Scuba diving exposes the human body to a uniquely challenging environment defined by hyperbaric pressure, breathing compressed gas, and the thermal stress of cold water. These conditions impose significant physiological demands that can turn a manageable health issue into a life-threatening emergency underwater. The physical laws governing gas behavior at depth mean that even minor medical vulnerabilities can have catastrophic consequences upon ascent or descent. Consequently, a rigorous medical screening process is mandatory, ensuring a diver is fit to withstand these environmental stressors.
Conditions Affecting the Respiratory System
Respiratory conditions are a primary concern because they can lead to pulmonary barotrauma, or lung overexpansion injury. This injury occurs when gas trapped in the lungs during ascent expands rapidly as pressure decreases, potentially rupturing alveolar tissue. The resulting risk is an Arterial Gas Embolism (AGE), where gas bubbles enter the bloodstream and travel to the brain, causing immediate stroke-like symptoms.
Active or severe asthma, especially if poorly controlled, is a major contraindication. Asthma causes localized airway narrowing, leading to gas trapping and preventing the uniform release of air during ascent. Similarly, Chronic Obstructive Pulmonary Disease (COPD) and emphysema create structural abnormalities, such as lung bullae, which are prone to rupture under pressure change.
A history of spontaneous pneumothorax (collapsed lung) is generally considered an absolute contraindication. This indicates a structural weakness in the lung lining, increasing the likelihood of recurrence or a tension pneumothorax upon ascent. Even after surgical intervention, the underlying risk often remains too high for safe diving.
Cardiovascular and Circulatory Restrictions
The combination of cold water and physical exertion places a substantial load on the cardiovascular system, making pre-existing heart conditions hazardous. Immersion causes a centralization of blood volume, shifting blood to the core and increasing the heart’s workload. This effect is compounded by cold, which can slow the heart rate and increase the risk of arrhythmias.
A history of Myocardial Infarction (heart attack) or untreated Coronary Artery Disease (CAD) poses an unacceptable risk due to the potential for sudden incapacitation underwater. Uncontrolled or severe hypertension (high blood pressure) is restrictive because the body’s response to cold and immersion can further elevate blood pressure. Significant, symptomatic cardiac arrhythmias that could lead to a loss of consciousness are absolute contraindications.
A major structural concern is the Patent Foramen Ovale (PFO), a small opening between the upper chambers of the heart present in about 20–25% of the population. In a diver, nitrogen bubbles normally filtered by the lungs can pass through a PFO directly into the arterial circulation. This phenomenon, known as a paradoxical embolism, increases the risk of Decompression Sickness (DCS) and stroke.
Neurological and Psychological Contraindications
Conditions affecting the central nervous system or mental stability are dangerous because they can lead to disorientation, impaired judgment, or sudden loss of consciousness. Any condition resulting in a seizure or unexplained syncope (fainting) is generally an absolute contraindication. An in-water seizure is almost certainly fatal, as the diver would lose the regulator and aspirate water.
A history of seizures requires a prolonged period, typically three to five years, of being seizure-free and off medication before diving can be considered. A previous stroke or Transient Ischemic Attack (TIA) generally disqualifies a diver due to underlying vascular issues and the increased risk of neurological DCS. Severe migraines, particularly those accompanied by an aura, are a concern because they are associated with a higher incidence of PFO.
Psychological conditions can also be catastrophic when combined with the isolation and physical demands of diving. Severe panic disorder, active psychosis, or debilitating claustrophobia are absolute contraindications because they increase the risk of an uncontrollable panic response. This anxiety can lead to rapid, uncontrolled ascent, a common cause of fatal pulmonary barotrauma.
Ear, Sinus, and Pressure Equalization Concerns
The air-filled cavities of the head, specifically the middle ear and sinuses, are highly susceptible to pressure changes, leading to barotrauma if equalization is impaired. Divers must equalize the pressure in their middle ear via the Eustachian tubes during descent to prevent a painful “squeeze.” Inability to perform this equalization, often due to chronic sinusitis, nasal polyps, or an upper respiratory infection, can cause middle ear or sinus barotrauma, including pain, bleeding, and eardrum rupture.
A perforated eardrum, or recent ear or sinus surgery, requires a medical hold until fully healed and cleared by a physician. A perforated drum allows cold water to enter the middle ear, potentially causing immediate vertigo and disorientation. Vestibular (balance) disorders, such as Meniere’s disease, cause episodes of severe vertigo, hearing loss, and tinnitus.
An episode of vertigo underwater, whether from Meniere’s disease or inner ear barotrauma, can be disorienting and lead to a loss of control, increasing the risk of drowning. Alternobaric Vertigo occurs when pressure equalization is asymmetrical between the two ears, resulting in temporary, severe dizziness typically during ascent.