Because oxygen is toxic at high concentrations. If you breathed in pure oxygen, you would eventually get very sick.
There is high concentrated oxygen treatments, or there is the standard oxygen treatments. The high concentrated is almost pure oxygen, people like doctors and scuba divers breathe pure oxygen almost every day.
No its not dangerous to breath pure oxygen(Under normal conditions).... Oxygen is the primary thing that you need in order to live..... Pure Oxygen(100%) however does become dangerous to breath at pressures greater than 1.6 atmospheres when diving in water (about 20 feet deep) at this depth pure oxygen becomes toxic and it causes Oxygen Narcosis resulting in unconscousness and other symptoms... This is why divers use different breathing mixtures for different depths, these mixtures contain different amounts of Oxygen,Nitorgen,helium ect...
Air contains 20% oxygen, When you go up a mountain the air is "thinner" and there is not enough oxygen so mountaineers breath oxygen from cylinders so that each breath has more than 20% oxygen, This helps them function at altitude. Divers DO NOT Carry Oxygen cylinders with them, they breath from cylinders containing compressed AIR. When you dive the "thickness" of the air increases, at 10 meters you are breathing air at TWICE atmospheric pressure. At depth the Oxygen becomes toxic. If you were to dive on pure Oxygen you could die if you went as deep as 10 meters.
The air we breathe is about 21% oxygen and 79% nitrogen. Less that one percent of our air is composed of trace gases like carbon dioxide and helium. This normal surface air is the breathing gas used by the vast majority of scuba divers. There are a number of reasons why recreational scuba divers do not dive using pure oxygen. The first, and most important, of these is that oxygen is toxic to humans at the elevated pressures experienced in diving. While the exact depth limit varies with a diver's physiology, a good rule of thumb us that pure oxygen should not be used below about 27 feet of sea water. At that depth, the partial pressure of oxygen makes it toxic to most divers. Pure oxygen IS used as the breathing gas in shallow water rebreathers like the Drager LAR V. These special purpose scuba rigs are used for combat swimmer delivery by highly trained combat divers such as US Navy SEALs. The oxygen toxicity depth limit still applies, even with these highly trained divers. Pure oxygen is also used as a decompression gas at shallow depths by technical and commercial divers who have used other breathing gases during the dive. Mixed gas rebreathers use pure oxygen as one of the two breathing gases which are mixed throughout a dive. Another diluent gas such as air is used to maintain optimum oxygen partial pressure for the diver to minimize both oxygen toxicity and decompression illness risk. Another reason standard surface air is typically used to fill recreational scuba tanks is convenience and expense. Normal air is easy to obtain and inexpensive. Oxygen is difficult to work with and expensive. Some recreational scuba divers do use a breathing gas called "Enriched Air Nitrox" (EANx). This is air to which oxygen has been added, increasing the percentage from 21% to as much as 40%. The increased oxygen reduces the amount of nitrogen, allowing longer dives without the risk of decompression illness. It is important to note that pure oxygen can be very dangerous to work with due to the risk of fire and explosion. Diving gear that is to be used with breathing gases containing elevated levels of oxygen is designed with special seal materials and non-petroleum-based lubricants.
Yes you can. But not for indefinitely long periods
if we breath pure oxygen it will cut our lungs like knife
One of the effects of a small amount of helium in the breath is to change the voice similar to a cartoon Donald Duck sound. Professional deep sea divers will breath a mixture of Oxygen and Helium this helps preventdecompression sickness known as the Bends. If you breath pure helium you would asphyxiate. (starve the brain of Oxygen and die)
It is a common misconception that humans absorb only oxygen when they breath. While Oxygen is the primary element that our bodies require when breating we can not breath it in pure concentration. Our own atmosphere is a Nitrogen/Oxygen mix that we inhale and require both elements. I'm not sure for the reason or chemistry behind it. However I do know that over time breathing pure oxygen will kill a person. So we need the mixed gas form similar to our atmosphere.
Not "life as we know it". Oxygen is poisonous, as all SCUBA divers know; below about 30 feet depth, pure oxygen will kill you, and a few people react badly to oxygen in normal pressures. And without nitrogen and carbon dioxide, plants would be unable to live.
For normal "sport" diving, scuba tanks are filled with just plain old air! That's just the mix of gases you're likely breathing right now, and it contains 78% Nitrogen, 21% Oxygen, and 1% other gases. But nitrogen can cause problems for divers ("bends" and "narcosis"). So for technical diving (below 100 feet), divers sometimes use Nitrox, less nitrogen than normal air, and more oxygen. But even pure oxygen can be dangerous when breathed under pressure. In fact, breathing pure oxygen while scuba diving below 25 feet can be fatal! So for very deep technical dives, the inert gas helium is often added into the gas mix; which can be heliox (oxygen & helium), or trimix (oxygen, helium, and nitrogen).