A surprise lesson…


There are plenty of incentives for taking part in a scuba diving ‘expedition,’ and some of them at least, may not obvious at first sight.

The best, and a motivation with all the enticement of a chocolate-chip cookie in the eyes of the average six-year old, is to learn something new. Coming a very close second – with the pulling power of a chocolate- dipped coconut macaroon in my world – is having a tightly-held, but wrong-headed notion, kicked into the trash, and replacing it with an idea backed by science, logic, and responsibly collected data. Again, a learning experience.

Earlier this year, Jill Heinerth and I were asked to put together a small expedition to do some work in the Bell Island Iron Ore Mine, in Conception Bay, Newfoundland. We had both been there before, and we were both huge fans of what the place has to offer: essentially, the chance to experience truly unique dives in an environment that screams history and heritage.

There were tonnes of other reasons to sign on for this particular expedition, but diving in what is essentially an underwater museum, was pretty high on the list.

A total surprise – and an unexpected bonus – was having my opinion about heated vests and their potential role in diver safety – specifically decompression stress – turned around about 180 degrees.

The lesson went something like this.

Dr. Neal W. Pollock was part of the Bell Island project, gathering data for his research at Divers Alert Network. Neal is research director there – at DAN – and a senior research associate at the Center for Hyperbaric Medicine and Environmental Physiology at Duke University Medical Centre. And when not pushing an ultrasonic transducer against your rib cage, he’s a handy guy to have around when the chatter turns to many things related to technical diving.

I’d seen Neal’s presentation on Thermal Physiology and Protection at Rebreather Forum 3.0, in Florida a few years before. But, frankly did not really grasp his message. Then, sprawled on the floor of the Bell Island Museum and watching a whole stream of gas bubbles race around in my heart after a dive, helped me – and others on the expedition – listen a little harder to what he was telling us.

A link to his presentation at RB 3 is at the bottom of this page, and if you dive at all, you’d do well to watch it, but the Coles Notes version is this.

If, like me, you figure the safest way to dive is to be warm throughout, you may want to rethink your approach.

Dr. Pollock, suggests that there are three issues to think about when we consider thermal protection. Number one, at depth we must be able to function; secondly, we must take into account the effect of temperature on our decompression stress; and lastly we need to consider comfort.

Based on a “small but significant” study conducted by the Navy Experimental Diving Unit, Dr. Pollock explained: “Divers tend to put the emphasis on the wrong thing… comfort.” And comfort, according to the data, and Dr. Pollock, “should only be a distant third.”

The NEDU study found that the fewest instances of DCS occurred in divers who started cold, and finished warm. Probably the exact opposite of what happens on a significant number of technical dives. Worth noting is that for this study, subjects wore no thermal protection and worked in water at 36 degrees (warm), and 27 degrees (cold).

Significant also was that there were zero cases of DCS in 80 “cold start, warm end” dives, but in warm/cold dives, probably the situation for many long dives, seven out of 32 (22 percent) resulted in DCS. The dive profile incidentally was a seemingly benign 120 fsw with 30 minutes of bottom time and 91 minutes of deco!

When the dive profile was appreciably altered to be more aggressive – 120 fsw for 70 minutes of bottom time but with an unchanged 91 minutes of deco – the results had very similar implications: cold/warm dives resulted in 0.1 percent DCS.

Of interest to those of us who own and use heated drysuit under garments, warm/warm dives following the same profile, returned a 17 percent instance of DCS (four cases in 24 dives).


A slide from Dr. Neal Pollock’s presentation

For technical divers, there are many, many factors with a role to play in decompression stress, but consider this. With a heated vest, several things might happen. The vest is turned on and keeps you warm throughout your dive. The vest is turned on to start the dive, and then is turned off or runs out of power at the end of the dive. The vest is turned off to begin the dive and left off when you start to get chilled as bottom time passes, and only turned on during the latter stages of decompression (probably NOT the most common practice).

Based on the NEDU study, each has possible consequences, and not all of them positive. It would seem that the best option in terms of thermal status, is to start cold and end warm.

As Neal remarked, the NEDU study was across a very small population (73 divers), and one has to take that into consideration when assessing its value and relevance. However, my personal observation of what was happening in my heart after a 90-minute warm/warm dive – thanks to the transthoracic echocardiogram being orchestrated by Dr. Pollock about 20 minutes after I surfaced – is that I won’t do that again.



View Neal’s RB3 presentation here… and in addition to a much more complete interpretation of the NEDU study, he covers several other related and significant issues… watch it.








6 thoughts on “A surprise lesson…

  1. I guess if you start warm, you’ll on gas quicker than if you were cold, and if your cold during your deco, you’ll off gas slower. And the inverse applies, if you start cold, you’ll on gas slower and if your warm during deco, you’ll off gas quicker. Makes sense when you really think about it. Good lesson though and I certainly find myself starting warm or hot prior to getting in the water, and colder during deco until I get above the thermocline. The ideal is not always possible, but having the knowledge to manage your situation, perhaps extending your deco beyond your computer or tables would help in that 20 foot range?

    • Good link Thomas. However, I disagree that it is “difficult to extract something useful for [the] recreational diver.” from Dr. Pollock’s presentation. Listening to the entire slide show, taught me something. And, looking at my bubble grade after the dive in question, certainly showed me something of value.

      Safe diving to you!

    • The divers in the NEDU study were wearing shorts and T-shirts. Being in 80-degree water for 2 hours is going to be chilly. Hell, I’ve gotten chilled in a 3mm wetsuit in 83-degree water doing a 2-hr dive in Florida in June.

