What is pollution?

We have seen that hazards are things that endanger human health or life, but hazards can also be harmful to our environment. Pollution is the introduction of contaminants into an environment causing harm, instability or disorder to the ecosystem. (An ecosystem includes all the living organisms (plants, animals, microorganisms) and their physical environment and the interactions between them.) Pollution can be also defined as the presence of a substance in a medium or environment that results in a change to its ‘natural' state, potentially causing an adverse effect. Pollution, however, is not simply the introduction of contaminants. There is always a response in the form of modification or change in the environment. From this standpoint, pollution is the harm that results because substances are present where they would not normally be found, or because they are present in larger than normal quantities.

Contaminants are not necessarily pollutants. A contaminant is a minor substance, material or agent that is unwanted in the environment and may or may not be harmful. A pollutant is a contaminant which, due to its properties or amount or concentration, causes harm. Gases (carbon monoxide, ozone, nitrogen dioxides), chemical vapours, dust particles, fumes and liquid chemicals (pesticides, solvents, drugs, acids, etc.) are examples of potential pollutants of air and water ecosystems.

In nature, the environment has an inherent capacity to clean itself through self-cleaning processes. Natural environmental processes have the ability to deal with many pollutants and correct most imbalances if given enough time. For example, self-cleaning processes in a river could involve:

• Dilution: this takes place when a small amount of a chemical in sewage enters a large flowing river and the pollutant is diluted in the water.
• Oxygenation: this process occurs through mixing of air with water which introduces oxygen that can then be used by aquatic (water-living) plants and animals. Microorganisms consume oxygen when they break down organic matter.
• Sedimentation: this takes place when larger particles settle out at the bottom of the river.
• Biodegradation: this takes place when organic matter is broken down by microorganisms. Organic matter means everything that is derived from living organisms. In a river this could be human and animal waste, decaying plant material, etc.