Broadly speaking, the term "pollution" may be used in several circumstances, but invariably it refers to the worsening of some aspects of our environment, from our habitat to interpersonal relationships.

	Fig 1: After 45 years of space exploration, near-Earth space is littered with more than 7,000 man-made objects, or mission-related debris, which move at speeds of about 28,000 km/h. This "space debris" now presents a serious hazard to satellites. (Credit:  ARES NASA/Johonson Space Center)

Pollution can refer to the falsification of evidence in a trial to hamper the identification of penal responsibility. The same term may refer both to the decline of collective behaviour in following dubious and illegal social rules and, from an aesthetic point of view, to landscape degradation from landfills, illegal garbage dumping, or illegal urbanization. From a physical point of view, we can mention acoustic pollution in industrial, urban or natural settings, thermal pollution (when, for example, the temperature of  a receiving body of surface water increases due to a discharge into it), and rubbish tips or auto junkyards are said to constitute aesthetic or visual pollution.

Living organisms can also be defined as pollutants, when a non-indigenous species (plant or animal) enters a habitat and modifies the existing equilibrium among the organisms of the affected ecosystem (sea, lake, river).
Non-indigenous, including transgenic species (GMOs), may bring about a particular version of pollution in the vegetal kingdom: so-called genetic pollution. This term refers to the uncontrolled diffusion of genes (or transgenes) into genomes of plants of the same type  or even unrelated species where such genes are not present in nature.  For example, a grass modified to resist herbicides could pollinate conventional grass many miles away, creating weeds immune to the most widely used weed-killer, with obvious consequences for crops. Genetic pollution is at the basis of the debate on the use of GMOs in agriculture.
In a stricter sense, the term pollution is commonly referred to the release into the environment of man-made or natural chemicals (pollutants) capable of altering it considerably from a physical, chemical, biological, and ecological point of view.

Pollution may derive from natural events (forest fires, volcanic eruptions, algal blooms and anoxic conditions in coastal systems, etc.), or from anthropic sources: war (e.g. use of depleted uranium weapons), transport (oil spills into the sea from tanker accidents, etc.), use of obsolete industrial technology (e.g. environmental problems caused by the use of natural resources beyond the regenerative capacity at source and/or when waste is overproduced or inadequately treated).

Fig. 2: Caulerpa taxifolia: in the 1980s this alien alga invaded the Northern Tyrrhenian Sea after an accidental release into the sea from the Montecarlo aquarium. It was recently observed along Croatian coasts, probably transported there by ships' ballast waters. (Credit: IZOR ( Institute of Oceanography and fisheries))

Fig. 3: Satellite picture of Etna's plume during the eruption of July 2002. Along its path, volcanic ash causes serious damage not only to people's health but also to road and air traffic. (Credit: Kathy Strabala and NASA)

The Webweavers: Last modified Tue, 20 Jul 2005 10:09:35 GMT