Oxygen is essential for human respiration.
People, who have difficulty in breathing, are provided with oxygen to help them eg. oxygen tent used in hospitals to supplied oxygen to patients, or an astronaut carrying oxygen to the moon to help them breathe.
Figure 3: Oxygen tent
Accessed from: http://en.wikipedia.org/wiki/File:Caleb_David.jpg
Can humans breathe 100% oxygen?
The answer is no. Even divers do not breathe pure oxygen at any depth.
Humans breathe air that is 21 percent oxygen. If you think that breathing 100 percent oxygen would be good for you - think again! It can be harmful. Pure oxygen is generally bad, and sometimes toxic.
What do deep sea divers breathe?
Oxygen content in a divers breathing mix is dependent on the depth.
The deeper a diver goes, the less oxygen they want in your mix or the less exposure you want to that mix. Over the years from trial and error, dive tables have been their mix.There are different gass mixtures and a diver has to study table to safely dive to a certain depth using a certain gas mix for a certain amount of time. Divers have to know what they can and cannot do.
Oxygen isn't the only important part in a breathing mix, and divers can't breathe 100% Oxygen. So, there has to be another gas in their tank. This gas can be either nitrogen or helium, depending on the dive.
The most common gas mix scuba divers use is exactly what you're breathing right now. 21% oxygen, 78% nitrogen and 1% other gases. NOTE: The nitrogen contanct causes a narcotic effect under pressure.
To go any deeper than casual diving, one needs to start reducing the oxygen % in that mix ( nitrogen too since it's narcotic and it's effects just get worse) and replace it with a gas that you can breathe and won't cause any narcotic effect or be toxic at depth. The gas of choice is helium since humans can inhale it and it doesn't get narcotic until you're VERY deep. (Divers on communications gear sound like they just sucked on a party balloon).
This gas blend is commonly called Tri mix. It has Oxygen, helium and a little bit of nitrogen.
Nitrox is one other gas mix that divers use. It's almost like "super charged" air. It has more, instead of less oxygen as a percentage of volume. Any diving gas that has more than 21% oxygen by volume can be considered Nitrox or enriched air. Oxygen is toxic to a diver at depth - that's why this mix is used at shallow depths and the higher the oxygen content, the shallower your maximum depth can be.
Nitrox can be anywhere from 32% to max. 50% oxygen by volume. The 50% mix is used for things like shallow water decompression - helps rid the body of absorbed nitrogen a little quicker than normal. The other mixes are actually used for the dive.
The advantage using enriched air is that there is a reduced nitrogen content without the expense of helium ( it's costly) which in turn helps limit the amount of nitrogen your body takes on. This gives divers a longer time at the bottom before they need to either head up or start a decompression schedule.
What is contained in the gas bubbles exhaled by divers?
The exhaled bubbles consist of mostly carbon dioxide, some oxygen and some nitrogen - not all of the nitrogen you inhaled though! Some Nitrogen gets trapped in your body and divers need to be careful of this. Some used trimix, helium. Divers exhalations bubble up through the water column until they break surface and mix with the atmosphere.
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