Fit to Dive

Why fitness to dive matters

Here is the fact that reframes the whole topic: the diver who dies of decompression sickness (DCS, "the bends") is rare. The diver who has a cardiac event underwater, often mistaken for a drowning, is not. The Divers Alert Network (DAN), which tracks diving fatalities, finds cardiac causes among the leading killers of divers, with the share rising sharply in divers over 45. So "fitness to dive" is mostly about your heart, your lungs, and a handful of chronic conditions, far more than about your decompression maths.

Get properly assessed

The single most useful thing most divers can do is get a real dive medical from a physician with dive-medicine training (through UHMS, DAN, or an equivalent), rather than ticking a self-declaration form and moving on.

• Complete an honest dive-medical questionnaire. The system only works on accurate answers; hiding a condition mostly endangers you.
• If you are over 45, or have cardiovascular risk factors, a resting ECG (a heart-rhythm trace) is reasonable, and an exercise stress test adds a lot if your exercise tolerance is borderline or you get symptoms on exertion.
• Tell the doctor everything you take, including over-the-counter decongestants and supplements, not just prescriptions.

The rest of this article is the "why" behind that conversation.

Conditions that matter, and why

Patent foramen ovale (PFO)

The foramen ovale is an opening between the heart's two upper chambers that everyone has before birth and that normally seals afterwards. In roughly one adult in four (about 25 to 30%) it stays open: a patent foramen ovale. It is a normal variant, not a disease. The diving relevance is specific: bubbles that form in the veins are normally filtered out by the lungs, but an open PFO can let them cross straight into the arterial side and travel to the brain, spinal cord, or heart.

The 2015 DAN/UHMS PFO workshop found that divers with a PFO have higher rates of neurological decompression illness. A PFO is not an automatic disqualification. Divers with a known significant PFO are generally advised to dive more conservatively, avoid profiles that throw off lots of venous bubbles, and discuss the risk with a dive physician. Closure is considered only in specific cases of recurrent, unexplained neurological hits, and is not a routine preventive step.

Asthma

The core worry with asthma is gas trapping. On ascent, the gas in your lungs has to vent freely; if airways are narrowed by bronchospasm (triggered by cold, exercise, or allergens), gas can't get out fast enough, and the lung can overexpand. That can cause pulmonary barotrauma and, at worst, arterial gas embolism. Exercise-triggered asthma is especially relevant because diving is physically demanding.

What matters here is assessment, not a number you can check yourself. A dive physician looks at how well-controlled the asthma is and, importantly, tests for hidden airway twitchiness with a challenge test (such as methacholine), which can be abnormal even when ordinary lung-function testing looks normal. Well-controlled mild asthma may be compatible with diving after that evaluation; poorly controlled asthma, frequent reliever use, or exercise-induced symptoms are reasons to hold off until it is sorted out. This is a "see a dive doctor" condition, not a "read a threshold off a blog" one.

ADHD medications

A long-standing question in dive medicine was whether stimulants (methylphenidate: Ritalin, Concerta) might lower the threshold for CNS oxygen toxicity, the convulsion that high oxygen pressures can trigger underwater. A 2024 study (Gur et al.) found that therapeutic doses did not raise CNS oxygen-toxicity risk in controlled hyperbaric exposure. That is reassuring, but it does not make stimulant use trivially safe: the cardiovascular effects (raised heart rate and blood pressure) still warrant evaluation, particularly for technical or rebreather (CCR) diving where oxygen pressures are pushed harder. Get cleared if you are on one.

Diabetes

In 2005 a DAN/UHMS workshop ended the old blanket ban on insulin-treated divers, based on years of data showing well-managed diabetic divers were already diving safely. The real underwater risks are a low blood sugar (hypoglycaemia) impairing judgement or causing loss of consciousness, and the difficulty of self-rescue, and cold, exercise, and stress all burn glucose faster.

Crucially, this is done under a dive physician, on a structured protocol, not improvised. That protocol sets blood-glucose targets with repeated checks before every dive (cancel if sugar is low or falling), conservative depth and time limits, no overhead or mandatory-stop diving, a briefed buddy, glucose carried on every dive, and good long-term control (an HbA1c, the three-month average blood-sugar marker, within target). Well-controlled diabetes with no significant complications can be workable under those conditions; poorly controlled diabetes, or diabetes with eye, kidney, nerve, or vascular complications, remains a contraindication.

