Geomedicine

In earlier chapters (particularly chapters 16 and 17, which dealt with pollution), we considered some of
the adverse health effects of anthropogenic substances and the human activities that have caused a
redistribution of some naturally occurring substances. Geomedicine, however, encompasses not only
pollution, but also investigation of the broader relationships between the natural geologic environment
and the health of or occurrence of disease in the people, animals, and plants living in that environment.

Basic Principles of Geomedicine

Trace Elements

Just as most of the earth’s crust is composed of only a few major elements, so, too, are most organisms,
though the elements involved are, for the most part, different. More than 99 percent of the human body
is made of the six elements oxygen, carbon, hydrogen, nitrogen, calcium, and phosphorus (table 20.1). In
fact, we are about 60 percent water. Most other elements do occur in the body, as they do in rocks, but
only at very low concentrations, as trace elements. Trace elements may be defined loosely as those
found in concentrations of about 10 to 100 ppm or less. The major constituents of the human body are
obviously critical to life and health. However, to a great extent, geomedicine is the study of the effects of
the presence or absence of the trace elements, or compounds present in equally minute amounts.

Geomedicine has roots that go back centuries; papers appeared in the nineteenth century noting
geographical patterns in distribution of disease. However, many areas of the subject are relatively new—
for several related reasons. One is that it was necessary to develop instruments sufficiently sophisticated
and sensitive to measure extremely low concentrations of elements before their distribution could be
determined or studied. Another reason is that it has historically been easier to recognize the effects of the
few major constituents than of any one of the many minor ones in people and in other organisms. To
take an agricultural example, farmers realized that nitrogen, phosphorus, and potassium are key nutrients
required for plant growth long before it became apparent that the trace element boron is required for the
proper growth of beets. The importance of iron in the human diet has been realized since the seventeenth
century, but for many of the essential trace elements listed in table 20.2, recognition has come only
within the last few decades.

The number of trace elements and their varying effects from organism to organism can make the
isolation and identification of the function of any one a slow and complex task. A further complication is
that the same element can have quite different effects, depending on the concentration in which it occurs
or is consumed. This is a well-recognized phenomenon with many drugs. For instance, small doses of
the drug digitalin, found in foxglove plants, can be helpful in treating heart problems, but large doses can
be fatal, as any murder-mystery fan who has encountered a foxglove-leaves-in-the-salad plot is aware.

Dose-Response Curves

Such variability in effects of trace elements can be described by a dose-response curve, on which the
positive or negative effects of a trace element are plotted as a function of dosage. Several hypothetical
examples are shown in figure 20.1. In case A, the element in question would have no known beneficial
or harmful effects in low concentrations but would be toxic in high concentrations and fatal in very high
doses. Lead and mercury might be examples of this kind of trace element. In case B, the element is
needed to avoid deficiency, but benefits only increase up to a point, after which no particular benefit or
harm is derived from consuming higher levels. An element like calcium might be an example: It is
necessary to the proper development of bones and teeth, and it would be hard to “overdose” on this
major element in the body. Case C, perhaps the most common among the essential trace elements of
table 20.2, is one in which a small amount of the element is necessary for optimum health, but too much
can be toxic or even fatal. Copper and molybdenum are examples (figure 20.2). We will look at
additional examples in more detail shortly.

As a practical matter, there are limits on the extent to which detailed dose-response curves can be
constructed for humans. The limits are related to the difficulties in isolating the effects of individual
trace elements (see later in this chapter) and in having representative human populations exposed to the
full range of doses of interest. Controlled experiments on plant or animal populations are possible, but
there is then the difficulty of trying to extrapolate the results to humans with any confidence. It is also
true that different individuals, and persons of different ages, may respond somewhat differently to any
chemical, trace elements included, so any dose-response curve must either be specific to a particular
population, or it must be only an approximation.

Geology, Trace Elements, and Health

Controls on Elemental Intake

The ultimate source of the body’s trace elements is the earth—generally, rocks. The various pathways
through which the trace elements find their way into the body are summarized schematically in figure
20.3. The concentrations of trace elements in rocks vary by rock type and location and are a fundamental
control on the availability of trace elements to humans (table 20.3).

