Immune Suppression and Diving

What diving does to your immune system

After long, deep, or repetitive dives, some divers feel run-down, mildly unwell, or just "off," even when the dive went perfectly. Part of that is probably the immune system: diving shifts the body's inflammation and immune markers for a while, even on dives that never cause decompression sickness (DCS, "the bends").

What's happening in the blood

Every dive, even a shallow, within-limits recreational one, produces small bubbles in the veins on ascent (venous gas emboli, the bubbles picked up by Doppler ultrasound). Most are far too small to cause DCS, but they are big enough to brush against the endothelium (the inner lining of your blood vessels). That contact appears to nudge the same early-response systems that switch on during infection or injury: inflammation and clotting.

The result is a low-grade inflammatory signal. Studies of divers have measured raised levels of inflammatory messengers (cytokines, the chemical signals immune cells use), including IL-6, which acts on the brain to produce that fatigued, slightly-ill feeling, and TNF-alpha, which amplifies the response. These are the same cytokines that make you feel rough during a cold or after hard exercise, which is likely why post-dive malaise feels like a mild bug coming on.

On top of that, breathing oxygen at raised pressure (more so on nitrox or rich decompression gas) generates reactive oxygen species (ROS): unstable molecules your antioxidant defences normally mop up. Pile on enough, over long or repeated dives, and the cleanup can fall behind, adding to the overall oxidative load. (The chemistry is its own article: oxygen toxicity and ROS.)

The temporary suppression part

Alongside that rise in inflammation, parts of the immune system seem to be turned down for a while. Studies of divers after diving have reported shifts such as a higher neutrophil-to-lymphocyte ratio (a general stress marker) and reduced activity in some lymphocyte populations. The pattern matches the body's usual response to acute physical stress: the fast, blunt "innate" defences ramp up while the slower, targeted "adaptive" ones dip briefly. Whether that dip is large enough to matter for real-world infection risk is the open question above, but it is a reasonable reason not to stack hard diving on top of a body that is already fighting something.

The one that actually matters: diving while ill

This is where the science stops being theoretical. An active infection already has your inflammation markers high and your blood vessel linings irritated. Adding dive bubbles to that primed state is the concern: it may lower the threshold at which a given bubble load turns into symptomatic DCS, and a fever's dehydration already impairs gas transport.

Adaptation, and overload

There is an upside. Divers who rest properly between dives and dive days often adapt over a career: calmer inflammatory responses to the same exposure and quicker recovery. But that adaptation needs time. Short surface intervals, poor sleep, heavy cold exposure, and bad nutrition stop the recovery cycle from completing. Without it, the stress from each day plausibly carries into the next as a kind of "fatigue debt" (persistent tiredness, feeling run-down), which is the everyday version of what the markers hint at. This is the same compounding the multi-day oxygen and post-dive fatigue articles describe from other angles.

Recovering well

The biology above is uncertain in its details, but the practical advice it points to is not, and it costs nothing:

• Don't dive sick. A fever or chest infection is a hard stop, not a "push through."
• Build in rest days on multi-day and liveaboard trips. They are physiology, not luxury.
• Sleep. Immune rebalancing is mostly sleep-driven, so protect it.
• Hydrate, and lean on antioxidant-rich food (fruit, vegetables, nuts) rather than supplements.
• Go easy on the alcohol. It wrecks sleep, worsens dehydration, and suppresses immune cells, which is exactly the opposite of recovery, and squarely at odds with post-dive drinking culture.
• Skip the hard workout right after diving. Heavy exercise soon after a dive can raise the risk of bubbles crossing into the arteries (Madden et al. 2013), as well as adding to the load your body is already clearing.

Related reading

• Decompression Sickness: What It Is and Why It Happens

• Predisposing Factors for DCS

• Thermal Stress in Diving

• Post-Dive Fatigue

References

• Obad A, Palada I, Valic Z, et al. The effects of acute oral antioxidants on diving-induced alterations in human cardiovascular function. Journal of Physiology 2007;578(3):859–870.
• Madden D, Thom SR, Milovanova TN, et al. Exercise before scuba diving ameliorates decompression-induced neutrophil activation. Medicine & Science in Sports & Exercise 2014;46(10):1928–1935. doi:10.1249/MSS.0000000000000319
• Madden D, Lozo M, Dujic Z, Ljubkovic M. Exercise after SCUBA diving increases the incidence of arterial gas embolism. Journal of Applied Physiology 2013;115(5):716–722.
• Germonpré P, Balestra C. Preconditioning to reduce decompression stress in scuba divers. Aerospace Medicine and Human Performance 2017;88(2):114–120. doi:10.3357/AMHP.4642.2017
• Bosco G, Yang ZJ, Nandi J, et al. Effects of hyperbaric oxygen on antioxidant enzymes in skeletal muscle during ischaemia and reperfusion. Clinical and Experimental Pharmacology and Physiology 2007;34(1-2):70–76.
• Denoble PJ (ed.). Fitness to Dive. DAN/UHMS Consensus Workshop Proceedings, 2015.

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