Pollution is primarily understood as a contamination of nature and earth's resources by human activity. Agricultural, industrial and transport activities seem to be the main sources of pollution today. These activities result in the release of chemical, physical or biological substances into water, air and soil, which interfere with the ecosystem and also have an adverse effect on human health, and the health of every other living creature on the planet. In fact, with the threat of global warming looming large, the entire planet seems to be at a great risk.

Pollutants are of two types: Biodegradable waste, which doesn't pose much risk unless the waste is generated faster than the environment can decompose it. Non-degradable pollutants are the most dangerous as they do not decompose at all. These pollutants mingle with the environment, and affect the soil, the food chain and threaten to affect everything on earth. With the recent rise in population and industrial activity, these non-degradable pollutants have accumulated dangerously. Several chemicals like polychlorinated biphenyls (PCBs) etc. and radioactive materials have accumulated in the environment and threaten to enter the food chain.

Further more, researchers have classified several kinds of pollution based on the facet of the environment that it affects. Air pollution refers to emission of toxic gaseous compounds like carbon dioxide, sulphur dioxide and nitrous oxides into the air. Air pollution is the result of industrial smoke, fuel combustion, emission from vehicles, etc. This is perhaps the worst kind of pollution as it directly affects the air we inhale. City smog over large industrial cities reduces air quality, attacks plants, animal life and causes a number of respiratory diseases among humans. These pollutants spread over vast spaces, accumulate, and often mix with rain, forming what we call acid rain. Acid rain is known to destroy entire lakes, forests and even whole ecosystems. The worst effect yet of air pollution is global warming: an overall increase in the temperature of the atmosphere which melts ice caps, raises the sea level and threatens destruction.

Water pollution is the release of urban sewage into seas, rivers and other water bodies. These affect the oxygen levels of the water, killing aquatic plant and animal life. Even agricultural activities have destroyed micro-organisms and life under the soil.
All these pollutants whether solid, liquid or gaseous have proved immensely hazardous to the environment, eliminating certain species of animals and plants completely, and permanently affecting the quality of natural resources.