      Most of the tissues in the human body are perfusion limited, and that makes this study very relevant when you consider that perfusion is directly influenced by thermal conditions. Also, decompression stress is influenced by many factors like exertion, hydration, immune response, dive profile, thermal conditions, etc. Being cold during deco might just be that thing that tips the scales and turns deco stress into deco sickness.

      Based on this NEDU study, I try to only use my heater during the later stages of deco, and usually deal with the cold at the bottom as long as it doesn’t impede my performance or ruin the dive for me.

      Thanks for the post Steve.

  2. We have an interesting lake dive where I live that is fresh water on top 16 meters and salt water on the bottom. The fresh water layer is approx. 30C and the salt water layer is hottest at 18 meters (39C) then cools as you go deeper. No one is 100% sure, but it is assumed that a natural hotspring is causing the warmer temperatures. The lake used to be the last dive of the day, but there were several cases of skin bends reported. My thoughts were that divers would spend more time in the 39C water because the 30 degree water felt much colder until they acclimated thus spent more time at deeper depths. My other thought was that your circulation was increased at depth in the hot water and then decreased in the shallow colder water when your body is off-gassing. Changing the dive to the first dive of the day has (so far) eliminated cases of skin bends.

  3. There was a response to the article referenced in Diving and Hyperbaric Medicine above by Thomas. The citation and text of the response follow:

    Pollock NW. “Re: Don’t dive cold when you don’t have to.” Diving Hyperb Med. 2015 Sep; 45(3): 209.

    The letter by Clarke et al.1 unfortunately misrepresents the work at the US Navy Experimental Diving Unit (NEDU) to which it refers2, and delivers a confused picture of the physiological impact of thermal status on decompression stress.

    A series of earlier reports outline the importance of thermal status. Being warm during a dive results in higher post-dive Doppler bubble scores.3 Hot water suits are associated with a higher rate of decompression sickness (DCS) than passively insulated drysuits.4 And post-dive cooling can prolong the risk window for developing symptoms of skin bends.5

    The NEDU chamber study provided an elegant design to further assess the impact of thermal stress. Dives to 37 msw (120 fsw) were divided into descent/bottom and ascent/stop phases, prolonging the latter so bottom times could be increased if results allowed without compromising the experimental structure. The water temperature was held at either 36C (97F; ‘warm’) or 27C (80F; ‘cold’). The ‘warm/cold’ exposure, with a bottom time of 30 minutes, yielded a DCS rate of 22% (7/32 subject-exposures). The ‘cold/warm’ bottom time was increased to 70 minutes and still yielded a DCS rate of only 1.3% (2/158).2 Even if the effects are exaggerated by the prolonged ascent/stop phase, the dramatic results demand serious attention.

    Contrary to the claim made by Clarke et al.1, the high temperature employed in the NEDU study could almost certainly be maintained at the skin by a number of active heating garments available to the diving public. Hot water suits are not required for the effect. And the ‘cold’ study temperature (better described as ‘cool’) is clearly well within the range experienced by divers.

    The statement by Clarke et al.1 that “the Navy uses their extensive mathematical expertise to select the one dive profile that, in their estimation, is the most likely to identify a difference in decompression risk…” is frankly baffling. Use of a single dive depth in no way invalidates the relevance to other dive profiles. Similarly, it is not reasonable to characterize skin temperatures lower than those produced in the study as “venturing into the unknown” and thereby invalidating the results.

    Scientific method does encourage the confirmation of findings. This goal, however, does not diminish the value of singular, well-designed studies. The NEDU study is certainly one of these, most valuable in reminding divers that factors beyond the pressure-time profile will affect decompression risk.

    Divers must have adequate thermal protection to function effectively (physically and cognitively) throughout a dive. However, excessive warming during the descent/bottom phase increases inert gas uptake and can compromise decompression safety. Practically, while it may be optimal for divers to be cool or cold during the descent/bottom phase, it is prudent to recommend a thermoneutral range and avoidance of any excessive warming.

    Being cool during the ascent/stop phase inhibits inert gas elimination and can compromise safety but sudden warming should be limited to avoid reducing the gas solubility of superficial tissues that could promote localized bubble formation and symptoms of skin bends.

    Active heating systems are attractive, but they have the potential to create the worst decompression stress condition; excessive heating during the descent/bottom phase and cooling during the ascent/stop phase (if they fail part way through a dive). The risk is also still elevated if the systems work throughout a dive.2,4 Gerth et al.2 were able to increase the bottom time to 70 minutes for both the ‘cold-warm’ and ‘warm-warm’ conditions, but the rate of DCS was significantly lower for the ‘cold-warm’ condition (1.3% [2/158] vs. 17% [4/24], respectively). This lesson is relevant to any diving exposure.

    Ultimately, divers need to be aware of the potential impact of thermal status. Thermal protection should preserve clear thinking and physical performance, but unnecessary warming should be avoided. For many, passive systems will provide adequate and appropriate protection. For those who need or choose active warming systems, thoughtful use is vital. Further research is required to quantify the hazards and be able to incorporate thermal status into decompression algorithms in a meaningful way.


    1. Clarke JR, Moon RE, Chimiak MJ, Stinton R, Van Hoesen KB, Lang MA. “Don’t dive cold when you don’t have to” (letter). Diving Hyperb Med. 2015; 45(1): 62.

    2. Gerth WA, Ruterbusch VL, Long ET. The influence of thermal exposure on diver susceptibility to decompression sickness. NEDU Report TR 06-07. November, 2007; 70 pp.

    3. Dunford R, Hayward J. Venous gas bubble production following cold stress during a decompression dive. Undersea Biomed Res. 1981; 8(1): 41-9.

    4. 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.

    5. Mekjavic IM, Kakitsuba N. Effect of peripheral temperature on the formation of venous gas bubbles. Undersea Biomed Res. 1989; 16(5): 391-401.

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