Air consists of a mixture of O2 + N2 + a trace of carbon dioxide (CO2), and minute amounts of rare gases. Rare gases such as Neon (Ne), Argon (Ar) and Xenon (Xe), and Hydrogen (H2) exist in trace amounts only.
The approximate composition of air is:
Some less reputable suppliers of air fills for scuba tanks provide free additives to the compressed air, such as dust, oil, hydrocarbons, rust, water vapor and carbon monoxide (CO).
This is a colorless, odorless, tasteless gas which is indistinguishable from air to breathe. It is essential for metabolism and maintenance of life yet in quantities exceeding those in air it is toxic to man. Its proportion in air (21% or more specifically, a partial pressure of 0.21 ATA at sea level) is critical. A little more than this causes O2 toxicity, a little less will not support human life. For this reason most gas mixtures breathed by deep divers contain an inert gas – usually either N2 or helium (He), mixed with O2 to ensure that the O2 composition is maintained at a partial pressure close to 0.2 ATA (0.16 – 0.40 ATA).
O2 supports combustion vigorously and can cause normally non-flammable substances (such as the occupants of a recompression chamber) to burn brilliantly if it is present at a sufficiently high partial pressure. Divers should be aware of the potentially explosive and combustible properties of oxygen, as
they may require to use it in first-aid, or be inadvisably enticed into diving with high oxygen mixtures.
Nitrogen – N2
This gas, which is the major constituent of air, is also colorless, odorless and tasteless. N2 dissolves well in body fluids and tissues, causing narcosis at depth and decompression sickness when it bubbles out of solution, after ascent.
It is termed an “inert gas” because it does not take part in human biochemical processes. The Creator appears to have included this gas in air to prevent us from developing O2 toxicity, and to reduce the fire hazard.
Divers vary this N2/O2 ratio (in Nitrox, oxygen enriched air or mixed gas diving) in an attempt to improve on nature, extend diving durations, and reduce narcosis.
Carbon Dioxide – CO2
This gas is also colorless, odorless and is said to be tasteless. However if a diver inhales a mouthful of CO2 from a buoyancy vest inflated from a CO2 cartridge it will be found to taste very nasty, due to its formation of carbonic acid in water.
CO2 is a by-product of cellular metabolism and we exhale approximately 5% of CO2 in our breath. If a diver re-breathes some of his exhaled gas by using faulty breathing equipment or an excessively long snorkel the CO2 will accumulate in the body leading to toxicity.
These effects are discussed further here.
Carbon Monoxide – CO
This gas is colorless, odorless and tasteless. It cannot be detected by a diver and even in trace amounts can cause loss of consciousness or death.
It is usually produced as a product of incomplete combustion of carbon containing compounds and is a constituent of internal combustion engine exhausts and cigarette smoke.
Air contaminated by carbon monoxide, if supplied in scuba cylinders or by surface supply to divers, may have lethal results
Helium – He
This is a colorless, odorless, tasteless gas, which is very light and very expensive. It is obtained from underground natural gas sources found in North America and elsewhere.
It is used to dilute O2 in gas mixtures breathed at great depths because it has little tendency to produce narcosis (e.g. Heliox may be 90% He + 10% O2, or any other proportion). Due to its very low density it readily escapes through small leaks in pipes and valves making it difficult to retain. It is also a very effective conductor of heat, causing serious problems with hypothermia.
The low density of He alters the normal process of speech production causing “Donald Duck” like speech when a diver breathes this gas
Hydrogen – H2
This is a very lightweight gas that can replace N2 to reduce narcosis at depth. Unfortunately it can combine explosively with O2 and the resultant water (H2O) is not sufficient to ‘put out the diver’.
It is sometimes used with very low O2 percentages, at great depths, by skilled professional divers. It shares many problems with He.
Neon – Ne, Argon – Ar,
Radon – Rn, and Xenon – Xe
These are more biologically inert gases which are present only in trace amounts in the atmosphere. They are of no importance to recreational divers.
Because of lubrication needs in the compressor, oil vapors and hydrocarbons can be produced which may then contaminate the air supply. See
Related articles by Scuba Diving Sarasota
- Scuba Diver Running Out of Air and Needing a Back-Up Air Supply (scuba-diving.suite101.com)
- What is Nitrogen? (brainz.org)
- Fire 101 (neatorama.com)