And here it is, everything you ever wanted to know about ODSs (ozone depleting substances) but were afraid to ask.

CFCs

CFCs (chlorofluorocarbons or freons) are chemical compounds made up from carbon, fluorine and chlorine atoms. They used to be widely used, especially as coolants, propellants and cleaning solvents, but their use was prohibited by the Montreal Protocol (1989), as once in the atmosphere they contribute to destruction of the ozone layer.

When they are exposed to sunlight they break up, giving off chlorine atoms, which catalyse (assist) reactions in which ozone is destroyed.

CFCs are not the only chemicals which have this effect on the ozone layer – collectively chemicals which do this are know as ODSs (ozone depleting substances).

CFCs are also powerful greenhouse gases.

HCFC

HCFCs (hydrochlorofluorocarbons) are similar in composition to CFCs, but some hydrogen atoms have not been replaced by fluorine or chlorine. They are used as CFC substitutes, as they only have about 2-10% of the ozone deleting effect of CFCs. They also hang around for 80% less time before decomposing.

Since CFCs were banned in 1994, HCFCs have been more widely used

HFC

HFC (hydrofluorocarbons) have been widely used since 1994. They are thought to have no impact on the ozone layer as they contain no chlorine, but they are a greenhouse gas.

Ozone

Ozone (O₃) is a gaseous compound consisting of three oxygen atoms. It is formed when ultra violet radiation reacts with normal oxygen (O₂).

In the upper atmosphere (mainly the stratosphere, 10-50km above the earth’s surface) it helps stop harmful ultra-violet radiation reaching the earth’s surface.

Greenhouse Gases

When sunlight hits the earth’s surface some radiation is reflected back into the air. Because the radiation is of longer wavelengths than that coming from the sun, it is more easily absorbed by certain gases in the atmosphere, known as greenhouse gases. This causes the atmosphere to warm up, and also the greenhouse gases re-emit radiation back towards the earth, as well as up into space.

Gases causing the greenhouse effect are water vapour (between a third – 2 thirds of the total effect), carbon dioxide (up to one quarter of the effect), methane (up to 10%) and ozone (up to 7%).

Here is a full list of greenhouse gases.