to this and get stronger, so if someone trained for a sport at these high altitudes, then came back down to their normal altitude the lungs would be stronger and more efficient