During a dive to 20 metres, what aspect of dive planning is critical to prevent decompression sickness?

The aspect of dive planning that is critical to preventing decompression sickness is the depth and duration of the dive. Understanding these factors is essential because they directly influence the amount of nitrogen that gets absorbed into the body. As divers go deeper, the pressure increases, causing nitrogen from the breathing gas to dissolve more in bodily tissues. The longer the duration at a specific depth, the more nitrogen accumulates.

Proper planning involves determining the maximum depth you plan to reach and how long you intend to stay at that depth. By adhering to recommended limits for both depth and time, divers can significantly reduce their risk of nitrogen narcosis and potential decompression sickness upon ascent. It's vital to use dive tables or dive computers to track these parameters, ensuring that the ascent rate is controlled and safe.

While aspects such as steady ascent rates, awareness of other divers, and the use of specific breathing gases are important for overall dive safety, they do not have the same direct impact on the physiological absorption of nitrogen and the prevention of decompression sickness as depth and duration do.