What is the primary concern when a diver ascends after deep diving?

The primary concern when a diver ascends after deep diving is decompression sickness. This condition, often referred to as "the bends," occurs due to the formation of nitrogen bubbles in the body when a diver ascends too quickly. As divers descend, their bodies absorb nitrogen from the breathing gas at higher pressures. During a rapid ascent, the pressure decreases rapidly, and nitrogen that has been dissolved in the tissues can come out of solution and form bubbles. These bubbles can cause a range of symptoms, from mild joint pain to severe neurological issues, depending on where they form and how many bubbles are present.

To prevent decompression sickness, divers must follow specific ascent rates and safety stops during their ascent. Proper dive planning, adherence to dive tables or computers, and understanding ascent procedures are crucial for avoiding this potentially life-threatening condition.

The other options, while they are important aspects of diving, do not rank as the primary concern specifically associated with the ascent phase after deep diving. Nitrogen absorption is a part of the context leading to decompression sickness, carbon dioxide buildup is typically associated with issues during the dive itself or improper breathing patterns, and hypoxia risk, while relevant to diving, relates more to insufficient oxygen levels rather than the ascent process.