This is the first in a short series of pieces about gases and gas behavior.
Nitrogen is a colorless, odorless, tasteless and mostly inert gas — lithium and magnesium will burn in a nitrogen atmosphere but for our purposes, nitrogen is close to chemically inert. It makes up roughly 78 percent of Earth’s atmosphere by volume, and for the trivia buffs, nitrogen is slightly less dense than oxygen (about 87 percent as dense) and at room temperature and pressure has a mass of 1.25 grams per litre. It is not quite as easy to compress as oxygen. At low pressures — less than 20 bar or so — the difference is minor but becomes more and more apparent at pressures commonly used in scuba diving.
Nitrogen is important to scuba divers for a couple of reasons, because although it’s chemically inert, it does react biologically. As the diver descends and the partial pressure of nitrogen increases, more and more nitrogen dissolves in the bloodstream and from there diffuses into various tissues in a diver’s body. Rapid decompression (specifically in the case of a diver ascending too quickly) can cause nitrogen bubbles to form in the bloodstream, nerves, joints, and other sensitive or vital areas, which in turn can lead to potentially fatal, and certainly debilitating, decompression sickness.
The other reason nitrogen is important is narcosis. On the surface, nitrogen is metabolically inert — we function just fine with it at these levels and just fine without it, but when it’s inhaled at partial pressures in excess of about 3.0 to 3.3 bar — encountered at depths below 30 metres — nitrogen begins to act as an anesthetic agent. This nitrogen narcosis is a temporary semi-anesthetized state of mental impairment. Judgment can be compromised and reaction times slowed.
For some divers, mild narcosis manifests itself as a benign sense of euphoria, and for others the effect is like the arrival of the four horsemen of the apocalypse. Narcosis has been likened to an alcoholic buzz, nitrous oxide (laughing gas), sedatives and having one’s head stuffed with cotton balls. At extreme depths, narcosis can cause hallucinations and unconsciousness.
The intensity and perception of narcosis varies from diver-to-diver and day-to-day. Two similarly experienced and conditioned divers, using similar equipment and bottom gas, may come back from a dive with very different stories about what they saw and how they felt. To a third-party observer, they may respond equally appropriately to outside stimuli and conduct themselves with similar results, but during debriefing one may explain he felt narced while the other will say he felt fine. The next day, same conditions and same depth, the roles may be reversed. This begs a series of questions.
The biophysics of nitrogen narcosis are pretty much solid state. The actual changes made to the nervous system would suggest a constant… not completely understood but probably linear. There are some interesting studies suggesting that multi-day exposure to high pressures of nitrogen(1), lessens these changes, but even if we buy into this concept, it does not account fully for the dramatic variations in the risk and severity of narcosis that divers experience. The only logical explanation is that factors aside from nitrogen partial pressure play an important role in narcotic loading. These factors certainly include stressors such as cold, poor visibility, carbon dioxide retention, mental stress, task-loading, tiredness and poor cardiovascular fitness.
Many divers, myself included, report that mental alertness is compromised diving in cold water and diving following a rough night’s sleep… in a cramped bunk on a boat in high seas for example.
Another factor worsening the effects of narcosis may be mental pre-conditioning — divers who have been told that narcosis will be debilitating report severe narcosis at shallow depths than does the general community. The influence of this perception shift and other factors such as poor breathing habits (skip breathing) can make a huge difference to a diver’s enjoyment and ability to execute a dive safely.
We can therefore take as read that narcosis is a factor in diving and it’s as real as gravity. Its effects have to be accounted for during every dive. Each diver should develop a personal test for narcosis. Because of the nature of the beast, I like to run a little diagnostic from time to time regardless of depth and even when using trimix. Mine is the classic “fingers test” taught in many open water classes. My buddy and I will periodically show each other a number of fingers, and the response is a show of one less if five or more fingers are shown first and one more if that number is less than five. For example, if my buddy holds up nine fingers, I’ll display eight and follow that with an OK sign. I might then display three fingers and expect four back followed by an OK sign. If either of us makes a mess of the arithmetic, we suspect narcosis… and take the necessary precautions.
I suggest that divers getting into advanced open circuit diving select a personal limit for nitrogen partial pressure and stick to it as rigorously as they do to an oxygen partial pressure. Time and experience may affect your choices… you may increase or decrease your nitrogen depth as you fill more logbooks… but do the in-field experiments and start doing the research. My personal comfort-zone in most of the waters in which I dive is 3.1 or 3.2 bar of nitrogen. I’ll put up with more if circumstances dictate, but this level — about the same narcotic load as diving air to 30 metres — is my personal benchmark.
PARAMETERS OF BEHAVIORAL ADAPTATION TO NITROGEN NARCOSIS. Authors: Walsh, JM Abstract of the Undersea and Hyperbaric Medical Society, Inc. Annual Scientific Meeting held May 10-11, 1974.