Why is slow ascent critical following deep dives?

A slow ascent after deep dives is critical primarily to minimize the risk of gas bubbles forming. During a dive, especially at greater depths, the body absorbs gases, primarily nitrogen, due to the increased pressure. If a diver ascends too quickly, the pressure decreases rapidly, which can lead to the formation of nitrogen bubbles in the bloodstream and tissues. This condition is known as decompression sickness or "the bends," which can be life-threatening.

A gradual ascent allows for a proper decompression process where the dissolved gases are released from the body safely and effectively, thereby reducing the likelihood of bubble formation.

Other factors may be related to diving health and safety, like visibility or air consumption, but ensuring a slow ascent is key to physiological safety after deep dives.