Descent speed

Question:
People (divers and instructors) always talk about a safe ascent rate (10m/min), what about descent, is there a safety limit on that and why?

Answer:
Besides deep commercial diving, with the use of breathing mixtures different than air, where there can be a problem with High Pressure Nervous Syndrome (HPNS) the known limits regarding speed of descent are related to:
A: pressure equalization in the middle ear and paranasal sinus cavities
B: for quite deep diving, a certain higher susceptibility of certain individuals to nitrogen narcosis when the exposure time is very fast.
Generally speaking the issue of speed of descent is a self limited one, as the speed that you can physiologically obtain, while equalizing and even swimming down relatively fast is generally slow enough to be physiologically safe.
On another level, the slower the descent, the smaller the inert gas uptake at the maximal partial pressures and the less exposure. This may be significant in case of a square, table dive, calculated on the assumption of the entire time spent at the maximal depth.