Biological magnification and the ozone layer

Farmers spray pesticides to protect crops. Some of it washes into the soil and the water. Plants take it up along with water and minerals, and from the plants it enters food chains.

Many of these chemicals are not broken down. Picture a factory, or a field, beside a pond: the chemical gets into the water plants; small fish eat many plants, big fish eat many small fish, and a person eats many big fish, again and again. At each step the chemical builds up. This is biological magnification.

So the energy in a food chain goes down at each level, but the concentration of such chemicals goes up. Humans are at the top of most food chains, so we collect the most. Even washing fruit and vegetables cannot always remove it.

Ozone (O₃) is a molecule of three oxygen atoms. High up in the atmosphere, UV radiation from the Sun splits oxygen molecules (O₂) into single atoms, and a single atom then joins an O₂ molecule:

O₂ → O + O, then O + O₂ → O₃

Near the ground, ozone is a deadly poison. But high up, the ozone layer is a shield: it stops much of the Sun's harmful UV radiation, which causes skin cancer in human beings.

In the 1980s, the amount of ozone began to fall sharply. The main cause was CFCs (chlorofluorocarbons), used in refrigerators and fire extinguishers. In 1987 the United Nations Environment Programme (UNEP) got countries to agree to freeze CFC production at 1986 levels. Manufacturers must now make CFC-free refrigerators.