Ozone has always had a few natural enemies in the atmosphere. One of these is nitrogen (N). Over the past century, chlorine has become another. Due to human activity, its concentration has increased to the point of breaking the delicate balance that exists in the stratosphere.

Let's look in detail at how CFCs, by combining with ozone molecules, cause the thinning of the ozone layer:

Chlorine enters the stratosphere via CFCs (chlorofluorocarbons). CFCs (compounds made up of chlorine, fluorine and carbon) have a fairly long lifespan, which facilitates their accumulation, and they do not react readily with other substances. In fact, they are broken down only by solar radiation, which splits the molecule and thereby releases chlorine (Cl).

Once free, chlorine is able to react with ozone (O3) by taking one oxygen molecule (O) from it, forming chlorine monoxide (ClO) and releasing oxygen (O2).

Cl+O3 = ClO + O2

When a chlorine monoxide molecule (ClO) encounters another oxygen molecule (O), it splits apart, once again releasing chlorine (Cl), which is free to "destroy" another ozone molecule (O3), thus completing the catalytic cycle of chlorine.

ClO + O = Cl + O2

Industrial production of CFCs began in the 1920s, and the perpetuation of this cycle over the years has resulted in an average 3% reduction in ozone.

Fortunately, chlorine too has its "natural enemies", such as methane (CH4), thanks to which the natural ozone layer will be able to reform over a period of approximately 50 years from the moment CFC use and that of similar substances
ceases completely worldwide.

The term "hole" stems from the fact that the reduction is not uniform across the entire planet, but is instead concentrated predominantly over the Antarctic continent, with reductions that have reached as much as 70%. The latest data are fortunately encouraging: the ozone hole is closing.