Heart conditions and exercise tolerance

Diving asks more of the heart than people expect. Immersion alone shifts blood toward the chest; cold tightens vessels and raises the load on the heart; breathing dense gas under pressure adds respiratory work. In an emergency (a surface swim against current, towing an unconscious buddy) the demand spikes to the level of hard exercise.

Dive physicians often frame fitness in METs (a unit of exertion: sitting is 1 MET, a brisk run is around 10). Recreational diving wants comfortable tolerance of moderate exertion without chest pain, abnormal rhythm, or breathlessness; technical diving wants more, tested formally. The point for you is not to self-score but to know that any history of angina, heart attack (MI), coronary disease, arrhythmia, heart failure, hypertrophic cardiomyopathy, or uncontrolled high blood pressure needs evaluation first. Well-controlled blood pressure on stable medication is usually fine, but the medication matters: beta-blockers blunt your heart-rate response and can mask the early warning signs of trouble.

COVID-19 and return to diving

A 2021 DAN/UHMS workshop set tiered return-to-diving criteria after COVID, because the illness can leave behind lung scarring and air-trapping, heart-muscle inflammation, clotting changes, and lingering effects on blood vessels, all of which interact badly with diving. In short: a mild case usually means a short layoff and a gradual return once you are fully symptom-free with normal exercise tolerance; a moderate or severe case needs formal cardiopulmonary evaluation before going back, and anyone who had significant COVID pneumonia should have their lungs imaged (a chest CT) first, because those air-trapping changes behave like asthma on ascent. Lingering "long COVID" brain fog is itself a safety issue for dive decisions.

Building real dive fitness

Cardiovascular fitness

Aerobic fitness decides how much work you can sustain before you start building up CO₂ and fatiguing. Fitter divers breathe less gas for the same effort, handle current and emergencies with less strain, clear post-dive nitrogen and inflammation faster, and are less likely to make foggy, CO₂-driven decisions at depth. A reasonable base is 150 to 240 minutes a week of moderate aerobic work (swim, cycle, row, loaded walks), with some vigorous work if you dive demanding conditions. Avoid hard training in the 24 hours before a dive, and avoid strenuous exercise for several hours afterwards, when an elevated heart rate can encourage bubbles.

Strength, mobility, and breathing

Strength work pays off in carrying gear, boarding ladders in swell, and staying controlled underwater: think core stability (planks, bird-dogs), leg and carry strength (squats, lunges, loaded carries), and grip and tank handling (hangs, farmer's carries). Mobility and single-leg balance transfer straight to boat work and confined spaces. And breathing drills (slow diaphragmatic breathing, long exhales) improve gas consumption and cut the hyperventilation that worsens narcosis and wrecks buoyancy.

Questions worth asking your dive doctor

• Which specific test settles my question (for asthma, an airway challenge; for the heart, a stress test; for a suspected PFO, a bubble study)?
• Given my condition, what depth, exposure, and conditions should I stay within?
• Do any of my medications, prescription or over-the-counter, change anything?
• What would a "come back and get re-checked" warning sign look like for me?

Related reading

• Decompression Sickness

• Pulmonary Barotrauma & AGE

• Immune Suppression and Diving

• Post-Dive Fatigue

References

• Pollock NW, Uguccioni DM, Dear GdeL (eds.). Diabetes and Recreational Diving: Guidelines for the Future. DAN/UHMS Workshop Proceedings, 2005.
• Denoble PJ, Holm JR (eds.). Patent Foramen Ovale and Fitness to Dive Consensus Workshop Proceedings. DAN/UHMS, 2015.
• Germonpré P, Lafère P, Portier W, et al. Increased risk of decompression sickness when diving with a right-to-left shunt. Frontiers in Physiology 2021;12:763408. doi:10.3389/fphys.2021.763408
• Chimiak J, Moon R (eds.). Cardiopulmonary Considerations for Divers Recovered from COVID-19 Infections Workshop Proceedings. DAN/UHMS, 2021.
• Gur I, Arieli Y, Matsliah Y, et al. Methylphenidate and the risk of acute central nervous system oxygen toxicity. Diving and Hyperbaric Medicine 2024;54(3):168–175.
• Marroni A, Kot J, Pieri M, Pelliccia R, Balestra C. Identification of DCS risk factors in recreational diving: multifactorial model based on the DAN DSL Database 2024. International Maritime Health 2026;77(1):1–12. doi:10.5603/imh.108038
• DAN Annual Diving Report, 2021 edition (reporting 2019 data). Divers Alert Network.
• UHMS. Fitness to Dive. Undersea and Hyperbaric Medical Society position statement.

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