Scuba diving is a demanding activity that places unique physiological stress on the body, particularly the respiratory and circulatory systems. The underwater environment requires the body to manage significant pressure changes and efficiently exchange gases in the lungs. While a scuba certification does not automatically disqualify every smoker, the habit introduces major health risks amplified by the physics of diving. The primary concern is how smoking compromises lung function and blood quality, making a diver far more susceptible to life-threatening pressure-related injuries.
How Smoking Affects Blood Oxygen Levels
Smoking immediately compromises the blood’s ability to transport oxygen, a state known as functional anemia. This acute effect is primarily due to inhaling carbon monoxide (CO), a gas present in cigarette smoke. CO has an affinity for hemoglobin—the oxygen-carrying molecule in red blood cells—that is over 200 times stronger than oxygen.
When CO binds to hemoglobin, it forms carboxyhemoglobin (COHb), taking up space needed for oxygen transport. Heavy smokers can have up to 15 percent of their hemoglobin bound to CO, drastically lowering the oxygen supply throughout the body. This reduction in oxygen delivery forces the heart to work harder and decreases a diver’s exercise tolerance, which can become dangerous when facing unexpected physical exertion underwater.
The Danger of Air Trapping and Lung Injury
The most immediate and severe risk for a smoker who dives stems from the structural damage smoking inflicts on the lungs. Chronic smoking leads to conditions like emphysema and chronic bronchitis, which damage the tiny air sacs (alveoli) and small airways. This damage creates areas of non-compliant lung tissue and increased mucus production, leading to localized “air trapping.”
Air trapping occurs when air enters a lung area but cannot escape freely upon exhalation or ascent. Diving physics is governed by Boyle’s Law: as surrounding pressure decreases during ascent, the volume of gas must increase proportionally. If a diver ascends, the trapped air in damaged areas expands rapidly, causing a lung rupture known as pulmonary barotrauma.
This rupture is a medical emergency because it can lead to an Arterial Gas Embolism (AGE). In an AGE, gas bubbles enter the bloodstream and travel directly to the brain, blocking blood flow and causing immediate stroke-like symptoms or unconsciousness. The structural damage caused by long-term smoking creates a physical precondition for this life-threatening event.
Increased Risk of Decompression Sickness
Smoking significantly increases a diver’s susceptibility to Decompression Sickness (DCS), commonly called “the bends,” through two distinct mechanisms: circulation and gas exchange efficiency. Nicotine acts as a vasoconstrictor, causing small blood vessels throughout the body to narrow. This constriction slows blood flow to the extremities and tissues.
Impaired circulation means that inert gases, primarily nitrogen, are not efficiently off-gassed from the tissues back into the blood during ascent and decompression stops. The second mechanism relates to the quality of gas exchange in the lungs, known as ventilation-perfusion (V/Q) matching. Smoking causes a V/Q mismatch, making the overall process of gas removal inefficient. This inefficiency results in the diver retaining a higher residual nitrogen load, which increases the likelihood of bubble formation and DCS symptoms.
Medical Recommendations for Smoking Divers
The risks associated with smoking and diving require a clear medical strategy for anyone who continues the habit. The immediate risk of carbon monoxide poisoning is acutely reversible. The half-life of carboxyhemoglobin is about five to six hours, meaning CO levels can return to that of a non-smoker within 24 to 48 hours of cessation. Therefore, temporary abstinence before a dive can mitigate the risk of acute oxygen deprivation.
However, temporary cessation does not address the structural damage to the lungs that causes air trapping and the risk of barotrauma. Emphysema and the associated damage to the alveoli are non-reversible, even after quitting. Quitting only prevents further deterioration and slows the progression of the disease. For any diver with a history of smoking, particularly those over 45, a mandatory medical clearance is advised from a physician experienced in hyperbaric medicine.