Which type of dive profile poses the highest risk for DCS?

A deep first dive followed by shallower dives poses the highest risk for decompression sickness (DCS) due to several factors related to nitrogen absorption and elimination.

During a deep dive, the body absorbs a significant amount of nitrogen because of increased pressure at greater depths. When divers then transition to shallower dives, the nitrogen that is dissolved in the body does not immediately come out of solution. As a result, if divers ascend too quickly or do not allow sufficient time at deeper depths for nitrogen elimination, this can lead to the formation of nitrogen bubbles in the bloodstream and tissues, which causes DCS.

The risk increases with the initial deep dive because the amount of nitrogen absorbed is proportionally greater than that from shallower dives. Following a deep dive with additional shallower dives means that there is ongoing nitrogen uptake from each subsequent dive without allowing adequate time for the previously absorbed nitrogen to off-gas effectively.

In contrast, a short, shallow dive, a series of long, shallow dives, or a single shallow dive would generally involve less nitrogen absorption and therefore lower risk of DCS, as these profiles allow better nitrogen elimination due to the reduced pressures and shorter duration under water.