Pollution is the entry of any substance or type of energy into a geographic area or habitat that is not normally there. It is commonly divided into several categories including air pollution, water pollution, soil contamination, light pollution, litter, plastic pollution, thermal pollution, radioactive contamination, noise pollution and visual pollution. Any of these various forms of pollution can adversely affect the health (physical and/or mental) and activities of humans and other organisms.
The contaminants can be in the form of solids, particles, liquids, gases, sound, light, heat, electromagnetic radiation and radioactivity. Their emission can be from natural sources such as ash from a forest fire or volcanic eruption, or it can be a direct or indirect result of human activity.
Pollution from human activity began hundreds of thousands of years ago with the discarding of wastes and the use of fire. It intensified several thousand years ago with the start of mining and metalworking, but it was generally very small in scale and highly localized. However, accompanying the Industrial Revolution in the mid-eighteenth century and the worldwide industrialization that followed, it has continued to accelerate in both scale and diversity.
This acceleration became particularly great during the past hundred years or so with the rapid growth in the use of fossil fuels and the large-scale production of petrochemicals and automobiles. Such pollution, including the massive release of carbon dioxide and other greenhouse gases into the atmosphere, is the most fundamental, although not the only, cause of the environmental crisis that the world is now facing.