We said earlier that pollution always has a source and a recipient. The pathway of pollution is the way the pollutant moves from the source, enters into the environment, and finally how it reaches the human body or other recipient. The pathway between source and recipient can take several different forms depending on the type of pollutant. Primary recipients for pollution are water, air, and soil. Pollutants usually reach humans through the consumption of contaminated and polluted water and food, and breathing polluted air.

Once released into the environment, the worst effects of many pollutants are reduced by one or more of the following processes:

• Dispersion – smoke disperses into the air and is no longer noticeable away from the source.
• Dilution – soluble pollutants are diluted in the water of a river or lake.
• Deposition – some suspended solids carried in a river settle (are deposited) on the river bed.
• Degradation – some substances break down (degrade) by natural processes into different, simpler substances that are not polluting.

In each case the effect is to reduce the concentration of the pollutant. Concentration is a measure of the amount of the substance in a known volume of water or air. The units used for water pollutants are usually milligrams per litre (mg/l, also written as mg l-1), although sometimes you may see ppm which stands for ‘parts per million’.

These processes do not apply to all pollutants. There are some persistent pollutants which remain intact when released into the environment because they do not break down by natural processes. These are described in Study Session 8.