Air

Atmospheric air is a gaseous mixture consisting of different gases (see table below). In hyperbaric practice it is accurate enough to speak of air as a mixture of ~ 21% oxygen + ~ 79% nitrogen (including ~ 1% of the noble gas argon, which behaves similarly to nitrogen). The fraction of CO2 is negligible. CO2 is only important in expired gas, where the CO2 fraction at atmospheric (= normobaric) pressure is ~ 4%.

_Table 1.1-3. Components of air_

_Table 1.1-3. Components of air_

nitrogen (N2)

78.1

oxygen (O2)

20.93

carbon dioxide (CO2)

0.038 (see above)

argon (Ar)

0.93

neon (Ne)

0.0018

helium (He)

0.00053

krypton (Kr)

0.00011

hydrogen (H2)

0.00005

xenon (Xe)

0.000008

ozone (O3)

0.000002

water vapour (H2O)

(see below)

Water vapour is a very variable component of air. At higher temperatures air may contain higher amounts of water vapour. The unit '% of relative humidity' is temperature dependent. Like all other gases in the air mixture water vapour produces a gas pressure (pH2O). At 37°C and 100% of relative humidity (= 100% saturation with water vapour) pH2O equals 47 mmHg.

1.4.1 Oxygen

Discovered by Joseph Priestley in 1774, oxygen at ambient temperature and pressure is a colourless, odourless and tasteless gas. It consists of a diatomic molecule with the chemical formula O2, and molecular weight 32. Oxygen is a major component of air and is necessary for aerobic respiration. It is the second largest single component of the Earth's atmosphere (20.947% by volume). Due to its electronegativity, oxygen forms chemical bonds with almost all other elements (which is the original definition of oxidation). The only elements to escape the possibility of oxidation are a few of the noble gases. The most famous of these oxides is dihydrogen oxide, or water (H2O). Oxygen promotes fire. For more details see later chapters.

1.4.2 Nitrogen

Discovered by Daniel Rutherford in 1772, nitrogen at ambient temperature and pressure is a colourless, odourless, tasteless and mostly unreactive (ie inert) diatomic non-metal gas. It consists of a diatomic molecule with the chemical formula N2, and molecular weight 28. Nitrogen is the largest single component of the Earth's atmosphere (78.084% by volume). Nitrogen is nearly insoluble in water, which is important for bubble formation in supersaturated tissues in decompression sickness.

1.4.3 Carbon dioxide

First described by Baptist van Helmont in the 17th century, carbon dioxide at ambient temperature and pressure is a colourless, odourless and tasteless gas, with molecular weight 44. CO2 is a chemical compound with two double bounds (O=C=O). As it is fully oxidized, it is not very reactive and particularly inflammable. CO2 is very soluble in water (0.145g CO2 in 100g H2O). When dissolved in water, about 1% of CO2 turns into carbonic acid, which in turn dissociates partly to form bicarbonate and carbonate ions. In 2004, the worldwide atmospheric concentration of CO2 was 0.038 %.

2. GAS LAWS

2.1 Boyle's Law

First described independently by Sir Robert Boyle (1627-1691) and Edme Mariotte (1620-1684), it is also called the 'Boyle-Mariotte Law':

'The product of pressure (p) and volume (V) in a confined amount of gas at equal temperature (T) remains constant.'

For a confined amount of gas in two different states, we can say:

where: 1 = state 1 of confined amount of gas 2 = state 2 of confined amount of gas

Figure 1.1-1. Principle of Boyle's law (Welslau, 2004)

Practical relevance: Inside hyperbaric chambers any confined gas volume in the human body and in (medical) equipment is subject to this law. In gas filled spaces with rigid walls, this effect has to be accommodated during compression to and decompression from higher pressures. This is most important between 1bar and 1.5bar (100kPa - 150kPa) where changes of pressure cause the biggest relative changes of volume.

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