Trace-element concentrations are modified by a variety of natural processes and deliberate and
accidental human activities. Rocks weather into soil, gaining, or more often losing, some of their
chemical constituents. Soils may lose some elements by leaching; agricultural chemicals and pollutants
may be added. Crops selectively remove from the soil the elements they require for growth; animals
intensify this selectivity by consuming only certain parts of plants for food. Methods of processing and
storing food further change its composition, and we then exercise preferences in diet, consuming only
some of the available range of foods and nutrients. The water we drink contains trace elements leached
from rock and soil and may also have been polluted or chemically treated. The air we breathe also is a
source of a few trace elements, outgassed from the earth, released during volcanic eruptions, or added
through pollution.

Superimposed on geologic processes are what might be called “cultural factors,” such as dietary choices
and pollution, that affect our elemental intake. Examples exist not only in the present but in the past.
Ancient Roman aristocrats used cosmetics containing lead and piped their water through lead pipes. It
has been hypothesized that the fall of the Roman Empire can be attributed at least in part to widespread
lead poisoning and correspondingly decreased fertility among the upper classes.
Within the last few centuries, the idea that geology might be related to health was initially suggested by
geographic correlation. Both Kentucky and Ireland are famed for their horses, and both have abundant
limestone bedrock; high levels of bone-strengthening calcium derived from the limestone in the soils,
grasses, and water of the two regions might be a key factor in explaining the animals’ vigor. The
frequency of occurrence of certain diseases has seemed to vary by location, to be more common in some
places than others. In a few cases, from the coincidence of distinctive chemical characteristics of rock or
soil and the distribution of disease, inferences have been drawn that the two might be related. Further
controlled experiments could then be undertaken to test the hypotheses.

Iodine

Iodine is necessary to the proper functioning of the thyroid gland. A lack of iodine leads to thyroid
enlargement and the deficiency disease known as goiter. Until the last century, the occurrence of goiter
was particularly common in the northern half of the United States, an area known as the “goiter belt.”
Subsequent analysis revealed that the soils in that area were relatively low in iodine; consequently, the
crops grown and the animals grazing on those lands constituted an iodine-deficient diet for people living
there. When their diets were supplemented with iodine, the occurrence of goiter decreased.

Iodine is not difficult to incorporate into the diet, once the need is identified. Saltwater fish are naturally
high in iodine because they live in the salty oceans in which iodine salts are dissolved. Moreover, for
half a century, it has been common practice to “iodize” table salt, adding a little sodium iodide or
potassium iodide salt to the sodium chloride. Routine use of iodized salt has virtually eliminated goiter
without the need for other special dietary precautions.

There may be a geologic reason for the dearth of iodine in northern U.S. soils, aside from any
deficiencies in the bedrocks. The northern part of the country was the portion most affected by
continental glaciation during the last Ice Age. When the ice melted, huge volumes of water must have
drained through the soils on their way to the sea. Iodine salts are quite soluble; perhaps, then, these
northern soils had much of their iodine leached out by the glacial meltwaters. (In the midcontinent
region, the lack of iodine may be compounded by distance from the ocean; sea spray puts some iodine
into the air, from which it falls on nearby land.)

The regional link between soil composition and goiter was particularly clear because, at the time the
connection was observed, most people were eating locally grown food. Nowadays, in this and many
other developed countries, food production is concentrated in a few areas, and the bulk of most people’s
food is not locally grown but is brought in from many places. This will make it more difficult to see
links between soil chemistry and health in the future, except in countries or areas where most food is still
locally produced.

Because extremely high doses of iodine also cause abnormal thyroid function, iodine is an example of
the “type C” trace element in figure 20.1. (Excess iodine intake through normal dietary sources,
however, is extremely improbable.)

Fluorine

Water chemistry, like soil chemistry, can influence health. Perhaps the best-documented example of a
positive relationship between a trace element in water and improved health is the effect of fluorine in
reducing the incidence and severity of dental caries (tooth decay). Teeth are composed of a calcium
phosphate mineral—apatite. Incorporation of some fluoride into its crystal structure makes the apatite
harder and, therefore, more resistant to decay. This, too, was recognized initially on the basis of regional
differences in the occurrence of tooth decay.

Fluorine is present in average continental crust at a concentration of several hundred ppm; its
concentration in natural waters is much lower, typically only a few ppm. Even such seemingly low
concentrations make a demonstrable difference in tooth-decay statistics. Persons whose drinking water
contains fluoride concentrations even as low as 1 ppm show a reduction in tooth-decay incidence. This
corresponds to a fluoride intake of 1 milligram per liter of water consumed; a milligram is one
thousandth of a gram, and a gram is about the mass of one raisin. Fluoride may also be important in
reducing the incidence of osteoporosis, a degenerative loss of bone mass commonly associated with old
age. Normal total daily fluoride intake ranges from 0.3 to 5 milligrams. (Up to 3 milligrams may be
contributed by food, but this varies widely with diet and with the soil chemistry where crops are grown.)

