What does exhaling during an emergency ascent help prevent?

Exhaling during an emergency ascent is crucial for preventing air embolism, which occurs when air bubbles enter the bloodstream, often as a result of rapid changes in pressure. When a diver ascends too quickly while holding their breath, the expansion of air in the lungs can lead to over-inflation and potentially rupture lung tissue. This can release air into the bloodstream, causing an embolism.

By consciously exhaling during the ascent, the diver helps to maintain lung volume, allowing the air to escape safely and reducing the risk of over-expansion. This practice promotes safer ascent procedures, as it balances the internal lung pressure with the decreasing external pressure, mitigating the chances of developing an air embolism.

The other choices, while related to diving safety, do not directly connect to the mechanics of exhaling during ascent. Pneumonia is not caused by pressure changes during diving, decompression sickness results from rapid ascent but manifests differently, and hypothermia is a concern in cold water rather than pressure-related lung injuries. Thus, exhaling is specifically aligned with preventing air embolism during emergency situations.