2
26 NAUI Nitrox Diver
SOME FACTS ABOUT
INDIVIDUAL GASES
Oxygen (O2)
Oxygen is one of the most abundant elements on earth.
It is present not only as a gas – as free oxygen in the
atmosphere and as an element in atmospheric carbon
dioxide; it is a constituent in many other compounds,
such as silicon dioxide (quartz and sand). By weight,
almost half of the composition of the earth’s outer crust
is oxygen in various compounds. Oxygen is highly
reactive and combines readily with most other elements.
As a gas, it is colorless, odorless, and tasteless. Oxygen is
our life-supporting gas and is the essential component
of all breathing gas mixtures. It is used in respiration,
combining with glucose in our cells to produce carbon
dioxide and water. Although it is essential for body
metabolism, when it is breathed in excessive amounts
under pressure, oxygen can be very toxic. Oxygen
supports combustion, and many substances will
spontaneously ignite or burn furiously in a pure or high
oxygen environment.
Much of what you will learn in this course is about
the risks of oxygen, the proper use of oxygen, setting
appropriate limits by controlling the oxygen fraction
in the nitrox you use and the depth to which you dive,
safety considerations in oxygen handling, and care of
scuba and air-fill equipment that will be exposed to high
concentrations of oxygen.
Nitrogen (N2)
Nitrogen is a largely inert gas. It is not used in
metabolism and serves essentially to dilute the oxygen
in the air we breathe. It is also colorless, odorless, and
tasteless. Although it does not combine easily with other
elements, it is a component in many organic compounds
and is in all living organisms. When breathed at higher
pressures, it has a pronounced anesthetic effect referred
to as nitrogen narcosis.
Nitrogen narcosis is one of the reasons that
recreational diving is limited to about 40 meters (130
feet). The mechanisms of narcosis are only partly
understood, and several factors seem to be involved–
including psychological predisposition, stress, and
anxiety, but hyperbaric nitrogen is the major contributor.
Narcosis can impair a diver’s ability to function at
greater depths, and severity increases with depth. A
favorite tongue-in-cheek “law” of divers is Martini’s
Law: “Breathing air, each 15 meters (50 feet) of depth
is equivalent to drinking one dry martini on an empty
stomach.” The symptoms of nitrogen narcosis also
parallel those often seen in alcoholic intoxication:
inability to concentrate and multi-task, easy distraction,
loss of motor ability and dexterity, mood changes, etc.
Sensitivity to narcosis varies from person to person and
time to time. There is no certain evidence that breathing
oxygen-enriched air reduces the risk of narcosis.
Theoretically it might, but clinical studies have produced
mixed results, perhaps because the individual differences
among human subjects outweigh the physics.
Argon (Ar)
Argon makes up about 1% of air. As noted above, it is
included as part of the nitrogen component in enriched
air nitrox calculations. Argon is one of the “noble gases”
meaning it is virtually completely inert, not combining
with other elements. (The other noble gases are helium,
neon, krypton, xenon, and radon.) It has a narcotic
potency slightly over twice that of nitrogen. Because
argon has lower heat conductivity than air, it is often
used as an inflation gas for dry suits to improve their
insulating properties.
Carbon Dioxide (CO2)
Carbon dioxide is colorless, odorless, and tasteless in
small quantities. It combines readily with water to form
carbonic acid, and at higher concentrations it has an
acidic taste and odor, which you can sometimes sense
when you sniff fizzing carbonated beverages. At high
concentrations (above about 10%), it can be extremely
toxic and cause convulsions and death. Carbon dioxide is
a natural byproduct of our respiration and combustion of
organic compounds.
The concentration of carbon dioxide in the
atmosphere actually varies depending on place and
season, and much of the concern about global warming
is due to the increasing fraction of carbon dioxide in the
atmosphere from burning fossil fuels and other sources.
The amount of carbon dioxide in the atmosphere
was about 280 parts per million (ppm) in the early
19th century. Currently, sampling at Mauna Loa,
which because of its favorable location and altitude is
considered to be a reliable indicator, showed a carbon
dioxide content of 316 ppm in 1959 increasing to 405
ppm in 2019, an increase of 28%!
Helium (He) & Neon (Ne)
Both helium and neon are less dense than nitrogen. They
have a very low narcotic potential and so are possible
candidates to reduce both the nitrogen content and
the oxygen content of the breathing gas in deep diving.
Both are expensive; neon is very expensive. Helium
is the mixing gas of choice for diving deeper than the
air depth range. You will learn more about helium if
you venture into technical diving. “Trimix” refers to a
mixture of oxygen, helium, and nitrogen. Neon is not