a slow, steady decline of about 4% per decade in the total volume of ozone in Earth's stratosphere (ozone layer), and a much larger, but seasonal, decrease in stratospheric ozone over Earth's polar regions during the same period.

The ozone layer is a layer present in the earth’s atmosphere that contains a high concentration of ozone. Since the late 1970s, much of this layer of protective stratospheric ozone has been steadily disappearing. There is a hole the approximate size of the continent of North America in the layer over Antarctica. Less dramatic but significant depletions have been recorded around the globe.

The primary cause of ozone depletion is chlorine-containing source gases. In the presence of UV light, these gases disassociate and release chlorine atoms. The chlorine atoms then go on to combine with oxygen in ozone and catalyse ozone layer destruction. A single chlorine atom is capable of destroying a large number of ozone atoms over a long period of time.

There are many different sources that generate gases that contain chlorine. Natural sources that are capable of generating include volcanoes and oceans. Volcanoes are powerful natural events and inject large quantities of hydrogen chloride into the atmosphere.  Oceans also produce large amounts of salt daily; salt contains chlorine. But both of these sources are too weak to have any influence on the ozone present in the stratosphere. The intensity of volcanic eruptions is not high enough for the hydrogen chloride to reach the stratosphere. Salt from the oceans is also deposited very low in the atmosphere and will have to be air borne for up to five years to have any impact on the ozone layer. Also, both of these are soluble in water.

On the other hand, Chlorofluorocarbons (CFCs), the biggest man-made cause of ozone depletion, do not dissolve in water. They are also far more stable and there are no known processes that remove them from the atmosphere. Thus they have a much bigger impact on ozone depletion that all natural sources combined.

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