I was asked to comment on this thread by Mpetryk. There is a lot of useful information and good discussion. I will pick a few bones, however.
First, the comparison of fatalities occurring in "inland waters" and "abroad" in the Fall 2009 Alert Diver 'Dive Slate' was improper. As was suggested by one person in this thread, the data capture by DAN is not the same for the two cases mentioned. Cases that occur in the US or Canada or those known to involve American citizens are actively followed. The Caribbean often gets a bit more attention, but with insufficient rigor to justify this as a meaningful discussion point. While the mention of "fatalities abroad" was completely inappropriate, the idea that swimming pool or tropical conditions do not adequately prepare divers for more complex environments is true. The fact that this subject is being discussed in the current thread is another positive sign.
I agree with the comments of some in this thread that absolute temperature is a red herring in terms of risk. Appropriate thermal protection removes most of the stress. There is likely to be a bit more load on the diver due to the increased mass (most important out of the water) and increased bulk and shifts in trim can become issues. These, though, are not huge factors for the prepared and healthy diver. Preparation is the key to avoiding trouble. The comment about cold water divers being safer when dropped into warm water conditions than the reverse is true for more reasons than temperature. For example, the warm water diver is likely to be less effective wearing three finger mitts for the first time than a diver used to wearing heavy mitts going barehanded.
Let's consider training. There has been a problem with a macho attitude in diving. Any instructor that shows the attitude of "I am great" and passes on some of that sense of "greatness" to his (sorry, but this will likely be gender specific) students has done them a disservice. Instructors need to be assiduous in helping students hone critical skills AND in making them realize that they are only getting a license to learn, not expertise. Several elements can relate directly to the current discussion. I will use mitts as the first example since they were mentioned above. Divers that will be wearing mitts in open water should begin to use them early on in pool training early so the sudden lost of dexterity is not an unnecessary stressor of open water training. This training evolution is a little harder to justify for tropical courses when the use of handwear is discouraged to minimize divers touching sensitive substrates. Warm water training should include some discussion of the added complexities of cold water diving so the candidate appreciates that additional challenges will arise when conditions get more complicated. The complications can come from changes in equipment, depth, visibility, current, overhead environments, etc.
Neutral buoyancy is a critical skill to put any diver on the right path. An instructor does a disservice to students when they are encouraged to settle on the bottom to practice skills. While it is not as easy to manage, practicing skills in the water column better prepares students for the real diving world. Similarly, many students are overweighted during dive training. This makes it easier to get them below the surface and/or to keep them on the bottom, but it is a tremendous disservice to their skill development. Any diver who argues for a given amount of weight because "I have always used that" was poorly trained. The impact of experience on buoyancy is critical. Novice divers should appreciate that critical and ongoing evaluation of weighting needs is important for comfort, performance and safety. The relaxed diver with a smaller lung volume (less anxiety) and greater ease of moving through the water can often lose a substantial amount of weight.
The neutral buoyancy discussion is not off topic. One comment in the string was blatantly misleading:
"a. Cold requires insulation -- insulation requires weight -- weight leads to overweighting and poor buoyancy control -- "
Yes, thicker neoprene or drysuits with increasing gas trapping due to greater insulation does require more external weight to achieve neutrality. The idea that this inevitably leads to "overweighting and poor buoyancy control" is nonsense. Adequate training, diligence and experience can preserve good neutral buoyancy with any reasonable equipment configuration. Cold water is not the problem; a lack of preparedness and competency may well be.
Time to turn to the hypothermia issue. Again, some good discussion with a bit of nonsense thrown in to remind us of the importance of fact checking. Sure, it is possible to get hypothermia in 80°F (27°C) water, but the person who expects to see it in divers probably also expects to win when buying a lottery ticket. The critical water temperature (temperature at which core temperature can be maintained for three hours by maximal vasoconstriction alone without shivering) varies as a function of body mass, the ratio of body mass to surface area, the thickness of skinfold insulation, and other lesser factors. The critical temperature for a semi-naked (i.e., bathing suit wearing), resting diver is 32-35°C (90-95°F). Light exercise decreased this to 29-33°C (84-91°F) Nakanishi et al. (1999). Craig and Dvorak (1968) found that 24°C (75°F) was the switchpoint temperature for core temperature protection through swimming. More heat was lost with swimming below this temperature but core temperature could be maintained by swimming efforts above this temperature. It is important to note that thermal stress will vary on an individual basis, but these numbers provide some foundation reference. My personal switchpoint for happily wearing dive skins instead of wetsuit is 82°F (28°C). I feel minimal thermal stress over the duration of a healthy length dive in water at this temperature. Yes, if I were lost at sea in water at this temperature I would ultimately have a problem with hypothermia, but this would only be a significant risk after many, many hours. Having additional protection is always desirable for bad days, but a rational understanding of true risk is important.
The article reached by link that talked about cold stress from Alaskan experience was misleading. It would have been more compelling if it were written by the expert instead of someone who had heard the expert lecture. Hypothermia is typically defined as a core temperature of 35°C (95°F) or less, not a temperature below 37°C (98.6°F). A core temperature between 35°C and an individual's normal temperature is not hypothermia. Many uninformed divers talk about being "hypothermic" when they should be saying "I'm cold." Typical cold research studies use a core temperature of 35°C (95°F) as an endpoint. The risk of life in an otherwise healthy individual is trivial at this point. It may feel uncomfortable but is of little significance. Again, for perspective, a good study evaluated survivors of serious cold exposure. Eight out of 26 had core temperatures at admission to hospital of 23.6±2.4°C (74.5±4.3°F). The critical factor for survival was no evidence of asphyxia (Farstad et al., 2001). In other words, if the airway is protected, we can recover from quite low core temperatures.
Even a modest amount of protection provided by a wetsuit or drysuit system prolongs tolerance and survival time dramatically. I frequently refer to an example of a diver wearing a drysuit with a substantial leak completing a 43 minute dive in -2°C (29°F) water (Pollock, 2007). The effect of core temperature of this quite stressful dive was surprisingly small. It should reinforce the point that more homework would be helpful for those who expect hypothermia to be a likely outcome of diving even under extreme conditions.
Neal W. Pollock, Ph.D.
Divers Alert Network
6 West Colony Place
Durham, NC 27705
and
Center for Hyperbaric Medicine and Environmental Physiology
Duke University Medical Center
P.O. Box 3823
Durham, NC 27710
References
Craig AB, Dvorak M. Thermal regulation during water immersion. J Appl Physiol 1968; 21: 1577-1585.
Farstad M, Andersen KS, Koller ME, Grong K, Segadal L, Husby P. Rewarming from accidental hypothermia by extracorporeal circulation: a retrospective study. Eur J Cardiothor Surg 2001; 20: 58-64.
Nakanishi Y, Kimura T, Yokoo Y. Maximal physiological responses to deep water running at thermoneutral temperature. Applied Human Science. 1999; 18(2): 31-5.
Pollock NW. Scientific diving in Antarctica: history and current practice. Diving Hyperb Med. 2007; 37(4): 204-211.
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