Clarke JR1, Moon RE2, Chimiak JM3, Stinton R4, Van Hoesen KB5, and Lang MA5,6.
1 US Navy Experimental Diving Unit, Panama City, Florida
2 Duke University, Durham, North Carolina
3 Divers Alert Network, Durham, North Carolina
4 Diving Unlimited International, Inc., San Diego, California
5 UC San Diego – Emergency Medicine, San Diego, California
6 OxyHeal Health Group, National City, California
The San Diego Center of Excellence in Diving at UC San Diego aims to help divers be effective consumers of scientific information through its “Healthy Divers in Healthy Oceans” mission. In this monograph we explore a research report from the Navy Experimental Diving Unit (NEDU) that is leading some divers to think they should be cold if they want to reduce decompression risk. That is a misinterpretation of the report, and may be causing divers to miss some of the joy of diving. There is no substitute for comfort and safety on a dive.
In 2007 NEDU published their often-cited report “The Influence of Thermal Exposure on Diver Susceptibility to Decompression Sickness” (Gerth et al., 2007). The authors, Drs. Wayne Gerth, Victor Ruterbusch, and Ed Long were questioning the conventional wisdom that cold at depth increases the risk of decompression illness. After conducting a very carefully designed experiment, they were shocked to find that exactly the opposite was true. Some degree of cooling was beneficial, as long as the diver was warm during ascent.
Discussion and Implications
There are some important caveats for the non-Navy diver to consider. First of all, it was anticipated that a diver would have a system for carefully controlling their temperature during the separate phases of bottom time and decompression. Most non-Navy divers do not have that sort of surface support.
Secondly, the “cold” water in the NEDU study was 80 °F (27 °C). For most of us, 80 °F (27 °C) is an ideal swimming pool temperature, not exactly what you are going to find in non-tropical oceans and lakes. The warm water was 97 °F (36 °C), also a temperature not likely to be available to recreational and technical divers.
When testing the effect of anything on decompression results, the Navy uses their extensive mathematical expertise to select the one dive profile that is, in their estimation, the most likely to identify a difference in decompression risk, if that difference in risk exists. In this case the profile selected was a 120 fsw (37 msw) dive with 25 to 70 min bottom time, decompressed on a US Navy Standard Air table for 120 fsw (37 msw) and 70 min bottom time. That table prescribes 91 minutes of decompression divided thusly: 30 fsw/9 min (9msw/9 min), 20 fsw/23 min (6 msw/23 min), 10 fsw/55 min (3 msw/55 min).
A total of 400 carefully controlled dives were conducted yielding 21 diagnosed cases of decompression sickness. Overwhelmingly, the lowest risk of decompression was found when divers were kept warm during decompression. The effects of a 9 °C increase in water temperature during decompression was comparable to the effects of halving bottom time.
That is of course a remarkable result, apparently remarkable enough to cause civilian divers to alter their behavior when performing decompression dives. However, before you decide to chill yourself on the bottom or increase your risk of becoming hypothermic, consider these facts.
Support for the San Diego Center of Excellence in Diving is provided by founding partners UC San Diego Health Sciences, UC San Diego Scripps Institution of Oceanography, OxyHeal Health Group, Divers Alert Network, Diving Unlimited International, Inc. and Scubapro.
Sterba JA. Thermal Problems: Prevention and Treatment, in P.B. Bennett and D.H. Elliot, eds., The Physiology and Medicine of Diving, 4th ed. (London: Saunders, 1993), pp. 301-341.