At sea level atmospheric pressure is 1 bar absolute (1 standard atmosphere =101 kPa=1.013 bars). The weight of the atmosphere exerts a pressure which will support a column of water 10 m high; 10 m under water the pressure on a diver is 200 kPa. The volume of gas in an early diving bell full of air at sea level is halved at 10 m according to Boyle’s law; at 20 m pressure is 300 kPa absolute and the gas is compressed into one third the volume.
Dry air is composed of roughly 21% oxygen, 78% nitrogen, and 1% other gases. According to Dalton’s law the partial pressure of oxygen at any depth will be 21% of the total pressure exerted by the air and the partial pressure of nitrogen will be 78% of total pressure.
Gases dissolve in the liquid with which they are in contact. Nitrogen is fat soluble and at sea level we have several litres dissolved in our bodies. If the partial pressure of nitrogen is doubled (by breathing air at 10 m depth) for long enough for equilibration to take place we will contain twice as many dissolved nitrogen molecules as at sea level.