Fluoride is not an unmixed blessing. Where natural water supplies are unusually high in fluoride, so that
two to eight times the normal dose of fluoride is consumed, teeth may become mottled with dark spots.
The teeth are still quite decay resistant, however; the spots are only a cosmetic problem. Of more
concern is the effect of very high fluoride consumption (twenty to forty times the normal dose), which
may trigger abnormal, excess bone development (bone sclerosis) and calcification of ligaments.
Fluoride, then, might be characterized by the dose-response curve shown in figure 20.4.

The prospect of reduced tooth decay is the principal motive behind fluoridation of public water supplies
where natural fluoride concentrations are low. Because extremely high doses of fluoride might be toxic
to humans (fluorine is an ingredient of some rat poisons), there has been some localized opposition to
fluoridation programs. However, the concern is a misplaced one, for the difference between a beneficial
and toxic dose is enormous. The usual concentration achieved through purposeful fluoridation is 1 ppm
(1 milligram per liter). A lethal dose of fluoride would be about 4 grams (4,000 milligrams). To take in
that much fluoride through fluoridated drinking water, one would not only have to consume 4,000 liters
(about 1,000 gallons) of water, but would have to do so within a day or so, because fluoride is not
accumulative in the body like some other elements, such as the heavy metals. Even to consume as much
as 20 milligrams per day of fluoride, which might cause bone sclerosis, would require the consumption
of 20 liters (5 gallons) of water, every day, for a period of time. That, too, is most improbable.

Zinc

Zinc is one of the heavy-metal elements which, nevertheless, appears to be a critical trace-element
nutrient, recognized as such since the 1930s. Zinc-deficiency symptoms can take many forms, including
dwarfism, dermatitis (skin disease), loss of taste sensitivity, and delay in the rate at which wounds heal.
Optimal zinc consumption should fall in the range of 5 to 40 milligrams per day. At doses above 150
milligrams per day, zinc excess may cause anemia; the very much higher dose of about 6,000 milligrams
per day is lethal in humans.

Zinc is present at concentrations of about 70 ppm in average crustal rock, but the concentration varies
widely with geology. So far, no regional correlation between geology (such as zinc content of water or
soil) and the occurrence of zinc deficiency has been found in the United States. However, there is some
concern that deficiencies may become more widespread in the future as certain incidental sources of
zinc, particularly galvanized items and water pipes, are phased out of our lives. In the absence of dietary
supplements, we are becoming increasingly dependent on plants as sources of necessary zinc, and these
do show clear regional variations in zinc content. In general, low-zinc sandy soils and soils rich in
calcium carbonate produce more zinc-deficient plants. A single plant species might show a range in zinc
content from only a few ppm on a zinc-poor soil to several thousand ppm elsewhere. While regional zinc
deficiencies closely correlated with local geology might be avoided by interregional food transport,
widespread zinc deficiencies could result if large proportions of some important food plants were grown
in low-zinc regions.

Selenium

Selenium is quite a rare metal, for which excess is more often a concern than is deficiency. Its
concentration in most rocks and soils is less than one-tenth part per million, and normal selenium intake
in humans is 0.006 to 0.2 milligrams per day. Lack of selenium has been shown to cause abnormalities
in many plants and animals; selenium deficiency symptoms in humans are not well documented except
in the Keshan region of southeastern China, where the most selenium-deficient soils known are found.
On the other hand, selenium toxicity (when more than about 5 milligrams per day is consumed) is well
recognized, reflected in development of cancers, malformation of nails and hair, depression,
nervousness, and other symptoms in humans.

While soil chemistry is a major control on selenium uptake in plants, different species have been shown
to vary widely in the degree to which they absorb selenium from the soil. The “locoweed” known in the
western United States, actually a group of plant species of the genus Astragalus, concentrates selenium
strongly. The informal name derives from the selenium poisoning that occurs in livestock that graze on
it: The animals develop a disease known as “blind staggers.” Other selenium-concentrating plants are
used in prospecting for ores, including uranium ores. Among human food crops, plants that concentrate
sulfur—which include cabbage, mustard, and onions—also concentrate the geochemically similar
selenium.

