h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Nitrogen Narcosis is caused by breathing any gas containing Nitrogen at partial pressures exceeding 0.79 bar, the effects come on gradually and are directly proportional to the partial pressure increase. There is no magic figure below which Nitrogen Narcosis occurs, nor any magical figure above which you are safe from the effects.
The 60m limit is not derived from Nitrogen Narcosis, but is a protection against the much more serious Oxygen Poisoning, High oxygen Partial Pressures are to be avoided as the main symptom is convulsions, which causes the diver to loose his / her regulator and drown. The other problem with oxygen poisoning compare to nitrogen poisoning (or narcosis) is that if you ascend to a lower partial pressure the symptoms of nitrogen narcosis reduce, where the symptom of oxygen poisoning don’t. The “safe” level for oxygen poisoning is a partial pressure of 1.4 bar or a depth on air of 55m, a contingency (absolute maximum) of 1.6 bar or a depth on air of 65m is also used.
Helium, Nitrox and Argon are used in Technical Diving to go below this limit. That I may take a shot at as a separate subject.

There was a BBC TV programme last week presented by Michael Portillo saying that Nitrogen was the most humane way to deliver Capital Punishment (!!) and having tried it himself I suppose we can believe him.
To change the subject slightly, does diving cause your joints damage? is this the worst side effect of scuba diving and how bad is it as I am thinking of taking it up (aged 46) ? thanks for any advice.

I used to dive, and I don't remember hearing anything about diving damaging joints, but it's been a while.

I don't think there are any 'side effects' of scuba diving as such - there are dangers, but there aren't any inevitable side effects. Apart from getting cold if diving in UK waters.