A Navy SEAL, who is getting involved in freediving, recently asked me if he could get decompression sickness (DCS) if he freedived after scuba diving. It’s a good question. I’ve read discussions on this subject and have tossed the idea about with extreme freedivers in the past. But before answering the SEAL’s question, I wanted to consult with one of my valued advisors, Divers Alert Network’s (DAN) Research Director, Dr. Neal Pollock.
Dr. Pollock provided a helpful article to me which is now available on the DiveWise site, and I encourage you to read it. The question is: Can repeated freedive descents and ascents change the ultimate destination of venous gas emboli (VGE) which might be present in the blood following scuba diving, increasing a freediver’s risk for DCS?
Before answering that question we must ask is it possible for freedivers to experience DCS at all? In a single extreme freedive, this seems unlikely, according to the scientists who studied hypothetical scenarios in which this might occur. However, if there are many repeated freedives with short surface intervals, this could, in theory, produce DCS. Nonetheless, reports of bubbles observed in breath-hold divers have been very few. DCS-like symptoms in breath-hold divers have been reported, but the causes are difficult to pinpoint and may involve pre-existing medical conditions, and not necessarily DCS.
When attempts were made to model the risk of DCS for breath-hold divers, it was found to be negligible for dives to depths of less than 330 feet. The risk is likely very low for most freedivers. Additionally, the evidence for the risk of DCS, if one engages in breath-hold diving after scuba diving, is scarce.
To quote Dr. Pollock, “Compressed-gas diving prior to freediving certainly increases the theoretical risk [of DCS]. High tissue concentrations of inert gas after compressed-gas dives could make the impact of the freediving important. While no experimental evidence exists, bubbles produced following the compressed-gas dive could migrate to more sensitive tissue when transiently compressed by the freedive. Similarly, the physiological stress of freediving could enhance pulmonary shunting, potentially increasing the risk or frequency of bubbles entering arterial circulation. The hazard might be greatest in the first part of the freedive when both bubble size and physical effort would be relatively high or at the end of the freedive if augmented shunting continued. Again, though, there is no evidence of these factors causing injury. Studying a relatively rare event like DCS is difficult; studying a second rare event on top of the first is much more difficult.”
For a more in-depth look at this topic, I encourage you to read the full article Could Breath-Hold Diving after Scuba Cause Decompression Sickness? by Petar Denoble, which includes an interesting discussion with Dr. Neal Pollock and Dr. Robert Wong.