Pollution is much more than an environmental challenge - it is a profound and pervasive threat that affects many aspects of human health and wellbeing.

The biggest risk factor, air pollution, contributed to 6.5 million premature deaths. This included pollution from outdoor sources, such as gases and particulate matter in the air, and in households, from burning wood or charcoal indoors.

The next largest risk factor, water pollution, accounted for 1.8 million deaths, while pollution in the workplace was linked to 800,000 deaths globally.

About 92% of these deaths occurred in poorer countries, with the greatest impact felt in places undergoing rapid economic development such as India, which had the fifth highest level of pollution deaths.

Water pollution is any chemical, physical or biological change in the quality of water that has a harmful effect on any living thing that drinks or uses or lives (in) it. When humans drink polluted water it often has serious effects on their health. Water pollution can also make water unsuited for the desired use.

What are the major water pollutants?

There are several classes of water pollutants. The first are disease-causing agents. These are bacteria, viruses, protozoa and parasitic worms that enter sewage systems and untreated waste.

A second category of water pollutants is oxygen-demanding wastes; wastes that can be decomposed by oxygen-requiring bacteria. When large populations of decomposing bacteria are converting these wastes it can deplete oxygen levels in the water. This causes other organisms in the water, such as fish, to die.

A third class of water pollutants is water-soluble inorganic pollutants, such as acids, salts and toxic metals. Large quantities of these compounds will make water unfit to drink and will cause the death of aquatic life.

Another class of water pollutants are nutrients; they are water-soluble nitrates and phosphates that cause excessive growth of algae and other water plants, which deplete the water's oxygen supply. This kills fish and, when found in drinking water, can kill young children.

Water can also be polluted by a number of organic compounds such as oil, plastics and pesticides, which are harmful to humans and all plants and animals in the water.

A very dangerous category is suspended sediment, because it causes depletion in the water's light absorption and the particles spread dangerous compounds such as pesticides through the water.

Finally, water-soluble radioactive compounds can cause cancer, birth defects and genetic damage and are thus very dangerous water pollutant.