Decompression Sickness (DCS), commonly known as “the bends,” is a condition affecting divers due to changes in ambient pressure. It occurs when dissolved gases form bubbles upon ascent, causing pain and potentially severe injury. Scuba divers must manage their ascent to avoid this risk, yet freedivers, who often descend to significant depths, rarely experience the condition. This difference is rooted in fundamental physiological and physical distinctions between the two forms of diving.
How Decompression Sickness Occurs
The mechanism behind decompression sickness is governed by Henry’s Law. This law states that the amount of gas dissolved in a liquid is directly proportional to the pressure of that gas above the liquid. In diving, the inert gas nitrogen, which makes up about 78% of the air we breathe, dissolves into the diver’s blood and tissues as the surrounding water pressure increases with depth.
The problem arises when pressure is rapidly reduced during ascent. If the ascent is too fast, nitrogen comes out of solution too quickly, forming bubbles in the tissues and bloodstream, much like opening a shaken soda bottle. These bubbles can obstruct blood flow or damage nerve tissue, leading to DCS symptoms. The risk is directly related to the amount of nitrogen absorbed and the speed of the pressure reduction.
The Role of Time and Compressed Air
The primary factor protecting the freediver is the absence of a continuous, high-pressure gas source. A scuba diver breathes compressed air from a tank, which delivers air at the same high pressure as the surrounding water. This continuous supply forces a large volume of nitrogen into the tissues over the course of a dive, leading to significant nitrogen saturation.
Freedivers, by contrast, descend on a single breath of surface air. The total amount of nitrogen available for absorption is limited to the volume of air in their lungs at the start of the dive. While the pressure at depth increases the partial pressure of nitrogen in that single breath, the exposure time is extremely short, typically lasting only a few minutes. This brief exposure does not allow enough time for the body’s slower tissues to absorb a dangerous amount of nitrogen.
For a scuba diver, the duration of the dive is measured in tens of minutes or hours, sufficient time for nitrogen to saturate the tissues. The freediver’s dive time is limited by their breath-hold capacity, which is too short for the necessary saturation to occur. The combination of a single breath and short duration prevents the buildup of excess dissolved nitrogen that would lead to bubble formation upon ascent. While it is technically possible for a freediver to experience mild DCS after multiple, deep dives with insufficient surface intervals, the risk is negligible compared to a scuba diver.
The Body’s Dive Response
The human body possesses innate physiological adaptations, collectively known as the Mammalian Dive Reflex, which aids the freediver. This reflex is triggered by breath-holding and the immersion of the face in cold water.
Bradycardia and Vasoconstriction
One of the main components is bradycardia, which causes the heart rate to slow down significantly, sometimes by 20% to 50% in trained individuals. This reduction conserves oxygen by lowering the body’s metabolic rate.
Simultaneously, peripheral vasoconstriction occurs, narrowing blood vessels in the extremities. This action shunts oxygenated blood away from the limbs and toward the core organs, including the heart and brain, prioritizing their function during the breath-hold.
The Blood Shift
A third component is the “blood shift,” important for deep freediving. As the diver descends, increasing water pressure compresses the air in the lungs, reducing their volume. To prevent the lungs from collapsing, blood plasma and red blood cells are drawn from the extremities into the chest cavity and the capillaries surrounding the lungs’ air sacs. This fluid shift helps equalize the pressure inside the chest with the external water pressure, preventing lung squeeze. This mechanism also reduces the surface area available for nitrogen exchange, further limiting gas saturation.
Safety and Risk in Freediving
While physiological and physical factors largely eliminate the risk of decompression sickness for freedivers, the sport presents other unique and serious hazards. The primary danger is hypoxic blackout, a loss of consciousness due to a lack of oxygen to the brain. This often occurs near the surface during the final stage of ascent, known as shallow water blackout.
The rapid drop in ambient pressure during the final meters of ascent causes the partial pressure of oxygen in the lungs to fall quickly, leading to sudden unconsciousness. Freedivers also face the risk of pulmonary barotrauma, or lung squeeze, if they descend too quickly or attempt depths beyond their physical adaptation. These risks underscore the necessity of proper training, conservative depth progression, and strict adherence to the “one up, one down” buddy system.