nitrogen poisoning

I have not heard of nitrogen being a problem for freedivers. Nitrogen narcosis only occurs at depth while breathing compressed air. However I have heard of advanced freedivers having oxygen poisoning at depth.

"nitrogen poisoning"??? No - that doesn't happen IMO. Nitrogen narcosis is a similar feeling to being drunk and occurs at depth. It goes away as you come up. I have been "narced" as it is commonly called when SCUBA diving below 40m but not when freediving around that depth. Nitrogen narcosis can happen freediving but you need to be at pretty serious depths to experience that when freediving - e.g. the 100m+ divers talk of it.

The other thing with Nitrogen compressed into the blood stream when breathing compressed air at depth is that more and more nitrogen gets compressed into your blood as you go deeper and as time passes. SCUBA divers need to come up slowly and at times have "decompression stops" to allow the gas to gradually come out of their bloodstream to avoid getting "the bends". This is because if they return to the surface too quickly the gas will come out of solution and forms bubbles (similar to when you open a bottle of champagne with the quick release of pressure). These gas bubbles can collect round joints and cause the diver to be in pain and bent over hence the common name. Being bent is NOT good.

Freedivers can get "the bends" but this is only a real risk for those doing serious depths and multiple dives over a day. There is much more knowledge round the bends risk for SCUBA divers than there is for free divers. AIDA have guidelines round surface intervals between deep freedives etc. but other than the simple "surface interval = double your last dive time" rule of thumb for dives less than 30m most folks don't need to be concerned. Beyond 30m you should apply different logic (longer) and then boyond say 60m / 70m you should probably do one dive a day (I haven't looked up the guidelines lately so don't take that as gospel - I have never dived that deep).

gohard is basicly correct. There are some nuances that most divers don't need to worry about.

Its possible ( a friends personal experience) to get bent doing 55-60 ft dives, given enough dives and long enough bottom time. Spearos who have very long dive times(aspetto) are most at risk. My buddy brought on his case by flying back from the Bahamas in an unpressurized airplane. There have been some other cases, mostly spearos in very deep water. Also, adding moderate depth freediving after scuba diving puts the diver at relatively high risk.

Nitrogen narcosis is subtle. Its effects begin as soon as the diver starts to descend and are measurable in scuba divers in less than 30 meters, although the effects are not obvious until deeper. Divers doing FRC, report having much clearer heads at depths of 30 meters or so than they used to when diving full lung. This looks very much like less narcosis. In freedivers, N2 narcosis and CO2 narcosis are intertwined, hard to separate the effects. CO2 is probably more important

cdavis said:

Divers doing FRC, report having much clearer heads at depths of 30 meters or so than they used to when diving full lung. This looks very much like less narcosis.

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Does anybody seriously get narked at 30m? Shallowest I've felt it was doing bottom time in 60m after a very fast descent. Suspect when FRC divers talk about having clear heads they mean on the surface after the dive.

Seriously narked, I don't think so, but the science is pretty well established. Narcotic effect starts at the surface and builds as you go deeper. I suspect it is a geometric increase, but don't know for sure. Its been a while since I reviewed the research(as in many years), but in pressure chamber experiments, narcosis had a measurable effect at something less than 30 meters. The diver isn't affected enough to recognize it until deeper. The insidious thing about narcosis is that it reduces the diver's awareness that he is becoming impaired.

On the clear head, I mean at depth and I'm pretty sure others do as well. The first few times I did 90 -100 at about 1/2 lung, it really got my attention because it was so different. Not at all sure this is a N2 narcosis effect, could be somehow wrapped up in blood shift and C02,but it sure is suspicious.

There have also been numerous comments that an FRC diver is less hypoxic and therefore more clear headed at the end of a long dive compared to full lung. My lousy co2 tolerance wouldn't let me push far enough to get hypoxic diving full lung, so can't comment on that.

I don't notice any difference down to about 60m FRC. All that research is for SCUBA, I've never heard anybody claim to be narked as shallow as 30m when freediving. I can get narked at the surface if I do a hard CO2 table - but narcosis at depth seems to be way more about descent speed, since a fast VWT descent produces even less CO2 than an FRC one yet has me away with the fairies.

I'm definitely not less hypoxic on FRC - very much doubt I could do 100m without blacking out.

Mark Jeffery said:

However I have heard of advanced freedivers having oxygen poisoning at depth.

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Don't think this happens either. A while back I did wonder if the shakes I sometimes got at depth were related to O2 toxicity but the partial pressure wasn't really high enough (tops out at about 2atm) and the duration was too short. Far more likely to be CO2 related.