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The risk of decompression illness depends on many aspects and the risk does vary with the type of diving. So, what are the rates reported?
The rate among recreational divers is approximately 10 to 20 hits per 100,000 dives. This rate is derived from the reported cases of DCI and an estimate of the total number of dives from various studies.
Previous studies have reported various numbers per 100,000 dives: 12.9 in the Caribbean; 13.4 in Okinawa; 9.57 in British Columbia; and 10.5 at an inland diving centre in England. There are various factors and uncertainties around each of these estimates relating to data collected or other events, such as missed safety stops, time-depth violations, etc.
Other studies at Scapa Flow, Scotland estimated 20, 46 and 87.4 per 100,000 using different periods and different methods. This is a vastly different number to surveys by DAN Europe on new members which reports 1.9 per 100,000 dives.
The data presented confirms the assertion that it is difficult to calculate the exact rate of DCI in recreational divers. Studies have also shown different rates of DCI for tropical water versus cold water diving, and in different locations around the world. The errors in the reporting can existing in three key areas: underreporting of DCI; errors in diagnosing DCI; and errors in the estimate of the population of dives.
The errors listed above can lead to both over and under estimates of risk. However, the studies show that the risk of DCI in recreational sport diving is low. Even considering this, you should do all you can to minimize your risk: shallower dives; ascending slowly; and performing deep stops (if required) or safety stops.
The original article ‘What is the risk of decompression illness’ by Barbara Leigh is available here: http://divingresearch.qofe.com/page2/page4/page4.html