Every day we eat, drink and breathe minerals and trace elements, never giving a thought to what moves from the environment and into our bodies. For most of us this interaction with natural materials is harmless, perhaps even beneficial, supplying us with essential nutrients. However, for some, the interaction with minerals and trace elements can have devastating, even fatal effects. These interactions are the realm of medical geology, a fast-growing field that not only involves geoscientists but also medical, public health, veterinary, agricultural, environmental and biological scientists. Medical geology is the study of the effects of geologic materials and processes on human, animal and plant health, with both good and possibly hazardous results.
In its broadest sense, medical geology studies exposure to or deficiency of trace elements and minerals; inhalation of ambient and anthropogenic mineral dusts and volcanic emissions; transportation, modification and concentration of organic compounds; and exposure to radionuclides, microbes and pathogens.
The name of the discipline may be new, but the impacts of geologic materials on human health have been recognized for thousands of years. Mercury, cadmium and selenium levels were measured from preserved, 7,000-year-old human hair in the Karluk Archaeological Site in Kodiak, Alaska; although the health implications of these data are difficult to determine due to the possibility of addition or degradation over time. Inhaled soot particles were detected in preserved lung tissue from the Tyrolean Iceman, which is at least 5,000 years old. This person may have suffered from respiratory ailments after he inhaled tiny mineral crystals, including quartz grains.
Hippocrates and other Hellenic writers recognized that environmental factors affected geographical distributions of human diseases 2,400 years ago. And in 300 B.C., Aristotle noted lead poisoning in miners. Rocks and minerals have also been used for thousands of years to treat various maladies such as the plague, smallpox and fevers.
Scientists began investigating the links between geologic substances and processes and medical conditions 300 years ago. Several decades ago, however, medical geology fell out of favor to some extent in the United States because of perceptions by some influential people that geologists were overstepping their bounds and were dabbling dangerously with epidemiology. Now, through partnerships such as that between the U.S. Geological Survey and the National Institute of Environmental Health Sciences - and because funding agencies are beginning to demonstrate a recognition of the value in multidisciplinary research - the field is flourishing. Currently there are many collaborative investigations between geoscientists and biomedical and public health researchers worldwide, embracing a wide range of medical geology issues.
We present here three recent examples of medical geology studies that illustrate the impacts of minerals and trace elements on human health, and that also underscore the opportunities for geoscientists to make additional contributions to our society in this realm.
Dusts have long been linked to human health problems. One example is the link between disease and certain dusts containing asbestos. Another is the recognition that transport of dust, either within a small area or across an ocean, may contribute to ecological and human health problems. For example, Gene Shinn of the U.S. Geological Survey and others have found that dusts can transport pathogens, such as soil fungus spores, and transport human-produced or natural toxins such as pesticides or the heavy metals arsenic and mercury.
Advancements in analytical techniques, as well as interdisciplinary research linking earth sciences, ecological sciences and medical sciences, are providing new insights into the roles that dusts play in human disease on a local to global scale.
The story of asbestos illustrates not only the progress made, but also the unresolved questions remaining. Inhaling asbestos can cause asbestosis, a fibrosis of the lungs, as well as lung cancer and malignant mesothelioma. These and other potentially deleterious health effects of asbestos have been recognized and studied for decades. Much of the resulting regulatory and remediation focus in the 1970s and 1980s was on the morphology and size of asbestiform materials commonly found in industrial or commercial applications. These include the serpentine mineral chrysotile, the most commonly used, and the asbestiform varieties of several of the amphibole minerals, including grunerite, known commercially as amosite); reibeckite, known as crocidolite, or blue asbestos; anthophyllite, tremolite; and actinolite.
In the 1980s, earth scientists helped medical scientists to recognize that there was more than one type of material called asbestos, and that the different asbestos materials are not equally carcinogenic. Chrysotile asbestos, for example, is commonly regarded as being less carcinogenic than amphibole asbestos.
The last several years have seen renewed public attention on the potential health effects of asbestiform minerals that occur naturally as trace constituents in rocks or mineral deposits. For example, in 1999 the Seattle Post-Intelligencer brought nationwide media and scientific attention to asbestos-related health problems in residents of Libby, Mont. Many residents have diseases that have since been attributed to their exposure to amphibole asbestos minerals. The minerals were naturally intergrown with the vermiculite mined and processed at Libby. The mine has since closed (see also page 29, this issue).
