Don’t Dive Cold When You Don’t Have To

Navy Experimental Diving UnitClarke 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.

  1. Do you have a way of keeping yourself warm, for instance with a hot water suit, during decompression? If not, the study results do not apply to you.
  2. Of the many possible decompression schedules, the Navy tested only one schedule, the one considered to be the best for demonstrating a thermal influence on decompression risk. Although it seems reasonable that this result could be extrapolated to other dive profiles, such extrapolation is always risky. It may simply not hold for the particular dive you plan to make, especially if that dive is deeper and longer than tested.
  3. Most commercial decompression computers do not adhere to the U.S. Navy Air Tables; few recreational dives are square profiles. Furthermore, additional conservatism is usually added to commercial algorithms. NEDU is not able to test the effects of diver skin temperature on all proprietary decompression tables, nor should they. That is not their mission.
  4. The scientific method requires research to be replicated before test results can be proven or generalized. However, due to the labor and expense involved in the NEDU dive series, it seems unlikely that any experiments that would determine the relevance of these results to recreational or technical diving will ever be performed. As such, it may raise as many questions as it answers. For instance, the original question remains; if you become chilled on a dive, how does that affect your overall risk of decompression illness compared to remaining comfortably warm? Unfortunately, that question may never be answered fully.
  5. Thermoneutral temperatures for swim suited divers are reported to be 93 °F to 97 °F (34 to 36 °C) for divers at rest and 90 °F (32 °C) during light to moderate work (Sterba, 1993). So a skin temperature of 80 °F (27 °C) is indeed cold for long duration dives. If your skin temperature is less than 80 °F (27 °C), then you are venturing into the unknown; NEDU’s results may not apply.

In summary, beer and some types of wine are best chilled. Arguably, divers are not.

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.

Gerth WA, Ruterbusch VL, Long ET. The Influence of Thermal Exposure on Diver Susceptibility to Decompression Sickness. NEDU Technical Report 06-07, November 2007.
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.

4 replies
  1. Landen
    Landen says:

    Best part of the article was the closing, “In summary, beer and some types of wine are best chilled. Arguably, divers are not”.

  2. Toscano
    Toscano says:

    Wonder if dives at waters around 5 degrees celsius , with dey suits , with decompression may present some problems with DD , for the hands are protected with good gloves but not dry ones

  3. Rosemary E Lunn
    Rosemary E Lunn says:

    Dr Neal W Pollock wears many hats. He gained his doctorate in exercise physiology / environmental physiology at Florida State University. Amongst other things, he is Research Director at Divers Alert Network, a Senior Research Associate at the Center for Hyperbaric Medicine and Environmental Physiology at Duke University Medical Center, a member of the Board of the American Academy of Underwater Sciences Foundation and a member of Editorial Board of Diving and Hyperbaric Medicine.

    Neal specialises in thermal stress and diving and lectures throughout the world. He has talked at EUROTEK and TEKDiveUSA, and spoke extensively about this US Navy Experimental Unit research at Rebreather Forum 3. (Not entirely sure how divers are interpreting the NEDU research to think they should be cold if they want to reduce decompression risk).

    Neal explains quite clearly about diving warm / cold, cold / warm etc in these videos. These films are free to access, so pass them on to any interested diver.

    Part one;
    Part two;

    In addition you can access the conference proceedings from this international rebreather symposium for free. (TDI were a key part of this event). Just follow this link; and read what Neal Pollock has to say on how to dive temperate water diving whilst reducing your decompression stress.

    FYI Dr Pollock also recommends that you dive heated devices (i.e. heated vests) in a thoughtful manner. So off /low during the first half of the dive, turning the heated device on or up during the ascent phase of the dive.

  4. Neal Pollock
    Neal Pollock says:

    This post is, unfortunately, extremely misleading. The NEDU study (Gerth et al. 2007) was very well designed. It shows the practical importance that thermal status can have on decompression risk. This reinforces earlier reports of elevated risk of DCS in North Sea divers wearing hot water suits vs. drysuits with passive insulation (Shields and Lee 1986).

    Divers need to have adequate thermal protection to function properly (physically and cognitively). This operational imperative was never in question. The suggestion that a cool state equates to hypothermia is a ridiculous distraction that seems like an effort to distract from the value of the study. By definition, hypothermia is a core temperature below 35C. Divers can get cold; but even the poorest thermal protection will keep them from becoming hypothermic. The smart interpretation of the study results are that sufficient warming is appropriate, but excessive warming during the gas uptake phase can increase the decompression hazard.

    The single depth provided control; it does not invalidate the ability to apply these findings to other exposure patterns.

    Pointing out that most recreational divers do not follow square profiles is another red herring. Dive computers (which most divers rely on) compute based on actual exposure profiles. More critically, no dive computer measures skin or core temperature and no computer algorithm is yet able to directly compute the decompression risk of variable thermal status. Dive computers provide a rudimentary estimate of decompression status; it is up to the diver to adjust the exposure based on many factors, with thermal status being a very important one.

    Replication of scientific finding is important, but to suggest that the findings of a solid study are not valid until that point is achieved is quite lame. The NEDU investigators did not overpromise; they highlighted the potential importance of thermal issues. Encouraging divers to ignore the results some of the best work available to the community is disingenuous. The glib disdain of the closing statement is similarly disappointing.

    Shields TG, Lee WB. The Incidence of Decompression Sickness Arising from Commercial Offshore Air-Diving Operations in the UK Sector of the North Sea during 1982/83. Dept of Energy and Robert Gordon’s Institute of Technology: UK, 1986.


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