[TreeRingCounter]

For those curious, 40m is a standard cutoff for rec diving because

A) oxygen partial pressure with compressed air reaches 1.05atm, with 1.2-1.4atm posing a serious risk of oxygen-induced seizures (extremely dangerous)

B) nitrogen partial pressure becomes high enough that it becomes narcotic (every gas gets you high at enough pressure, even noble gasses).

[nradov]

There is virtually zero risk of an oxygen toxicity seizure in the 1.2 - 1.4 atm range. I don't think there has ever been a confirmed case of a sport diver toxing at that level. We normally target about 1.2 atm for the working phase of the dive (reduce slightly for long exposures) and 1.6 for deco (with back gas breaks for long oxygen stops).

Narcosis is an issue with nitrogen and oxygen (as well as most other gasses). Helium is the exception. It is a noble gas and has no real narcotic effect; rather the opposite. Other noble gasses such as argon are much more narcotic than nitrogen and aren't used as diving breathing gasses (outside of a few limited experiments).

The other problem with nitrogen and oxygen is that they are relatively dense and so at greater depths they cause increased work of breathing, which in turn causes CO2 accumulation. CO2 is itself highly narcotic, and has other negative physiological effects. So for deeper dives we add progressively more helium to the mix.