Thailand – Australia – United Kingdom

Technical Diving – How safe is it?

Big Blue Tech explains the safety regarding technical diving.

Koh Tao, Thailand

Deep (100m) heliox bounce diving in the oil industry was briefly fashionable in the 1960’s, before it was realized that upwards of 20% of dives resulted in decompression illness (DCI). Even earlier (the 1920’s) the US made the first helium dives but again, a high incidence of DCI resulted. So what has changed now that many thousands of divers seemingly make safe and uneventful deep technical dives annually? Is it as safe as the technical training agencies purport it to be?

Technical diving as we know it had it’s origins with the cave divers of Florida and Europe. Bill Hamilton, the well known decompression modeler, produced schedules which enabled US cave divers to dive to over 70m using trimix (a mixture of oxygen, helium and nitrogen) and return safely to the surface breathing different “deco” mixes on the way up. This practice forms the basis of all deep technical diving today.

Technical diving, whilst still being a recreation, is clearly different to the hundreds of thousands of recreational SCUBA dives performed for pleasure around the world every year. Technical divers plan to dive beyond what are normally accepted as recreational limits, use gas mixtures other than air, frequently undergo prolonged staged decompression, often dive in an overhead environment (caves, wrecks and under the ice) and employ more sophisticated technology to fulfill their objective (such as closed circuit rebreathers, dive propulsion vehicles and other redundant safety equipment). The nature of the dives they undertake means that self sufficiency and equipment redundancy is crucial, so that in the event of mishap or accident the surface can be reached without assistance from another diver. So although the buddy system is still strongly advocated by most, there is a strong belief in the ability to save ones self in the event a problem arises. In the overhead environment, or when an artificial overhead is in place due to decompression obligation, it is crucial that problems can be sorted out underwater without recourse to an emergency ascent. Such diving requires good understanding of diving and decompression pathophysiology and a sound understanding of dive equipment and the diving environment.

Hence many technical divers appear to be highly motivated, focused and dedicated individuals. The equipment, training and gas costs are not insignificant, so as a group they are older on average than recreational open water divers i.e. many have reached a point in their lives where this type of diving is affordable to them. In this time, they have also gained sufficient diving experience to be comfortable enough in the water. The technical diving community also contains at one end of the spectrum, a small group who are constantly pushing the limits of the sport by diving further into caves, deeper onto wrecks and trying new equipment technologies often modified to meet their specific needs. No one would argue that this group whilst pushing the limits of the sport is certainly incurring significant risk to do so. Like the pioneer cave divers of the ‘70s, deaths in this group are not uncommon, but for those who follow the lessons have been hard learned and make it much safer. Nowadays, the majority of “technical” divers perform countless dives with out incident.

But despite the well trodden path that today’s tech divers follow, despite the high quality of training and equipment available and the almost obsessive nature of many of these divers, just how safe is it compared with standard recreational air diving? Many would suggest that the risks remain too great, because when things go wrong during a technical dive, the surface is just too far away to hope for a good outcome. To try and find a measure of the risk, I have looked to several sources. Various websites delight in naming the ten most dangerous adventure sports. The following are consistently mentioned although the basis for some of the claims is not clear: Base jumping (probably a clear winner!), free diving, cave diving, bull riding, mountain and ice climbing, big-wave surfing, rock fishing, street luging and extreme snow skiing. Activities involving bicycles also rate highly. “Technical” diving per se doesn’t rate a mention. Base jumping sources in the USA suggest from 5-15 participants die per annum in the USA, however the number involved in the sport is not known. In 2002, 24 individuals died whilst rock climbing in the USA. An Austrian source revealed that 1170 people died in the mountains from 1986-1997, giving a risk of 6.7/100 000 participants or 0.007% for rock and ice climbing. Since 1960, over 500 divers have died in caves in Florida, Mexico and the Caribbean which averages out to around 11 deaths per annum, although one would suspect that many more occurred in the early days of the sport before good training and equipment advances were established. As the denominator is not known, a percentage risk cannot be given. In the UK, the overall estimated risk of dying whilst diving is 1:200 000 or 0.0005%. The BSAC have maintained excellent records on diving incidents and fatalities for many years, published each year in the NDC Diving Incidents Reports by Brian Cumming(1). The UK averages 16.3 fatalities per annum, of which a high percentage regularly involve deep diving. This quote from the 2003 report seems representative of most years: “The number of incidents reported in the greater than 50m range is 11 and this is lower than previous years. However 5 of these 11 were fatal incidents, clearly indicating the risks associated with deep diving. There has been a moderation in the number of incidents related to very deep diving, but there is still a clear and very strong correlation between increasing depth and increasing risk. 4 incidents involved depths of greater than 60m and 3 of these were fatalities.” And from 2002: “However the number of incidents reported in the greater than 50m ranges continues to grow. 22 incidents involve dives to greater than 50m and this is the highest number ever recorded in this category. Of these 22, 6 (27%) were fatalities. The message is very clear – Diving at depth brings much greater risk and deep incidents are far more likely to be serious ones”.

