Northwest Dive Club

I was just reviewing altitude diving procedures and found this statement:

Nitrogen narcosis might not be experienced by an Advanced Open Water diver who only dives to 100 feet normally. However, put that diver down 100 feet starting at 9,000 feet elevation and they will experience it to some degree.

Wait, what?

It's my understanding that the increased theoretical depth when planning an altitude dive (e.g., 100' ffw at sea level = 117 ffw at 5000') compensates for the lower atmospheric surface pressure. The objective of the deeper theoretical depth is to adjust the NDLs accordingly, not because you really are deeper.

Returning to the question of narcosis, it is also my understanding that nitrogen narcosis is a result of increased nitrogen partial pressures on the CNS. If that is the case, then there shouldn't be any increased narcosis susceptibility at altitude; if anything, it should be less because the absolute pressure is also less.

What am I missing here? Is increased narcosis at altitude really a concern (and if so, why?), or is this just a myth?

That doesn't seem right.

You're right, Narcosis is brought on by partial pressure (Henry's Law), so you really should see no real difference in Narcosis.

And you already know other things can influence narcosis, cold, dark, CO2 loading etc.

Decompression is a whole different story due to your saturation level at the time of the dive at altitude.