Metals have been used for centuries and are fundamental to major industries, yet some have the potential to damage human health and disturb the balance of environmental systems if they are allowed to reach excessive concentrations in air, water, soil, or food.

Only a few metals are important environmentally: those most likely to cause concern include copper, cadmium, mercury, tin, lead, vanadium, chromium, molybdenum, manganese, cobalt and nickel. In addition, the metalloids (including antimony, arsenic and selenium), which have some metallic properties, may cause environmental problems; uranium, plutonium and other actinides also have metallic properties, and are a cause for concern.

A metal is regarded as toxic if it injures the growth or metabolism of cells when it is present above a given concentration. Almost all metals are toxic at high concentrations, and some are severe poisons even at very low concentrations. Copper, for example, is a micronutrient, a necessary constituent of all organisms, but if the copper intake is increased above the proper level, it becomes highly toxic. Like copper, each metal has an optimum range of concentration, in excess of which the element is toxic. The toxicity of a metal depends on its route of administration and the chemical compound with which it is bound. The combining of a metal with an organic compound may either increase or decrease its toxic effects on cells. On the other hand, the combination of a metal with sulphur to form a sulphide results in a less toxic compound than the corresponding hydroxide or oxide, because the sulphide is less soluble in body fluids than the oxide. Toxicity generally results: when an excessive concentration is presented to an organism over a prolonged period of time; when the metal is presented in an unusual biochemical form; or when the metal is presented to an organism by way of an unusual route of intake. Less well understood, but perhaps of equal significance, are the carcinogenic and teratogenic properties of some metals.

An economic, social and political system theoretically based on common property and an equal distribution of income and wealth, communism may be seen as temporary or part of an evolutionary process, or as inherently repressive and a menace which can only be avoided if the system is prevented democratically or otherwise from obtaining power. Many opponents of the system see no alternative to the latter interpretation.

The people of small rural towns are unequipped to meet the needs of their town's future. The people of such communities may be willing to engage in the community's restoration but they are hampered by their narrow range of practical skills.

The variety of natural life forms, whether eco-regions, habitats, species or gene pools, is being endangered by human activity. This biodiversity ensures the regeneration of harvested resources and the maintenance of ecological processes, whether as a vital part of world heritage or for its own sake. It also provides resources for the development and improvement of domesticated crops and livestock, for recreation and tourism, and for research and education.

At the broadest level, biodiversity loss is driven by economic systems and policies that fail to value properly the environment and its resources, legal and institutional systems that promote unsustainable exploitation, and inequity in ownership and access to natural resources, including the benefits from their use. While some species are under direct threat, for example from hunting, poaching and illegal trade, the major threats come from changes in land use leading to the destruction, alteration or fragmentation of habitats.

A chronic illness usually develops slowly, lasts for an indefinite period of time, can deteriorate overall health and usually requires intervention to resolve.  Many chronic diseases are uncertain in prognosis. Cure is problematic or impossible, so treatment concentrates in making the patient comfortable or relieving pain. Sometimes patients suffer from multiple diseases; and the side effects of medical treatments can lead to additional chronicity. Long-time use of drugs, routine monitoring and crisis requiring hospitalization make chronic diseases expensive. The extent to which patients succumb to the various effects of chronic disease depends upon numerous factors, in particular their psycho-social support, financial status, childhood experiences, sense of humour and determination to push on.