What must be assessed before planning an ascent to altitude after a dive?

The surface interval before ascending is crucial to assess after a dive due to the implications of nitrogen absorption in the body and the potential risk of decompression sickness. During a dive, divers absorb nitrogen from the compressed air they breathe; this nitrogen is stored in the body's tissues. When ascending to altitude, the surrounding pressure decreases, and if divers have not allowed sufficient time for the nitrogen to off-gas (i.e., leave the body), they risk forming nitrogen bubbles that can lead to decompression sickness.

Determining an appropriate surface interval involves considering several factors, including the depth and duration of the dive, as well as any previous dives that day. Following standard dive tables or algorithms guides divers on how long they should wait on the surface before ascending to altitude to minimize risk.

Other factors, such as the depth of the dive, the number of divers present, and visibility conditions underwater, while important considerations in general diving safety and planning, do not directly influence the timing of ascent in relation to surface intervals and altitude changes.