What is the primary safety concern when ascending after a dive?

The primary safety concern when ascending after a dive is decompression sickness. This condition occurs when a diver ascends too quickly, leading to nitrogen bubbles forming in the bloodstream and tissues due to a rapid reduction in pressure. As divers breathe compressed air at deeper depths, nitrogen dissolves into their body tissues. If they ascend too quickly, the nitrogen does not have enough time to safely leave the body through exhalation, which can result in pain, injury, or even fatal outcomes.

Decompression procedures, including safety stops and controlled ascent rates, are critical to minimizing the risk of decompression sickness. This is why divers are trained to ascend slowly and follow established safety protocols after a dive. The other concerns, while relevant in different contexts, do not pose the same level of immediate risk during ascent as decompression sickness does.