1. Nitrox? EANx? Enriched air?
Everyone of us has breathed nitrox. Nitrox means a mix of nitrogen and oxygen and the word "nitrox" doesn't tell the relations of those elements. So air is nitrox too because it contains (basically) nitrogen (79%) and oxygen (21%). The base of word nitrox is the expression Nx/Ox, where N is the chemical sign of nitrogen and O is the chemical sign of oxygen. So basically nitrox can be any mix of nitrogen and oxygen containing 1 to 99 % of oxygen (and rest is nitrogen).
Enriched air oxygen means a nitrox mix which has more oxygen than air has, so then the mix has 22-99% of oxygen. Enriched air nitrox also has a shorter name, EANx, where x is the percentage of oxygen, ie. EAN40 (40% oxygen in the mix). Also EANx40 label is used sometimes (the letter "x" is preserved in the label). Usually people mean EANx when they say "nitrox", and so do I further here.
However, it is important to the relations of enriched air nitrox and nitrox, because nitrox can also be a "hypoxic" mixture containing less oxygen than air! Then we say that the mix is hypoxic, ie. it has less oxygen than 21%. Hyperoxic nitrox is EANx, and normoxic nitrox is basically air, 21% of oxygen and 79% of nitrogen. Hypoxic mixtures are not recreational diving, but air and EANx usually are. For more information about different gases, take a look at http://techdiver.ws/exotic_gases.shtml.
2. History of nitrox diving
In about the year 1890, scientists discovered the benefits of enriched air in diving in theory. The first real mass diving tests were conducted during the World War I and a little before. During WW II it was used much more, and the first open circuit EANx equipment were used.
In the year 1979 USA's National Underwater and Atmospheric Administration (NOAA) published EANx diving tables.
International Association of Nitrox and Technical Divers (IANTD) was established in the year 1985 (USA) and IANTD UK in 1992 (Europe). ANDI and TDI were also quick in the markets.
In 1995 PADI (Professional Association of Diving Instructors) took nitrox training as a part of it's training program, and the course certifies divers to dive nitrox dives within the recreational limits and with blends of 22%-40% of oxygen.
3. Is nitrox safer than regular air?
This is one of the most popular myths regarding nitrox diving. People usually refer to the avoidance of DCS (decompression sickness) when they claim EANx to be safer than air.
Let's take a look at the diving accident statistics! DAN (Divers Alert Network) has statistics of DCS cases in the USA, which is based on DEMA/NUADC and PADI New Diver Benchmark reports. According to them, the probability to get DCS by air diving is about 0.004% (1 to 25000). Based on this number, and on the PADI Undersea Journal 3/97 magazine, the probability to get DCS by air diving even once during 500 dives is 1.98% (1 to 50). The risk is 1.49% in nitrox diving in the same amount of dives. So... so far the difference is marginal, even though nitrox seems to be a little safer.
But an accident doesn't have to be DCS only. And when diving with nitrox there are several other concerns too that air diving doesn't have. Gas blending, analyzing errors, oxygen clean equipments, CNS clock etc. may also be a source of accidents. In this point I have to say that nitrox diving is not dangerous when done right, but these points have to be counted when people talk about safety. So nitrox diving is safe, but not safer than air, generally speaking.
4. Myths
Besides the safety myth above, there are several myths regarding nitrox diving:
People who don't know much about nitrox think that it is used for deeper dives. Well, it is not. But it is mostly used on mid deep dives, ranging from 20-40 meters (~60-130 feet). The more oxygen you have in the mix, the shallower will be the MOD (maximum operating depth).
It is also a common misunderstanding to think that it causes less narcosis. This would be due to the fact that it has less narcotic nitrogen. But oxygen is also narcotic, and according to some references it is perhaps as narcotic as nitrogen, and together with nitrogen it could be even more narcotic (Bennett 1993, Inert Gas Narcosis, the Physiology and Medicine of Diving, 4th edition)!
There are also no advantages in rare embolia cases (The Undersea Journal 3/97).
5. Why should one dive with nitrox?
Simply: to gain more bottom time and to have shorter surface intervals. For example, if you dive two dives to 20 meters (66 feet) with a one hour surface interval, the no decompression limited bottom times will be 45 minutes and 24 min, respectively. Now, if do the same with EAN40, the bottom times rise to 98 min and 61 min, respectively. (PADI diving tables)
Another example: Two 25 min dives to 25 meters (83 feet). The surface time will be at least 2 h 19 min (in no decompression limits), but with EAN40 you don't even have to have the surface time! (PADI diving tables)
EANx is also used in technical and commercial diving to shorten the decompression times.
6. How much does it cost, where do I get it and who can dive with nitrox?
Nowadays nitrox doesn't cost much more than air, maybe 1.5 times as much roughly (Sometimes less, sometimes more. Ask your local diving center.). Several diving centers fill nitrox, but you have to be certified first.
Have good nitrox dives!
PADI Nitrox Enriched Air Diver Course
There are about 18 PADI Dive Centres in Singapore and only a handful of them has the rating of a PADI 5 Star IDC - one of the highest rating given to a dive centre. 
