Heavy Metals. Preindustrial Societies

Heavy metals are those elements that have metallic properties and a relative density greater than 4.5 grams per cubic centimeter. Copper, cadmium, lead, selenium, arsenic, zinc, nickel, mercury, and chromium are all heavy metals. They occur in nature in different forms, being present in rocks, soil, and living organisms; some (such as selenium, copper, and zinc) are micronutrients necessary for plant and animal survival.

The production and consumption of heavy metals has acquired a strategic importance in contemporary industrial societies, but their high toxicity makes them extremely dangerous for human health and the environment. Because they are not chemically or biologically degradable, heavy metals that are released into the environment can remain in the ecosystem for hundreds of years. They also enter the food chain with great ease and bioaccumlate, becoming increasingly concentrated (and hence dangerous) as one moves up the food chain.

Humans, located at the end of many food chains, are thus exposed to high concentrations of toxic metals. Environmental and work-related illnesses, diverse types of cancer, renal pathologies, anemia, difficulties and delays in growth, and cardiovascular and neurological problems are just some of the possible consequences of exposure to heavy metals.

The cycles and the sources of emission of heavy metals to the atmosphere can be both natural and human-generated processes. The most frequent natural processes are geologic and volcanic phenomena.

Those of human origin are almost always associated with mining activity, metallurgy, the chemical industry, combustion of fossil fuels, and disposal of urban and industrial waste. In general, in remote and inhabited regions, processes of natural character prevail, while in urban and industrial spaces, human processes are dominant. Once emitted, certain heavy metals can move easily through the air, water, and soil, affecting areas and ecosystems very distant from the point at which the heavy metals were initially generated.

Preindustrial Societies. Although serious environmental pollution caused by the emission of heavy metals is eminently a contemporary phenomenon, the problem has in fact existed as long as there has been mining and metallurgy. The seven basic metals of antiquity are gold, silver, iron, mercury, tin copper and lead; of those, three ( copper, lead, and mercury) are heavy metals. The primitive techniques employeed to obtain them caused important and unavoidable trace-metal emissions to the atmosphere.

Lead, which did not come into use until somewhat later (c. 3500 все), doesn't exist in a metallic state in nature, but mineral compounds containing lead as the main mineral (for example, galena, or lead sulphide) are abundant. Indeed, most silver mines are in fact galena mines, and the metallurgy of lead and silver have always been closely related.

The low melting point of lead (328° C), the ease with which can be worked, and its resistance to corrosion made it especially suitable for diverse domestic uses and for water pipes. Even in ancient days, some Greek and Roman authors associated the expansion of its use with possible harmful effects on health.

As for mercury, also known from antiquity (c. 1750 все), it too was identified early on as a poisonous substance. Generally extracted from cinnabar (monosulfide of mercury) by distillation, it was used in Greek and Roman times for gilding and was likely used by the Egyptians in religious ceremonies. (It has been found in Egyptian tombs dating from approximately 1500 все). From an industrial point of view, the importance of mercury lies in its abilty to form alloys (amalgamate) with other metals.

The last of the heavy metals known prior to the industrial revolution is arsenic. Widely distributed in Earth's crust, usually in association with other metals, is highly toxic and carcinogenic, although, administered in small doses, it has been used to combat certain illnesses (for example, syphilis).

It was identified as an element in the seventeenth century, but compunds of arsenic were known as early as the fourth century все and used in early copper smelting techniques. The Egyptians used arsenic-copper ores, natural alloys that improved the mechanical properties of copper artifacts.

Although it is difficult to track the evolution of heavy-metal pollution before the industrial revolution, it has been possible to obtain highly significant data by analyzing records coming from diverse types of natural deposits, especially bogs, aquatic sediments, and polar ice caps. In fact, this type of study has made it possible to reconstruct an accurate picture of paleopollution from ancient to contemporary times, with fluctuations also noted.

The available data show an increase in the emission of traces metals, especially copper and lead, in the Northern Hemisphere during the Roman empire (500 bce-300 ce). From the eleventh century, there is evidence of increased mining and metallurgic activities in Central Europe that intensified again in the sixteenth century. On the American continent, the introduction in the sixteenth century of the patio system (involving an amalgam of mercury) for the production of silver caused a spectacular increase in mercury contamination.