Where levels of soluble selenium in the soil are high, most varieties of vegetation grown on that soil will
also be high in selenium, whether those varieties particularly concentrate selenium or not. In the early
1930s, one farm in South Dakota, known locally as the Reed farm, had a history of frequent changes of
ownership. In time, each new owner usually filed a lawsuit charging misrepresentation against the
former owner. Chicken eggs did not hatch or the chicks were malformed; cattle and horses became sick.
Eventually, the trouble was traced to selenium-rich crops, especially wheat, grown on the farm. After the
problem was identified, the federal government bought up the most seriously affected farms in the
region.

Fortunately, the number of places in the world having soils that routinely produce deadly selenium-rich
crops is small. Selenium is somewhat concentrated in volcanic rocks and in the soils derived from them
(as in Hawaii, for instance) as well as in certain shales rich in sulfide. The desert plume plant of the
southwestern United States, in fact, requires selenium-rich soil, so its presence is a clue to the possible
hazards of the soil chemistry. Selenium is also more readily leached from the pedocal soils common to
the western United States. In early 1985, it was discovered that runoff drained from farmland in
California’s San Joaquin Valley and dumped in ponds in the Kesterson Wildlife Refuge (figure 20.5)
was causing deaths and deformities among the local duck and fish populations. The culprit appears to be
selenium in the water, leached from the farm soils.
Interestingly, the problems became acute only as a result of human activities. The area’s shale-derived
soils were naturally high in selenium (figure 20.6), but in this normally dry area, the selenium was
ordinarily relatively immobile (figure 20.7). Only with the extensive flooding of the land for the
cultivation of rice were high levels of selenium leached, washed into the ponds of Kesterson, and there
concentrated by evaporation, increasing the flux to those surface-water reservoirs, disrupting the
established geochemical cycle, and creating a hazard to wildlife. Selenium, like the heavy metals, can be
accumulative in the food chain, so selenium toxicity symptoms were noted in birds (at the top of an
algae → insects → fish → birds → chain) before toxic selenium concentrations were achieved
in the water of Kesterson.

Radon

The problem of radon as a pollutant of indoor air was discussed in chapter 17. A fundamental control on
the extent of the hazard is the underlying regional geology, and especially the concentration of uranium
in the rocks (figure 20.8). Rocks that may have relatively high uranium contents include granites, silicic
volcanic rocks, phosphate-rich sedimentary rocks, sulfide-rich shales, and metamorphic rocks formed
from any of these.

The quantity of a radioactive material may be expressed in units of curies, where one curie is the activity
(number of radioactive decay events per unit of time) of one gram of pure radium. (The unit is named for
Marie Curie, who discovered radium.) One curie = 37,000,000,000 disintegrations (decays) per second.
This would be an extremely intense and hazardous level of radiation; environmental radiation levels are
more often measured in picocuries (pCi), where a picocurie is one trillionth of a curie, or .037
disintegrations per second, or about 133 radioactive decay events per hour. Figure 20.9 illustrates the
range of radon activities in ground water, air in soil, and indoor and outdoor air.

The potentially carcinogenic effects of radon exposure were recognized earlier in populations exposed to
high radiation levels (e.g., uranium miners). As with any radiation exposure, it is difficult to assess
precisely the risk represented by low doses. Given the concentrating effects of well-sealed houses
relative to outdoor air, it is at least clear that the risk increases in such structures, and clearly the higher
the natural radiation levels resulting from the particular geologic setting, the higher the risk associated
with concentrating that radon.

Cases in Which Connections Are Less Clear

In the previous examples, a clear relationship could be established between geochemistry and health
because a specific chemical substance could be identified with particular deficiency or disease
symptoms. On the frontiers of medical geology, considerable efforts are being made to establish
the causative links more firmly where they are presently
unknown.

Radioactivity and Tobacco

Cigarette smoking has been associated with the increased occurrence of lung cancer. While certain
chemical components in tobacco have certainly been shown to be damaging, it has also been suggested
that agents of geologic origin may indirectly be partly responsible also. The phosphate fertilizers spread
on cropland tend to be somewhat enriched in uranium, as a consequence of the natural geochemistry of
uranium and phosphate. As noted in chapter 17, one product of uranium decay is radon gas. It is
hypothesized that this naturally occurring radon gas rises from the soil to be adsorbed onto tobacco
leaves, where it decays, in turn, to solid radioactive products, including isotopes of lead and bismuth
(figure 20.10). These radioactive particles would be inhaled with the smoke and could lodge in the lungs,
where the radiation from their decay could cause cancer. There is a good deal of speculation in this
scenario, but it is plausible and worth testing further.

