A 1 mm layer of pure ozone gas around the earth at normal temperature and pressure is equivalent to

One Dobson Unit represents a 0.01 mm thick layer of pure ozone at normal temperature and pressure.

A full 1 mm layer equals one hundred such 0.01 mm slices stacked together, giving one hundred Dobson Units.

A reading of ten Dobson Units would correspond to only a 0.1 mm ozone layer.

A single Dobson Unit would be just one 0.01 mm layer, far thinner than 1 mm.

A value of 0.1 Dobson Unit is smaller still, a mere 0.001 mm of ozone.

Average total column ozone over the globe is roughly three hundred Dobson Units.