Which factor primarily affects the risk of decompression sickness?

The primary factor that affects the risk of decompression sickness is the depth and duration of the dive. As a diver descends deeper into the water, the ambient pressure increases, which causes more nitrogen from the breathing gas to dissolve into the body's tissues. Prolonged exposure at greater depths allows for even more nitrogen absorption.

When a dive is followed by a rapid ascent, the dissolved nitrogen can form bubbles as the pressure decreases. This is what leads to decompression sickness, also known as "the bends." The longer a diver remains at depth, or the deeper they go, increases the total nitrogen load taken up by the body, thus raising the risk of developing decompression sickness after the dive if proper ascent protocols, such as safety stops, are not followed.

While other factors like water temperature, dive site location, and type of breathing gas do play roles in diving safety and physiology, they do not directly correlate to the risk of decompression sickness as critically as the depth and duration of the dive do.