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When we discuss, materials, substances, or compounds we are actually considering a group of individual elements linked together, for example, H2O, this is the chemical formula representing water. It consists of two atoms of the hydrogen element with one atom of the oxygen element. A larger example would be Benzene, C6H12, here we have six atoms of the carbon element and 12 atoms of the hydrogen element. Substances containing just these two elements are sometimes called hydrocarbons. The elements are arranged in a specific way within each compound depending on a variety of factors, Benzene is known as an "aromatic" hydrocarbon because of its odor, the elements are arranged in a hexagonal form and it is generally shown as: - Benzine Di-methyl mercury is an organometallic compound. Not surprisingly it has an organic part and a metallic part, its chemical formula is Hg(CH3)2. Hg is the chemical symbol for the element mercury, whilst the term "methyl" means one atom of carbon and three atoms of hydrogen, and there are two (di) methyl groups in this compound. Finally we have inorganic materials these do not contain carbon, e.g., NaCl this is the chemical formula for sodium chloride (table salt), where Na is the symbol for the element sodium and Cl is the symbol for the element chlorine. Sulphur dioxide is also an inorganic compound, SO2 where S is the symbol for sulphur and O2 indicates two atoms of the element oxygen. Bio-Degradation In nature there are many processes that help to degrade (breakdown) materials (Bio-degradation), if a substance reaches the elemental stage it can no longer be broken down into anything smaller, i.e. elements are the fundamental building blocks for life the universe and everything. Some pollutants are not broken down easily, or in some cases at all, under common environmental conditions and therefore become concentrated in particular areas, e.g., lakes, thus environmental problems are stored for the future. Sometimes the products of the degradation process are more damaging than the original compound. Whilst in other cases the pollutant only becomes polluting through its interaction with the environment, e.g. metallic mercury is relatively harmless, however bacteria and molds found in many river sediments are capable of converting this into methyl mercury which has serious toxic effects and is far more soluble and liable for uptake by wildlife and humans alike. Conclusion When considering pollutants we must consider not only the original substance but also its possible degradation products.