Depletion of the ozone layer

Ozone is a gas of triatomic molecules made of oxygen alone. About 25 km above the Earth's surface there is a very thin layer of ozone gas.

In the upper atmosphere, oxygen absorbs ultraviolet rays and forms atomic oxygen. This atomic oxygen is very reactive and joins oxygen molecules to form ozone. The ozone formed turns back into oxygen, setting up a natural balance. The ozone layer acts as a protective screen that stops the high-energy ultraviolet (UV) rays from the Sun reaching the Earth's surface.

But gases such as chlorofluorocarbons (CFC) and nitric oxide (NO) break down ozone molecules and destroy the layer. In the upper atmosphere, CFCs take in solar energy and form atomic chlorine, which reacts with ozone and breaks the molecules down; nitric oxide in the atmosphere does the same. As the layer thins, holes form in it, and as a result high-energy ultraviolet rays reach the Earth.

Some harmful effects of the ultraviolet rays reaching the Earth through the thinned layer:

• More cataracts of the eye.
• More deformities in living things, and more skin cancer.
• Lowered immunity.
• Poorer photosynthesis, so smaller harvests.

Why one chlorine atom does so much damage

When a chlorine atom breaks an ozone molecule, it is freed again at the end and goes on to break another — like a catalyst, it is not used up. One atom can destroy thousands of ozone molecules before it is finally removed. That is why even small amounts of CFCs, once used in refrigerators, air conditioners and aerosol sprays, did so much harm, and why the world agreed in the Montreal Protocol (see the last skill) to stop making them. The ozone layer is now slowly recovering.