Diving is a risky sport, which is just one of its many attractions. But the risks are calculated, assessed and — for the most part — avoided. We assess the dangers and explains how you can avoid them.

An inquisitive hammerhead shark cruised into view and a surge of adrenaline quickened my pulse and breathing rate. The sleek, grey eating machine glided majestically around, checking out the intruders to the underwater realm he patrolled.

Fortunately man is too much of an oddity to appear on his menu, and unprovoked attacks are rare. I edged back into the protection of the reef wall, following his smooth fly-passed with wide, unblinking eyes.

I had recently completed by first open water dive off the Brighton coast and gained my Sport Diver qualification. Now I was in the Straits of Tiran in the Red Sea. At 30m down a reef wall I was sucking air though the mouthpiece four times faster than on the surface — the result of the increased water pressure experienced at that depth. And, as an apprehensive near-novice, I was getting through my air supply even faster than the others in the group. In the excitement of the shark sighting I forgot to keep a check on my air contents gauge and the air flow soon began to tighten.

I breathed long and slow, trying to quell the rising panic when I saw the gauge registered a spent air supply. OK, I told myself, we‘re taught to handle this. I signalled “out of air” to my buddy who handed me his octopus rig — a spare hose and mouthpiece from his air cylinder. We ascended slowly without further incident.

The experience impressed on me the fact that the greatest dangers to a diver come from human error and not sharks or other denizens of the deep, although these aquatic nasties do exist and can shorten your diving career if you don’t treat them with respect.

Certain flora and fauna should be given a wide berth. Fire coral, for example, can scratch you and leave you with an angry graze that irritates for days, moray eels don’t invite a friendly handshake, the spins of sotone fish or lion fish can inflict a painful — even fatal — wound and stinging hydrios are also best left along.

Before you begin diving in open water you need a medical check-up and chest X-ray to show you do not have heart or respiratory problems. Then there is the training, designed to increase awareness and decrease hazards.

I soon learnt that the rules of diving are not simply niceties introduced to make the sport look more respectable or “official” — they minimise the risks.

You learn to never dive alone. Always have a buddy and keep a constant eye on each other, communicating frequently by using hand signals underwater. Check that each other’s equipment is functioning properly and that you know how to operate your buddy’s lifejacket and release his weight belt before you dive.

Look after equipment carefully, it will last longer and it is less likely to fail at a critical moment. Cylinders, regulators, valves and lifejackets should be serviced regularly. All your gear should be rinsed off with fresh water after a dive. Chlorine in swimming pools attacks the neoprene in wetsuits, while salt water corrodes metal buckles, zippers etc. An occasional squirt of light oil will keep metal parts rust-free.

Training involves theory as well as practical sessions. You don’t need a degree in physics to dive safely, but understanding a few of the fundamental principles related to diving will make the sport easier, safer and more fun. For example, how do you release the increasing pressure on your ears during the descent? What is decompression sickness or the “bends” and how do you avoid getting it?

Water pressure affects the body and dive equipment in a number of ways that need to be understood if you intend going further than a holiday resort “taster” dive. When you are underwater the surrounding water exerts pressure on you from all sides. This pressure increases rapidly with depth, and has a marked effect on any air you take down — in your lungs, ears, lifejacket and within your wet suit or dry suit.

You don’t need to know that someone called Boyle gave his name to the law, but you should know that when the pressure is doubled, the volume of gas is halved and vice versa. This amount of change occurs in the first 10m of water. In order to enable your lungs to expand normally as you descent, they must be supplied with air at the same pressure as the surrounding water. The regulator or demand valve supplies compressed air from the aqualung at the same pressure as the surrounding water.

As you descend, the volume of air in your inner ear decreases, creating a slight vacuum. This can become painful if not corrected. Some people can equalise the pressure either side of their eardrum simply by swallowing. Others hold their nose, close their mouth and blow gently. This opens the Eustachian tubes which link the inner ear to the nasal cavity.

As the air in your lifejacket and diving suit is compressed you become less buoyant. Unless you compensate for this by inflating the lifejacket a little, you will descend with increasing speed, and unless you watch the depth gauge, it is quite possible to go beyond your intended depth, and possibly deeper than the safe limit of sport diving.

The effects of Boyle’s Law are more critical when returning to the surface. Imagine filling your lungs with air at 10 metres, then surfacing without releasing any. The volume of air will double and you’ll probably burst a lung. The simple solution is to breath out during ascent.

Although harmless under normal surface pressure, the nitrogen and oxygen we breath become dangerous at the higher pressures experienced at depth. If you go deeper than 30 metres the concentrated nitrogen in the air supply can produce nitrogen narcosis or “the narcs”. You feel slightly drunk and light-headed — hence the term “rapture of the deep”. Luckily the narcs don’t leave you with a hangover, but you should be wary of their influence.

As the effect increases you become disorientated and your ability to reason or make sensible decisions diminish. Divers have been known to feel so confident and secure that they have taken out their regulators and offered them to passing fish. The effects soon disappear when you ascend.

And what about the infamous “bends” or decompression sickness. You risk suffering the bends if you dive beyond the time and depth limits outlined in the dive tables. Under pressure some of the nitrogen in the air dissolves into the bloodstream and body tissues. The greater the pressure and the longer you stay down, the more nitrogen is absorbed into your system.

As you surface the reverse occurs. Nitrogen is returned to the lung surfaces and breathed out. If you stay down too long or surface too fast, the excess nitrogen will not have time to be dispelled. Instead it forms bubbles of gas which become trapped in your blood and body tissues. These trapped bubbles expand as you surface and may block the blood circulation or damage the nervous system. If the flow of blood to the heart or brain is blocked, the result will be permanent injury, paralysis or death. The solution is to stay within the limits specified by the dive tables.

Another physical law involves heat conduction. As water is an efficient conductor of heat, it rapidly conducts heat away from the body. In water cooler than 20 degrees centigrade, you will need a diving suit to avoid becoming quickly chilled. Hypothermia can set in very quickly when the body gets too cold. You become lethargic, woolly-headed and less able to make decision that are in your best interest. The solution is to leave the water and warm up as quickly as possible.

Learning to dive in British waters may not be the most comfortable because of the cold water and often poor visibility, but it stands you in good stead for diving anywhere in the world. You’ll be more alert to the dangers of hypothermia, losing your buddy or becoming tangled in a fishing net or other obstacle.

As with all matters of safety, prevention is always the best cure. Most diving accidents can be avoided by good diving practice and regular maintenance of equipment. To be a good diver you must be able to keep calm if something goes wrong. This enables you to assess the situation and decide the most appropriate course of action.