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Medical Geology: An Opportunity for the Future

Our environment is the entire web of geological and biological interactions that characterize the relationship between life and the planet Earth. Essential and toxic elements in bedrock or soils may become a direct risk for human and animal health and may be the underlying cause of both deficiency and toxicity. Some naturally occurring elements are necessary for our wellbeing, while others are detrimental to our health. Naturally occurring elements can have detrimental effects on health when ingested in increasing quantities. Metals have always existed and will forever exist, but we cannot avoid the fact that the health of human beings and animals is influenced by metals and other elements in the environment. Geological processes along with human activities of all kinds have redistributed these from sites where they are fairly harmless to places where they adversely affect humans and animals. Geology may appear far removed from human health. However, rocks are the source of all the naturally occurring chemical elements found on the Earth. Many elements in the right quantities are essential for plant, animal, and human health. Most of these elements enter the human body via food and water in the diet and through the air that we breathe. Through the weathering processes, rocks break down to form soils on which crops and animals that constitute the food supply are raised. Drinking water moves through rocks and soils as part of the hydrological cycle. Much of the dust and some of the gases present in the atmosphere are the result of geological processes. Hence, a direct link exists between geochemistry and health due to ingestion and inhalation of chemical elements. Volcanism and related igneous activities are the principal processes that bring elements to the surface from deep inside the Earth. For example, the volcano Pinatubo ejected in just over 2 d in June 1991, about 10 billion tonnes of magma and 20 million tonnes of SO2 and the resulting aerosols influenced the global climate for 3 y. This event alone introduced 800 000 tonnes of zinc, 600 000 tonnes of copper, and 1000 tonnes of cadmium to the surface environment. In addition to this, 30 000 tonnes of nickel, 550 000 tonnes of chromium, and 800 tonnes of mercury were also added to the Earth's surface environment. Volcanic eruptions redistribute some of the harmful elements, such as arsenic, beryllium, cadmium, mercury, lead, radon, and uranium, plus most of the remaining elements, many of which may have still undetermined biological effects. It is also important to realize that there are on average 60 subaerial volcanoes erupting on the surface of the Earth at any given time, releasing various elements into the environment. Submarine volcanism is even more significant than that at continental margins, and it has been conservatively estimated that there are at least 3000 vent fields on the midocean ridges (1). One interesting fact is that about 50% of SO2 is of natural origin, mainly from volcanoes, and the other 50% is from human sources. The naturally occurring elements are not distributed evenly across the surface of the Earth, and problems can arise when element abundances are too low (deficiency) or too high (toxicity). The inability of the environment to provide the correct chemical balance can lead to serious health problems. The links between environment and health are particularly important for subsistence populations that are heavily dependent on the local environment for their food supply. Approximately 25 of the naturally occurring elements are known to be essential to plant and animal life in trace amounts, including Ca, Mg, Fe, Co, Cu, Zn, P, N, S, Se, I, and Mo. On the other hand, an excess of these elements can cause toxicity problems. Some elements such as As, Cd, Pb, Hg, and Al have no or limited biological function and are generally toxic to humans. Many of these elements are known as trace elements because they generally occur in minute (,g kg-l or ppm) concentrations in most soils. Trace element deficiencies in crops and animals are therefore commonplace over large areas of the world, and mineral supplementation programs are widely practiced in agriculture. Trace element deficiencies generally lead to poor crop and animal growth and to reproductive disorders in animals. These problems often have the greatest impact on poor populations who can least afford nutritional interventions for their animals.