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Essential Definitions
Pollution can be defined as the contamination of air, land or water that occurs when artificial or toxic substances are released into the natural environment. It follows, then, that a pollutant is any substance that will lead to harmful contamination if it is improperly released into the environment. The word improperly is an important part of this definition because some substances are pollutants only under certain circumstances. For example, carbon released into the atmosphere is a problematic greenhouse gas, but this same carbon is relatively harmless if extracted, condensed into a solid and stored deep underground.
The Three Types of Pollution
The definition of pollution indicates three general categories of pollutants: air, land and water. Most air pollutants are gases or tiny particles that are small enough to remain suspended in the atmosphere. The dominant sources of air pollution are the combustion reactions that power everything from lawnmowers to cargo ships. Water pollutants include a wide variety of manufactured chemicals, fertilizer residues, biological wastes, eroded soil nutrients and pathogenic organisms. The primary land pollutant is trash that is disposed of improperly; other examples are toxic agricultural chemicals and industrial wastes that diminish the soil's ability to grow healthy vegetation.
Acidic Air Pollution
Sulfur dioxide is a well-known air pollutant that demonstrates both the subtle risks of industrial activities and the diverse collection of problems that can result from a single pollutant. The main human activity related to sulfur dioxide emissions is the combustion of fossil fuels, especially coal. Sulfur dioxide is directly harmful to respiratory organs, but it also leads to a more far-reaching environmental concern--acid rain. Atmospheric reactions involving sulfur dioxide, oxygen and water produce sulfuric acid, which is carried to the Earth's surface by rainfall. This acidic rainwater may seriously interfere with various ecosystems and can even cause construction materials to deteriorate more quickly.
A Nutrient and a Pollutant
Phosphorus is essential for optimal plant growth, but it becomes a water pollutant when excessive quantities are washed into lakes, streams and ponds. Phosphorus is a limiting nutrient in most aquatic ecosystems, which means that even small additions of phosphorus can lead to problematic ecological imbalances. The primary consequence of phosphorus contamination is imbalanced growth of algae, which reproduces rapidly at the expense of other aquatic organisms. In severe cases, excessive algal growth results in oxygen depletion, leading to the death of fish. A significant source of phosphorus pollution is runoff water contaminated with improperly applied agricultural and residential fertilizers.