By Libby Sutcliffe
BBC News Online, South East
The death of two scuba divers off the Sussex coast may have prompted fears about the safety of the sport.
Divers normally use compressed air - a mix of nitrogen and oxygen
But according to the British Sub Aqua Club (BSAC) diving is safe as long as people have the right training, know their limits and follow advice.
The divers who died in the sea off Shoreham - Clyde Warner, 56, from Slough and Martin Reed, in his 50s, from Uxbridge - had been exploring a wreck at a depth of 63m on Tuesday.
One of their diving buddies suffered decompression sickness but was released after hospital treatment.
Normally scuba divers use compressed air which is a mix of nitrogen and oxygen - the main components of the air we breathe.
But advanced divers can use what is known as tri-mix which incorporates another gas, usually helium.
This allows divers to reach greater depths because the helium reduces the narcosis effect of nitrogen experienced by divers when they go deeper than about 35m.
Jim Watson, BSAC coaching and instructor training scheme manager, said the organisation recommends a depth of 50m when divers are using air.
With tri-mix the advice is to dive no deeper than 70m.
But he said: "They are recommendations rather than rules.
"One of the reasons we call them recommendations is because if you create a rule then human beings will try and step over it."
Overall there are about 250,000 divers in the UK completing more than two million dives a year, said Mr Watson.
Divers pre-plan their dives with computers or tables
Out of these the average number of fatalities taken from figures over the past decade is 17 per year.
Mr Watson said: "In relation to fatal diving accidents at least a third of those are caused by people who exceed either their own personal limits, their training or their experience.
"Or they disregard safety advice provided by the diving agencies.
"For another third we simply do not have enough information to know what happened and the other third we really have to put down to one of those unfortunate incidents."
The divers involved in the incident on Tuesday were said to be very experienced.
Bubbles can form
They had gone out on the boat the Michelle Mary, which they used as their base, to explore a wreck called the Bessell which is known as a tri-mix dive as it lies at 63m.
The skipper of the Michelle Mary confirmed on Wednesday the divers involved in the incident were using air rather than tri-mix.
For a dive that deep, Mr Watson said divers would have needed at least three decompression stops - to allow nitrogen in the body to be released.
This is to prevent decompression sickness - a condition which used to be known as the bends.
Unless enough time is allowed for nitrogen to be released, the bubbles which form can impair blood flow, damaging nerves and tissues.
Divers pre-plan their dives using tables, or more commonly dive computers, to gauge when they have to stop and for how long.
One of the scuba divers who died is thought to have not completed any of his stops while his buddy who survived managed only half of them, although it is not known why.
Despite what could be perceived as dangers with the sport Mr Watson said: "Diving can be very safe providing you follow your training and build up experience.
"It is one of the few sports out of any sports where a human being can go into a totally alien environment and survive."