Pollutants are substances which directly or indirectly damage us or the environment. Many of the compounds which are dangerous to the environment can also be harmful to us in the long-term.
Pollutants can cause the destruction of areas of the environment which are protective to us. CFCs were carefully chosen not to be damaging to humans, however a side effect causes an effect held to be very damaging to the environment. They diffuse in to the upper atmosphere where they stay for some time. When the sun's radiation does break them down, they turn in to highly-reactive radicals. These catalyse the break down of the ozone layer, which protects us from cancer-causing ultraviolet radiation from the sun. CFCs are especially damaging because one CFC molecule can cause the breakdown of many millions of ozone molecules.
Some pollutants imbalance environmental processes by causing an excess of a compound which is already present naturally. Carbon dioxide is already present in the atmosphere - indeed it is vital for life on earth. Carbon-dioxide helps keep the earth warm by trapping infra-red radiation, which would otherwise be reflected in to space. However if there is too much of it present the earth's temperature will rise too high. It is thought that this would cause many destructive effects including the flooding of many low-lying areas, and an imbalance of the earth's weather system. Another example of these compounds are nitrate-containing fertilizers. When these leak in to streams they cause plants and algae to grow too fast. This restricts light for plants on the river-bed and they decay. Microbes feed off the decaying plants and use up all the oxygen in the lake, causing fish and other plants to start dying off.
Many pollutants have a poisonous effect on the body. Carbon monoxide is an example of a substance which is damaging to humans. This compound is taken up in the body in preference to oxygen, causing the body to suffocate.
Some pollutants are not so dangerous by themselves until they combine with other naturally present compounds. The oxides of nitrogen and sulphur are released from impurities in fossil fuels when they are burnt. They react with water-vapour in the atmosphere to become acid rain. Acid rain damages buildings and makes lakes uninhabitable.
Compounds can be bad pollutants not just by how damaging they are but also because of other relevant factors, such as the length of time that they stay dangerous. Uranium is a radioactive element used in nuclear fission power plants. Once it has been used it is often highly radioactive, meaning even small traces are able to cause cancer and damage unborn children. It will stay like this for many millions of years and has to be kept under adequate storage. Because of the difficulties in safely containing this element many people think it is inevitable that radiation will escape and cause damage to the environment.