“The air that I breathe” - Hollies 1974 single, highest chart position 2.

Introduction

The air that surrounds us is a mixture of gases. Many children think that the most abundant gas in air is oxygen because they know that we need oxygen to stay alive. Because of its high media profile, the other constituent of air that they may of heard of is carbon dioxide although they are likely to be surprised at the tiny percentage of carbon dioxide in air. The main constituents of air are nitrogen (78%), oxygen (21%) argon (0.9%) and carbon dioxide (0.04%).

Figure 1 - Proportions of gases in the air (Green - nitrogen, Blue, oxygen, Yellow - Argon & Red - carbon dioxide)

These numbers are hard for children to picture so you can suggest that they think of a car park (Figure 1) with 100 vehicles, 78 green cars (nitrogen), 21 blue cars (oxygen) and 1 yellow car (argon). At this scale the proportion that is carbon dioxide would be represented by a Dinky-sized toy car (red, at about 1:40 scale.) The relative proportions of the different gases remain the same at higher altitudes although the  air is “thinner” (less dense) the further you move away from the Earth’s surface. For instance, at 2500 metres (8000 ft), you are only getting 66% of the oxygen in any breath compared to sea level. Anyone entering a controlled atmosphere room, such as those used for the storage of fruit, must wear breathing apparatus if the oxygen level is below 17%. Humans cannot survive in an atmosphere which contains much less than 17% oxygen.

This also applies to keeping a flame alight. It goes out when the available oxygen drops below a critical percentage, not when all the oxygen has been used up.

Figure 2 - Some fuel tanks are in the wings of aircraft

This information is used to prevent the danger of explosion in aircraft fuel tanks. Scientists came up with a surprisingly simple idea: if you reduce the amount of oxygen in the fuel tank to below 12%, even if a spark were present, burning wouldn't be supported.

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