Regional Variations in Heart Disease

In a number of instances, geographic variations in occurrences of particular symptoms are known, but
there is either no known parallel pattern in the geology or no clear connection recognized between the
known geologic variables and the particular health problem. For example, it has been known for two
decades that the hardness of public water supplies shows regional variations, rooted in geology, that are
broadly similar to the variations in death rate from certain types of heart disease (figure 20.11). In the
central United States, where limestone-rich aquifer systems contribute to relatively hard water, coronary
heart disease death rates are generally lower. Along the Atlantic coast, the water is much softer, and the
heart-disease death rates are relatively high. Why?

So far, no single answer has emerged. Is there something beneficial in hard water that protects against
heart disease in some way not yet recognized? Soft water is usually more acidic than hard water, and
acids more readily leach metals from pipes—does that result in consumption of one or more metals that
somehow cause heart disease? Many people living in hard-water areas use water softeners, which add
salt to the water, and sodium has been implicated in cardiovascular disease; but not everyone in hard-
water areas drinks softened water. Does the water chemistry, in fact, have any causative link with heart
disease at all? Not necessarily; the apparent connection might be pure geographic coincidence. Perhaps
the stress of living in East Coast cities rather than in more rural areas of the Midwest is a causative
factor. Perhaps no one has even proposed the right answer yet. Still, the similar geographic patterns in
heart disease and water hardness are there, and they are intriguing. Also, the same kind of geographic
correlation between these two particular parameters has been observed in twenty-seven studies in eight
countries, which strengthens the impression that some genuine cause-and-effect relationship between
water chemistry and the disease exists.

Cardiovascular Disease in Georgia

Another puzzling geographic pattern is the regional variation in cardiovascular-disease death rates in
Georgia, which corresponds to variation in the underlying geology (figure 20.12). The counties with the
lowest death rates are all in the Appalachian highlands in the northern part of the state. The counties with
the highest death rates lie wholly or partially within the Atlantic coastal plain to the south. This
observation inspired exhaustive geochemical investigations to try to find a geologic cause for the pattern.

Soils, plants, trees, and food crops were analyzed for thirty different elements and the analyses compared
between regions. In general, the soils of the coastal plain are more weathered and more extensively
leached. The highland soils are significantly higher in aluminum, barium, calcium, chromium, copper,
iron, potassium, magnesium, manganese, niobium, phosphorus, titanium, and vanadium, and somewhat
lower in zirconium. Several of these elements (chromium, copper, manganese, and vanadium) are
believed to have some beneficial effect on the cardiovascular system. However, no simple relationship
was found between soil chemistry and the chemistry of vegetables grown on that soil, and it is, after all,
the vegetables that are consumed, not the soil. The sources of drinking water in the region were so
numerous and varied, including both public and individual supplies, surface and subsurface sources, that
systematic sampling and analysis could not be undertaken. Yet the water might be a significant factor.
More sampling and further studies to confirm the influence of some of these trace elements on
cardiovascular function are needed before any straightforward explanation of the death-rate pattern in
terms of geology can be shown.

This study illustrates one of the special problems faced by medical geologists as opposed to laboratory
scientists. In the lab, it is possible to control experiments, to vary one parameter, such as the
concentration of one chemical element, at a time. In nature, dozens of chemical characteristics are likely
to be varying at once from place to place. It becomes correspondingly far more difficult to pick out the
influence of any one. The situation is further complicated by interactions between trace elements, as will
be shown shortly.

Other Intriguing Patterns

Other diseases show strong geographic variations that might be tied to geology. In Iran, the rate of
occurrence of cancer of the esophagus shows a regional pattern that strikingly mimics the distribution of
soil types (figure 20.13). Whether the soil chemistry has, in fact, any direct influence on the incidence of
the cancer is not known. Elsewhere, possible geochemical causes have been proposed to explain regional
variations in the frequency of occurrence of other forms of cancer and of gout. The incidence of stomach
cancer in Wales, for example, seems to be correlated with the amount of organic matter in the soils.
Whether this is a consequence of the tendency of organic matter to concentrate some potentially toxic
metals or of some other factor is not presently known.