Results of recent scientific studies at Libby underscore some of the uncertainties that remain about asbestos health effects. For example, the geologic occurrences of the different asbestos materials worldwide have not been catalogued, nor have they been compared to epidemiological data on disease occurrence. The potential for human exposures to dust released from these occurrences by road building, quarrying and natural weathering is thus poorly understood. The role that mineralogical characteristics - mineral composition, mineral solubilities in body fluids, surface charge, shape, and cleavage - play in the toxicity of individual asbestos minerals is also poorly understood.
Libby amphiboles show a wide range of mineral compositions - including richterite and winchite - and a wide range of textures - including true asbestos fibers and acicular crystals or cleavage fragments - that are not regulated as asbestos.
Another example of the links between dusts and human and ecological health
is the tie between soil fungus and various illnesses such as valley fever and
asthma. For example, landslides and their resulting dust clouds generated by
the 1994 Northridge earthquake triggered an outbreak of coccidioidomycosis,
or valley fever, among residents of the nearby Simi Valley in southern California.
The landslide released the soil fungus Coccidioides immitis (C. immitis),
which causes valley fever, into dust clouds, exposing people to the fungus.
Recently, C. immitis has been been found in populations of sea otters
off the southwestern coast of California, indicating that the soil fungus has
wide-ranging adverse ecological impacts. Many aspects of
C. immitis ecology are known, such as its occurrence in arid deserts of southwestern North America. Ongoing earth science research is helping us to understand even key geological uncertainties: the interplay among soil-forming processes, climate, and the geochemical characteristics and parental rock materials of soils in which C. immitis thrives.
You are what you eat
The human skeleton, a unique structure that defines our species, has two functions. It is the structural support for the muscles that permits our upright stance and mobility. Each of its more than 200 distinctly shaped bones is a storehouse that reflects our diets. The body processes the elements in our food and drink and a portion is deposited in our bone tissues. The skeleton records the environment in which it forms.
Hydroxylapatite, Ca)5(PO4)3(OH), is the mineral in bone. It is the main stiffening agent for these unique biological tissues and, because of its distinctive crystal structure, acts as a storehouse for a host of naturally occurring elements. Formation of the fine-grained, apatitic mineral requires an adequate supply of calcium and phosphorus, which are easily available from a normal diet and are metabolized by specialized cells that continually rejuvenate the bone tissues to keep our skeletons fully mineralized. Research has shown that both adequate diet and exercise are required for a viable skeleton.
The present focus on osteoporosis, the degeneration of bone usually associated with older and less active individuals, has led people to consume food and drink that has added calcium and vitamin D, the hormone the body needs for proper uptake of calcium. Actually, many other cations can substitute for calcium within the crystal structure of hydroxylapatite. This fact has interesting health implications.
Magnesium is one possible substitute for calcium. The percentage of magnesium in apatitic biominerals is usually less than one percent, an amount similar to that found in apatites in other rock reactions. But our nutritional requirements for magnesium are large because it is a key element and cofactor in several metabolic events besides the construction and maintenance of bone. The magnesium in bone mineral is small; but because it is continuously resorbed and redeposited in the bone tissues, magnesium can be made available for other metabolic events. That the bones can be a source of magnesium may become important when ingestion levels of the element are inadequate.
Strontium is another calcium substitute. Where strontium concentrations in nature are high, discrete apatite minerals form - such as strontium-apatite, (Sr, Ca)5(PO4)3(OH). Although not an essential element for human health, strontium is always present in miniscule amounts, a few parts per million, in bone mineral. The possibility that strontium might displace a portion of the calcium in bioapatite became a national controversey during nuclear bomb testing in New Mexico in the 1950s. Parents worried that strontium-90 from the fallout, following the same pathway as calcium, would be in the milk ingested by their children, deposit in their bones, and cause bone cancers. The amount of the radioactive element was determined at the time to be very small - less than 11 picocuries per gram of calcium in samples of milk taken across the country. Biological discrimination factors play a roll in the transfer of strontium. Calcium uptake is seven times greater than strontium uptake in the transfer from plants to cow, and another three times greater between fluids and precipitation of mineralized tissues. The small amounts of strontium-90 present in milk that could be deposited in children's bones would be swamped by the preferential partition of the calcium in bioapatite.
