The question of whether a person sinks when they drown is complex, governed by physics, body composition, and environment. Drowning is clinically defined as respiratory impairment resulting from submersion or immersion in liquid. The buoyancy of a living human is finely balanced, meaning small changes in density or volume can quickly determine whether a person floats or sinks.
The Science of Human Buoyancy
A living human body is naturally close to the density of water, which is why most people can float with minimal effort. The physical principle at work is Archimedes’ Principle: the upward buoyant force exerted on a body immersed in a fluid is equal to the weight of the fluid that the body displaces. If a body displaces a weight of water greater than its own weight, it floats.
The single largest factor contributing to a person’s buoyancy is the air contained within the lungs. At total lung capacity, nearly everyone is buoyant in both fresh and saltwater. This trapped air significantly decreases the body’s overall density, acting like a built-in flotation device.
Beyond lung volume, the body’s composition plays a role because different tissues have different densities. Fat tissue (0.9 g/ml) is less dense than water and therefore more buoyant. In contrast, muscle and bone tissue are denser than water (muscle is about 1.1 g/ml). A person with a higher body fat percentage will naturally be more buoyant than a highly muscular or lean individual.
Factors Determining Immediate Sinking or Floating
The shift from floating to sinking during a drowning incident is primarily caused by the loss of air from the lungs. Panic and the intense struggle to breathe often lead to rapid, uncontrolled exhalation, emptying the lungs of their buoyant air volume. This rapid loss of air immediately increases the average density of the person’s body, reducing the upward buoyant force.
A secondary, but more definitive, mechanism for sinking is the aspiration of water into the lungs. As a person attempts to breathe while submerged, water, which is denser than air, fills the lung space, replacing the air that provided buoyancy. This water replacement significantly increases the density of the chest cavity, pushing the body past the tipping point into negative buoyancy.
The type of water also plays a role in this immediate outcome. Saltwater is denser than freshwater, providing a greater buoyant force. While an unconscious body with low lung volume might sink in freshwater, the same body has a much higher chance of remaining near the surface in denser saltwater. Body position is also relevant, as a horizontal posture allows for maximum water displacement, while a vertical position, common in a struggle, makes it harder to stay afloat.
Why Bodies Eventually Rise
After a body sinks, it does not typically remain on the bottom indefinitely. This is due to the natural biological process of decomposition, which occurs post-mortem. Bacteria residing in the gastrointestinal tract begin to break down tissues, a process called putrefaction.
This breakdown generates large volumes of gas, primarily methane, hydrogen sulfide, and carbon dioxide. These gases accumulate within the abdominal and chest cavities, causing the body to bloat and expand. This expansion dramatically increases the body’s overall volume, which in turn lowers the body’s average density.
When the density of the deceased body becomes lower than the density of the surrounding water, the buoyant force overcomes the body’s weight, causing it to resurface, or “rise”. The time it takes for a body to rise is highly variable, but it is strongly influenced by water temperature. In warmer water, bacterial activity is higher, and gas production is faster, potentially causing a body to resurface within a few days. In very cold water, decomposition is severely slowed, which can delay resurfacing for weeks or months.