Why must scuba divers never hold their breath as they ascend to the surface?

Understanding the relationship between pressure and volume as articulated by Boyle's Law is crucial not only in laboratory settings but also in practical applications like scuba diving. In essence, Boyle's Law tells us that in a closed system at constant temperature, the pressure of a gas is inversely proportional to its volume. Mathematically, it's described by the equation \( PV = k \), where \( P \) stands for pressure, \( V \) represents volume, and \( k \) is a constant for a given amount of gas.

Imagine a balloon deep underwater: as you bring it closer to the surface, the pressure of the surrounding water decreases. Boyle's Law predicts that the balloon's volume will increase in response, because the external pressure on the balloon is lessening. For scuba divers, this concept is paramount when considering the behavior of air in their lungs during an ascent.

Lung over-expansion is a serious concern for scuba divers. The physiological effects of Boyle's Law manifest dramatically during an ascent. A diver's lungs at depth are filled with compressed air that expands as the surrounding water pressure drops. If a diver holds their breath while surfacing, this expansion can force the lungs to stretch beyond their natural capacity, risking severe injury.

Scuba divers may encounter different types of lung over-expansion injuries. For instance, a pulmonary barotrauma could occur, potentially tearing lung tissue and leading to an air embolism, where air bubbles can travel through the bloodstream and obstruct blood flow. Another risk is pneumothorax, which happens when air escapes from the lungs into the chest cavity, making it hard to breathe and possibly collapsing the lung.

The safety of scuba divers hinges upon adhering to diving best practices, particularly while ascending. A controlled ascent, at a recommended rate of no more than 30 feet per minute, allows the body and the air in the lungs to equilibrate with decreasing pressures. Divers are also trained to continuously breathe out during the ascent—never holding their breath—to avoid lung over-expansion.

Key Safety Tips

• Breathing steadily and exhaling on ascent to let expanding air escape.
• Equalizing ear and sinus pressure throughout the dive.
• Using a dive computer or depth gauge to monitor ascent rate.
• Performing safety stops as needed to allow dissolved gases to off-gas from body tissues.

These steps reduce the risk of decompression sickness—a condition where dissolved gases form bubbles inside the body—and other diving-related injuries.

Boyle's Law falls under the umbrella of gas laws, which are a fundamental part of understanding scuba diving as they explain how gases behave under different conditions of pressure, volume, and temperature. Other gas laws that divers must be mindful of include Charles's Law, which deals with temperature and volume, and Dalton's Law, which is about the pressure contributions of different gases in a mixture.

Knowledge of these principles helps divers comprehend why they experience certain physical sensations and potential hazards under water, such as the narcotic effects of nitrogen at depth (nitrogen narcosis) or the need to adjust the mix of breathing gases for deep dives (using trimix or nitrox instead of common air). Studying gas laws equips divers with the scientific understanding necessary to manage the invisible yet tangible elements that define the underwater environment.