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Recently we were having discussions at my dive club on the dangers of freediving after scuba dives. These have revealed that some members seem to think there is no possible physical or medical problem with the recompression and rapid decompression associated with freediving, when you have a skin full of dissolved gas. What is your advice?

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Although there isn't much definite evidence on this, freediving soon after a scuba dive could give rise to 2 problems. As you know, during a scuba dive, tissue loading with inert gases occurs at different rates, with some tissues taking longer to release gas once the diver has ascended. Microbubbles which have expanded on ascent after a scuba dive but have not been filtered out through the lungs can be recompressed by freediving, and potentially bypass the lung. They will then end up in the arterial circulation and predispose to arterial gas embolism.

Secondly, freediving is similar to bounce diving or sawtooth profiling, in that rapid decompression or ascents can reload the tissues or impede the diffusion of gas out of the body, thus increasing the risk of DCI. The actual amount of inert gas loading on a freedive is likely to be small so this effect is unlikely to be a big problem.

Nevertheless, the accepted wisdom is not to freedive at all until your computer has cleared. That way, you have the best available evidence that you are starting from as clean a slate as possible.

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Can DCI result from freediving in 4 to 5 metres of water? I was lobstering for 10 days in Australia last August and only had a real issue after this first day of freediving. Upon return to the dock, I almost fell over from dizziness and sight difficulty, and had to lie down for a couple of hours before I could walk around again. The following 9 days went mostly well, particularly with the use of SCUBA, but on occasion with freediving some dizziness at the surface would occur whenever I failed to release a little of my held breath on the way up. It seems that even a held breath may force some nitrogen into the blood stream, but I cannot find this to be a fact in any of the literature that I've researched since.

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This is an interesting one. In the 1950’s DCI after freediving was thought to be impossible, for 2 reasons: the amount of nitrogen contained in one breath was not considered sufficient to cause symptoms, and dives were too short to allow the nitrogen to dissolve into the blood. We now know that it is indeed possible. Studies were carried out in the 1960’s on pearl divers in French Polynesia, who dive to 30m or more 40 to 60 times a day (each dive lasting about 2 minutes with a 3 to 4 minute surface interval). These divers showed a high rate of Taravana (literally meaning “to fall crazily”) – a range of symptoms varying from vertigo and impaired vision to loss of consciousness and paralysis – consistent with DCI. Interestingly divers on neighbouring islands performed similar profile dives but with surface intervals of 12 to 15 minutes. Taravana there was almost unknown.

Your symptoms seem to have come on severely after the first day. US Navy tables at 10 metres on air give a bottom time of over 6 hours, so at maximum depths of 5 metres, you would have to be underwater for a huge amount of time, breathing compressed gas, to put you at risk of DCI. On single breaths, even with multiple dives, you would not accumulate enough nitrogen after one day to induce DCI. I suspect that another issue may have been the culprit here, possibly an ear barotrauma from repetitive equalising.