More data comes from the 2004 edition of DAN’s “Report on Decompression Illness, Diving Fatalities and Project Dive Exploration” (2), which reports on the year 2002 and some 5 year trends. Project Dive Exploration (PDE) collects data from recreational dives performed with specific types of computers, whether or not the dives result in injury or fatality. Some 70000 dives have been collected since 1995. In this database (2002 only), about 3% of the divers held a technical diving qualification. 2% of dives were performed with trimix or heliox. The exact number of technical dives performed is not known, but may be inferred to be around the 2-3% mark. Overall, 9-10% of divers dying were making technical dives i.e. technical dives are over represented in the fatality figures. I would emphasise that because of the lack of a denominator, these significance of these figures remains unclear.

The final area of interest is that of closed circuit rebreathers (CCRs). CCRs are complex pieces of equipment which demand great diligence, discipline and respect to dive safely. They are enabling tech divers to push back the boundaries of exploration even further, and allow the “average” tech diver to perform dives that were previously too complex or difficult. The most widely used electronic CCR in use is the Inspiration made by Ambient Pressure Diving which was released in 1999. Because it was the first readily available CCR and because of its huge popularity (estimated around 6000 units now worldwide), it has also been involved in the largest number of accidents and fatalities (23 according to one source (3), but possibly more). The unit itself is highly regarded by most in the technical diving community and is CE approved, however by virtue of the number in the marketplace; it has been involved in the most incidents. Should another of the popular CCRs be the market leader, it is likely they would be in the same unenviable position. The 23 fatalities from some 6000 units give a risk of 0.4% or 1 in 240 per owner. Compare this with the 0.007% for Austrian Rock climbers or 0.0005% from UK divers overall.

So what factors may make rebreather diving so dangerous? Closed circuit rebreathers (compared to semi closed rebreathers which are generally not used for deep diving excursions) represent fairly new technology in recreational diving. Breathing gas is recycled around a loop so that exhaled oxygen is available for reuse, so only oxygen required for metabolism is consumed. Carbon dioxide is removed by a soda lime scrubber and metabolized oxygen is replenished into the loop by mechanical, electronic or manual means. A diluent gas is required to maintain volume in the loop on descent, and expanding gas is vented during ascent. All very simple in principle. However the control over the crucial levels of the 2 important gases in the loop (oxygen and carbon dioxide) must be precise or the diver courts disaster. If the partial pressure of oxygen (PO2) stays consistently above 1.4, the risk of convulsion and drowning increases. If the PO2 in the loop falls to hypoxic levels, unheralded unconsciousness (and drowning) may result. Exhaustion of soda lime or a problem with gas flow through the scrubber will cause a disabling rise in carbon dioxide levels. Any loss of integrity of the unit may cause flooding which will render the unit completely unusable (a situation far less likely to occur on SCUBA). Without meticulous pre-dive checks and adherence to procedure, these life threatening occurrences may occur in any CCR unit. Hence the primary safety feature of all rebreathers, is the comprehensive checking of the equipment before a dive. Most fatalities arise because of a problem in checking or maintenance, rather than some inherent fault of the unit.

All of us accept risks on a daily basis. Every year many more than 100 000 people are killed by snakes(4), 960 by crocodiles, 14 by ostriches and 7 by sharks(5). Compare the paranoia about shark attack with the worry over ostriches! Keep it in perspective in other words! However, the (scanty) facts available at present do suggest that technical diving whether deep, wreck, cave or rebreather diving, does have an increased risk of death compared to recreational open water diving. This kind of diving is not for everyone, and to do it safely requires a large investment in time, training, personal fitness and attention to detail. In its present form, it should not become an inevitable progression of advanced open water training following a basic OW course. Certain individuals should never take up technical diving, and most will never wish to. Training agencies must continue to make potential tech divers aware of the risks and maintain standards to the highest level, to avoid the growing tendency for outsiders to over-regulate the sport. Dive shops should be clear at the outset that enrollment into technical dive training will not guarantee successful completion if a high standard is not met.

For the record, I am a passionate cave and rebreather diver. The acceptance of risk is a very personal decision, and for me I cannot imagine a life without diving. But, like all prospective tech divers should do, I have attempted to fully inform myself of the dangers in order to dive safely.

(1) Cumming B. NDC Diving Incident Reports. BSAC website

(2) DAN Report on Decompression Illness, Diving Fatalities and Project Dive Exploration: 2004 edition (Based on 2002 data). Divers Alert Network. ISBN 0-967 3066-5-5

(3) Personal communication, Stephen Hawkins. See

(4) Chippaux J-P. Snake-bites: appraisal of the global situation. Bull WHO 1998; 76(5):515-24.

(5) Stark, P. (2001). Last breath: Cautionary tales from the limits of human endurance. Ballantine.


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