Decompression Sickness

Decompression Sickness

Thermal Stress

A diver's thermal status has long been known to influence decompression risk. The impact is best appreciated by considering the two fundamental phases of every dive: the descent and bottom phase, when gas uptake occurs, and the ascent and stop phase, when gas elimination occurs.

Two Phases

During the descent and bottom phase of a dive, a relatively warm state results in increased inert gas uptake; this is equivalent to conducting a deeper and/or longer dive. On the other hand, if you can maintain a cool or thermoneutral state during your descent and bottom phase, you will effectively reduce your inert gas uptake. This beneficial effect will be further magnified if you exert yourself as little as possible during this phase.

During the ascent and stop phase of your dive, a relatively warm state will promote inert gas elimination, thus reducing overall decompression stress. On the other hand, a cool or cold state during this phase will reduce inert gas elimination, effectively prolonging and possibly increasing decompression stress.

The decompression hazard associated with hot water suits — which effectively establish a warm condition in both phases of a dive — was established in a study of North Sea divers conducted 30 years ago (Shields and Lee 1986). The impact of thermal status on decompression stress was even more elegantly demonstrated in a recent study conducted by the U.S. Navy (Gerth et al. 2007). The controlled conditions of a research study cannot be directly correlated with everyday diving practices, but the key message from these studies is the importance of thoughtful thermal status. Keeping neutral on your way down — certainly avoiding unnecessary overheating — and warm on your way up (approaching a cool-warm pattern) will reduce the risk of DCS in comparison to being warmer on your way down and cool on your way up (a warm-cool pattern).