As well as being concerned about the quantity of water available for humans, governments and international agencies are much concerned with its quality. Naturally occurring water is never pure, but contains a wide variety of dissolved substances, some of which are harmful to health.

The supply of clean water is also affected by how people dispose of their waste water. People living in areas where there is no sanitation system for the disposal of waste water have little choice but to throw it away into a river or onto the ground. Each litre of water disposed of in this way pollutes an average of eight litres of freshwater, and the UN estimates that the global human population pollutes 12 000 km³ of water annually in this way. Unless there is major investment in sanitation systems, this figure will have increased to 18 000 km³ by 2050 (UNESCO, 2003).

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