One of the most thoroughly researched ions that partition into apatite is fluorine. Biominerals formed in the presence of fluorine always contain a trace of the element. Some regions have concentrations as high as 14 parts per million of fluorine in their water, and these high water concentrations were shown in the early decades of the 20th century to be responsible for mottled teeth. However, in what is one of the most direct connections between geology and health, we have controlled studies showing that adding one part per million of fluorine to domestic water can actually improve dental health for children by reducing tooth decay.
The adage "you are what you eat" is most appropriate. Human nutritional requirements include calcium, phosphorous, magnesium and fluorine if we wish to maintain a healthy, mineralized skeletal and dental system. All nutrients come from Earth's rocks and minerals, the bailiwick of geologists. Generating interdisciplinary projects between geologists and medical researchers can benefit geologists personally and professionally, and can lead to improvements in global health.
From coal to chili peppers
Exposure to toxic levels of trace elements is one of the widespread forms of environmental health problems. Millions of people worldwide suffer health problems because they have been exposed to arsenic, lead, fluorine, mercury, uranium, etc. The devastation caused by excess arsenic in drinking water in Bangladesh, West Bengal India and elsewhere has been headline news. An estimated 25 to 75 million people are at risk of arsenosis in that region.
In Guizhou Province, China, the cool, damp autumn weather forces villagers to bring their harvests of chili peppers and corn indoors to dry. They hang the peppers over unvented stoves that, until the middle of the last century, had been fueled by wood. Due to the destruction of the forests, wood is now scarce so the villagers have turned to the plentiful outcrops of coal for heating, cooking and drying their harvests. But mineralizing solutions in this area have deposited enormous concentrations of arsenic - up to 35,000 parts per million - and other trace elements in these coals.
The chili peppers dried over these arsenic-rich coals are a key component of the villagers' diet and, unfortunately, their principal source of arsenic. Thousands of villagers are now suffering from arsenic poisoning and exhibit typical symptoms, including hyperpigmentation (flushed appearance, freckles), hyperkeratosis (scaly lesions on the skin, generally concentrated on the hands and feet), Bowen's disease (dark, horny, pre-cancerous lesions of the skin), and squamous cell carcinoma.
In the same region, health problems caused by the fluorine volatilized during domestic coal use are far more extensive than those caused by arsenic. More than 10 million people in Guizhou and surrounding areas suffer from various forms of fluorosis. Typical symptoms of fluorosis include dental fluorosis, or mottling of tooth enamel, and various forms of skeletal fluorosis including, osteosclerosis, limited movement of the joints, and outward manifestations such as knock-knees, bowlegs and spinal curvature. Fluorosis combined with nutritional deficiencies in children can result in severe bone deformation.
In Guizhou, fluorosis and arsenism are both contracted from eating foods dried over coal-burning stoves. In 1989, Baoshan Zheng and Ronggui Huang demonstrated that adsorption of fluorine by corn dried over unvented ovens burning coal high in fluorine (greater than 200 parts per million) is the probable cause of the extensive dental and skeletal fluorosis in southwest China. The problem is compounded by the use of clay as a binder for making briquettes. The clay used is a residue formed by the intense leaching of a limestone substrate that has created the area's karst topography. The residue contains a mean fluorine concentration of 903 parts per million.
American and Chinese geoscientists are helping minimize the villagers' exposure to arsenic and fluorine by mapping the vertical and lateral distribution of the elements in the mineralized coals; developing models for predicting the occurrence of mineralized coals; studying the combustion behavior of the different forms of arsenic and fluorine in the coal; and helping to develop simple, inexpensive field test kits for quickly determining concentrations in the coal. The situation in Guizhou illustrates the many opportunities for the geoscience community to team with medical scientists and help relieve human suffering.
Medical geology, a long-recognized but perhaps underutilized discipline, presents
the geoscience community with tremendous opportunities for collaborative work
with the biomedical and ecological research communities. Such collaborations
have great potential to help understand, mitigate and possibly eradicate environmental
health problems that have plagued humans for thousands of years.
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Shinn E.A., Smith G.W., Prospero J.M., Betzer P., Hayes M.L., Garrrison V., Barber R.T., 2000. "African dust and the demise of Carribean coral reefs." Geophysical Research Letters, 27: pp. 3029-32.
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