As long as a diver stays shallower than which depth, the concern for decompression sickness is minimal?

Staying shallower than 30 feet (9 meters) significantly reduces the risk of decompression sickness due to the physics of nitrogen absorption and elimination in the body during diving. At this depth, the ambient pressure is relatively low, which means the diver's body absorbs a lower amount of nitrogen compared to deeper depths.

Decompression sickness, also known as "the bends," occurs when dissolved gases, primarily nitrogen, come out of solution too rapidly as a diver ascends. At depths greater than 30 feet, there is an increased risk as nitrogen uptake becomes more pronounced due to higher pressures. The concern for decompression sickness begins to increase significantly as depths exceed this threshold, necessitating careful ascent protocols and, sometimes, staged decompressions.

The other depth options, while important for different aspects of diving, introduce greater levels of nitrogen absorption, heightening the risk of decompression sickness, and therefore require more careful attention to dive profiles and ascent rates.