The Dobson unit (DU) is a unit of measurement of the columnar density of a trace gas in the Earth's atmosphere. It originated, and continues to be widely used, as a measure of total-column ozone, which is dominated by ozone in the stratospheric ozone layer. It is given as the thickness (in units of 10 µm) of that layer which would be formed by the total gas in a column under standard temperature and pressure, sometimes referred to as a 'milli-atmo-centimeter.' For example, 300 DU of ozone brought down to the surface of the Earth at 0 °C would occupy a layer only 3 mm thick. One DU is 2.69×10 ozone molecules per square centimetre, or 2.69×10 per square metre. This is 0.4462 millimoles of ozone per square metre, or approximately 1.25 ppb of the average total column of air.