As one can imagine, divers are particularly dependent on the supply of breathing gas. Various combinations of breathing gases are used in diving. Although compressed air is the most common, the use of other mixtures for diving is increasing. There are two different methods of providing breathing gases that can be used in diving. They are the following:
     1.   The diver may be supplied with gas through an umbilical from a distant source
     2.   He or she may carry the breathing gas supply. This method is called scuba, which is an initialism for "Self-Contained Underwater Breathing Apparatus."
 

		Characteristics and Disadvantages
A I R		Air is a mixture of gases. It contains different percentages of nitrogen (78.084%), oxygen (20.946%), argon (.934%), carbon dioxide (.033%), and other gases (.003%). The chart on the right illustrates the composition of air.
O X Y G E N		Oxygen is a colorless, odorless, and tasteless gas that is only slightly soluble in water. It can be liquefied at -297.4°F (-183°C) at atmospheric pressure and will solidify when cooled to -361.1°F (-218.4°C). Oxygen is the only gas used by the human body, and it is essential to life. The other gases breathed from the atmosphere or breathed by divers in their gas mixtures serve only as vehicles and diluents for oxygen. Oxygen in the human organism supports a constant process of oxidation of food substances. As a result, energy is released. An insignificant change of the amount of oxygen present in the inhaled air does not influence the human organism. However, a volume decrease of about 84% causes oxygen deficiency. ___________________________________________________ WARNING: Oxygen is dangerous when excessive amounts are breathed under pressure; this harmful effect is called oxygen poisoning.
N I T R O G E N		Nitrogen is a colorless, odorless, and tasteless gas. It is chemically inert and is incapable of supporting life. Its boiling point is -320.8°F (- 196°C). Nitrogen is commonly used as a diluent for oxygen in diving gas mixtures but has several disadvantages compared with some other diving gases. At normal pressure nitrogen does not influence the human organism and an amount of about 1 liter is dissolved in the blood and the tissues. CO2. ___________________________________________________ WARNING: When nitrogen is breathed at increased partial pressures, it has a distinct anesthetic effect called "nitrogen narcosis," a condition characterized by loss of judgment and disorientation.
H E L I U M		Helium is found in the atmosphere only in trace amounts. It has the lowest boiling point of any known substance, -452.02°F (- 268.9°C). Helium is colorless, odorless, and tasteless and is used extensively as a diluent for oxygen in deep diving gas mixtures. Helium is used in breathing mixtures at depth because of its lower density and lack of narcotic effect. However, helium should never be used in diving or treatment without a full understanding of its physiological implications. ___________________________________________________ WARNING: Helium has some disadvantages but none as serious as those associated with nitrogen.          •   Breathing helium-oxygen mixtures causes a temporary distortion of speech (producing a Donald Duck-like voice), which hinders communication.          •   Helium also has high thermal conductivity, which causes rapid loss of body heat in divers breathing a helium mixture.
C A R B O N	D I O X I D E	Carbon dioxide is a gas produced by various natural processes such as animal metabolism, combustion, and fermentation. It is colorless, odorless, and tasteless. A person should not breathe air containing more than 0.1% CO2 by volume; divers must therefore be concerned with the partial pressure of the carbon dioxide in their breathing gases. In the case of closed- and semi-closed-circuit breathing systems, the removal of the excess CO2 generated by the diver's breathing is essential to diving safety. ___________________________________________________ WARNING: Although carbon dioxide generally is not considered poisonous, in excessive amounts it is harmful to divers and can even cause convulsions. Breathing CO2 at increased partial pressure may cause unconsciousness.
C A R B O N	M O N O X I D E	Carbon monoxide is colorless, odorless, and tasteless and therefore difficult to detect. It is produced by the incomplete combustion of hydrocarbons, which occurs in the exhaust systems of internal combustion engines. Carbon monoxide may also be produced by over-heated oil-lubricated compressors. When scuba cylinders are filled, care should be taken not to introduce CO from the exhaust system of the air compressor into the breathing gases. Proper precautions must be taken to ensure that all areas where cylinders are filled are adequately ventilated. The compressor's air intake must draw from an area where the atmosphere is free of contamination, such as automobile exhaust fumes. ___________________________________________________ WARNING: Carbon monoxide is a poisonous gas. A level of 20 parts per million of CO should not be exceeded in pressurized breathing systems.
O T H E R S		Argon, neon, and hydrogen have been used experimentally as diluents for oxygen in breathing gas mixtures, although these gases are not used routinely in diving operations. However, the results of recent research suggest that hydrogen-oxygen and helium- hydrogen-oxygen breathing mixtures may be used within the next decade in deep diving operations.

Water Vapors
The normal weight of water vapors in the air is considered 1–1.5%. They have a harmful effect on the human organism if a large amount is present. Water vapors affect heat-exchange mechanisms and irritate the respiratory system. Their deficiency affects the organism as well. It causes unnecessary release of moisture and also increases fatigue. 

Breathing provides constant exchange of gases, resulting in a significant difference between the inhaled and the exhaled air. During breathing in a relaxed state, the amount of oxygen present in exhaled air decreases from 21 to 16%, whereas the amount of carbon dioxide increases from .3 to 4.5%. The air becomes saturated with water vapors and the amount of nitrogen remains constant.

Both the amount of oxygen used by the human organism and the amount of carbon dioxide released depend on the physical tension as well as on the type and speed of work. The amounts of gases taking part in gas exchange vary with changes in the frequency and depth of breathing.

Moisture
Breathing gas must have sufficient moisture to be comfortable for the diver to breathe. Too much moisture in a system can increase breathing resistance and produce congestion; too little can cause an uncomfortable sensation of dehydration in the diver's mouth, throat, nasal passages, and sinus cavities (U.S. Navy 1988). Air or other breathing gases supplied from surface compressors or tanks can be assumed to be dry. This dryness can be reduced by removing the mouthpiece and rinsing the mouth with water or by having the diver introduce a small amount of water into his or her throat inside a full face mask. The use of gum or candy diving, to reduce dryness while diving can be dangerous, because these items may become lodged in the diver's throat. The mouthpiece should not be removed in water that may be polluted.

Condensation in Breathing Tubes or Mask
Expired gas contains moisture that may condense in the breathing tubes or mask. This water is easily blown out through the exhaust valve and generally presents no problem. However, in very cold water the condensate may freeze; if this freezing becomes serious enough to block the regulator mechanism, the dive should be aborted.

Fogging of the Mask
Condensation of expired moisture or evaporation from the skin may cause fogging of the face mask glass. Moistening the glass with saliva, liquid soap, or commercially available anti-fog compounds will reduce or prevent this difficulty. However, it should be noted that some of the ingredients in chemical defogging agents can cause keratitis (inflammation of the cornea) if improperly used. Wright (1982) has described two such cases; symptoms included severe burning, photophobia, tearing, and loss of vision, which Wright attributed to the use of excessive quantities of the defogging solution and inadequate rinsing of the mask.