Among divers, the three top risk factors for diving deaths are running out of air, buoyancy problems and rapid ascents - - and often times, these factors can happen concurrently. But what’s the biggest cause for those factors to happen - - the diver, his equipment or the overall dive environment?

To determine which has a greater impact, researchers Richard Vann, Petar Denoble and Richard Dunford of Divers Alert Network, along with Peter Buzzacott of the University of Western Australia, collected data via surveys taken by divers making dives from liveaboards, day boats and shore between 1995 and 2004 in the Caribbean, Grand Cayman and Scapa Flow. Overall, they collected data on 452,582 recreational dives with air or nitrox made by 5,046 adults. They published the results in the journal Diving and Hyperbaric Medicine.

The three problems - - air, buoyancy and ascent - - were classified as human (diver) or diving (environmental/ equipment) factors. Human factors included age, sex and certification level. Diving factors included dive time, maximum depth, air used, boat or shore dive, water temperatures and strenuousness of the dive.

Running out of air was reported by 65 divers (1.4%), 223 divers had buoyancy problems (4.7%) and 235 divers had rapid ascents (7%). Divers who reported running out of air at least once were slightly older than the average age of all divers (42) and more than twice as likely to be female. Divers with buoyancy problems were also likely to be female and slightly older, plus more likely to have just a basic diver certification. On the other hand, divers making rapid ascents were mostly male, slightly younger than the norm and more likely to have advanced certification or specialty training.

Based on diving factors, running out of air happened on slightly deeper, slightly shorter dives (averaging 22 feet for 45 minutes) from liveaboard or charter boats. Dives with buoyancy problems were more likely to have been made from boats, used air instead of nitrox, and were reported as strenuous. Rapid-ascent dives shared all the above factors, except they happened at shallower depths (an average of 21 feet).

So overall, older women ran out of air and reported buoyancy problems, while younger men had rapid ascents more often. It’s curious how the increase in certification status reduced the risk of buoyancy problems but increased the risk of rapid ascents. Researchers think it may be because divers are more attentive when diving deeper, or perhaps more carefree if they can see the surface when they start their final ascent.

The most interesting finding was that all three dive problems were associated with boat dives. The weak but significant association with shorter dive times is likely a consequence of being told when to return to the boat, but strenuous dives were strongly associated with all three dive problems. Perhaps divers are being taken to sites they later discover are more challenging than anticipated, especially older divers and women who, for example, may perceive a moderate current or long surface swim to be harder work than younger men do.

The high prevalence of divers ascending faster than a commonly recommended maximum rate (3.8 %) is cause for concern, especially as the majority of those who ascended too rapidly did not report it. It’s noteworthy that few people who reported a rapid ascent actually exceeded 60 feet per minute, and those who actually did rapid ascents later reported it on their survey as a problem. Of the 227 rapid-ascent dives recorded on dive computers, 88 percent were made by divers who were either unaware of their ascent rate, ascended rapidly during the dive at a time other than the final ascent, or may have defined rapid ascent differently from the researchers’ criteria. Therefore, at first glance, the prevalence of these problems happening appears relatively low. To put the findings into perspective, if this were an accurate estimate of the prevalence likely to be experienced by a dive boat taking 25 divers out for two dives a day for 200 days of each year, crew could expect divers to run out of air on 18 dives, 77 divers will report buoyancy problems, 63 divers will report rapid ascents and 49 dives will show a recorded rapid ascent on the computer. That makes a total of 201 problems per year, or an average of one event per dive day. So even a low prevalence as reported here should be cause for concern aboard dive boats of all sizes.

Therefore, dive instructors should give greater emphasis during basic training to monitoring gas reserves, effective buoyancy control techniques and the importance of ascending slowly, coupled with practical methods of gauging ascent rate (like monitoring depth-per-minute rather than looking up to the surface). Also, greater emphasis upon ascent rates during training should be used to reduce ascent speeds among future divers. And dive crew should advise divers before each dive to consider the potential for physical stress.