The thickness of the ozone layer is measured in:

The Dobson unit (DU) is the unit used to measure the total amount (column thickness) of ozone in a vertical column of the atmosphere above a point on the Earth's surface. It is named after G.M.B. Dobson (Gordon Miller Bourne Dobson), a British physicist and meteorologist at the University of Oxford, who built one of the first instruments to measure atmospheric ozone from the ground in the 1920s, the Dobson spectrophotometer.

One Dobson unit is the amount of ozone that would form a layer 0.01 millimetres thick if all the ozone in the column were brought down to the Earth's surface at standard temperature (0°C) and pressure (1 atmosphere). In other words, 100 DU corresponds to a layer of ozone just 1 millimetre thick.

The average thickness of the ozone layer around the world is about 300 DU, which would be only about 3 millimetres thick if compressed at the surface. Ozone levels vary with latitude, season and weather.

The "ozone hole" is defined as an area where the total ozone falls below 220 DU. The Antarctic ozone hole appears every year during the southern spring (September to November). It was first reported in 1985 by scientists of the British Antarctic Survey: Joseph Farman, Brian Gardiner and Jonathan Shanklin. It is caused mainly by chlorine and bromine released from chlorofluorocarbons (CFCs) and halons, which destroy ozone in the presence of polar stratospheric clouds and sunlight.

The ozone layer lies mainly in the stratosphere, at about 15-35 kilometres above the Earth. It absorbs most of the harmful ultraviolet-B (UV-B) and all of the UV-C radiation from the Sun, protecting living organisms.

Thanks to the Montreal Protocol (1987), which phased out ozone-depleting substances, the ozone layer is slowly recovering and is expected to return to 1980 levels by around 2066 over Antarctica. World Ozone Day is observed on 16 September.

Why not the other options?

• ✗Decibels measure the intensity of sound.
• ✗Fathoms measure the depth of water (1 fathom = 6 feet).
• ✗Pascals measure pressure.