There also are correlations with a less obvious geochemical basis. A recent study of death rates from one
type of heart disease in Europe suggested a correlation with the age of the rocks in the region (table
20.4). A given age group may comprise a wide variety of rock types, and the same rock type may be
found in regions of several ages. Thus, geology cannot be related in any simple way to the pattern of
death rates. Certainly, why the age of a rock, by itself, should make any particular difference to the
health of those living above it is not at all apparent. In this case, it may be easy, intuitively, to dismiss
geology—or at least rock age—as a probable causative factor. Where spatial correlations with chemical
factors are observed, however, it is more difficult to establish or refute a connection.

There are also situations in which expected regional correlations are absent. There are clear regional
variations in radon levels in rocks and soils, and consequently in homes. Yet so far, corresponding
consistent regional variations in occurrence of lung cancer have not been clearly documented, perhaps
because these variations are masked by other factors causing lung cancer, such as cigarette smoking.

Further Complicating Factors

Cause-and-Effect or Coincidence?

Medical geologists must distinguish those correlations that arise because of cause-and-effect from
apparent correlations that do not. A tobacco company pointed out several years ago during an advertising
campaign that more people die in bed than anywhere else, but that hardly means that it is dangerous to
go to bed. One might observe a much higher percentage of gray-haired listeners at a symphony concert
featuring classical music than at a rock concert, but it would surely be scientifically irresponsible to
conclude that classical music causes gray hair. Many obviously silly examples could be constructed to
prove the point that coincidence in time or space of two things does not at all indicate a cause-and-effect
relationship between them. Similarly, the fact that the geology of a particular area may seem to be
correlated with a higher or lower incidence of a particular disease does not by itself prove any causal
connection between the two.

Trace-Element Interactions

As noted earlier, genuine cause-and-effect relationships may be especially hard to recognize in natural
geologic systems, in which numerous mineralogic and geochemical parameters vary simultaneously and
in different ways. The picture is further clouded because the effect of a particular element often is
modified in the presence of other elements. An example is shown in figure 20.14. The incidence of
copper deficiency in cattle in central England appeared strongly correlated with underlying geology, with
most cases in or near areas underlain by shales. Yet the shales and the soils derived from them were not
markedly poor in copper. They were rich in molybdenum, as were many streams in that area, and this
was the source of the problem. Apparently, increased intake of molybdenum increased the need for
copper in the animals’ diets. Animals drinking from less-molybdenum-rich streams might show no signs
of copper deficiency even when the levels of copper in their blood were relatively low.

Other known and suspected interactions abound. Plants growing in high-phosphorus soils (including
those enriched by fertilizers) are frequently deficient in zinc. Uptake of high levels of zinc by plants may
limit the extent to which the plants can absorb cadmium; thus, application of zinc to cadmium-rich soils
may be helpful in controlling a potentially harmful accumulation of high cadmium concentrations in
food crops. Sulfur may “compete with” selenium during uptake in plants, but experimental results are
inconsistent: Applying sulfur-bearing fertilizers to low-selenium soils has depressed the levels of
selenium in plants even further, but on the other hand, efforts to prevent accumulation of high selenium
levels in plants growing on selenium-rich soils by applying sulfur have not worked well.

In animals, metabolic processes involving magnesium, potassium, and calcium have been shown to be
interrelated, and evidence indicates that the same is true in humans. Thus a deficiency in one element
may upset the balance of another. Copper is essential to the proper metabolism of iron; molybdenum
may also affect iron metabolism. Plant fiber may inhibit the absorption of zinc in the human digestive
tract. In plants, phosphorus may inhibit uptake of zinc, causing zinc deficiency in plants (and higher
organisms, including humans, consuming the plants); this can be a particular problem because
phosphorus is a major ingredient in fertilizer. Zinc and cadmium may compete in humans as well as in
plants; hence, increased zinc consumption may afford some protection against cadmium. Selenium may
protect against the harmful effects of some mercury compounds (but, of course, selenium in turn is
potentially toxic and thus should not be consumed indiscriminately).

The list of established and suspected interactions is extended year by year. Some of the interactions—
especially competition among elements—can be anticipated from the elements’ positions in the periodic
table (figure 20.15). Elements in the same column (sulfur (S) and selenium (Se), column VIa; zinc (Zn)
and cadmium (Cd), column IIIB; calcium (Ca) and magnesium (Mg), column IIa) have similar electronic
structures and may therefore show similar bonding characteristics. Other apparent interactions between
less similar elements (copper and molybdenum, for example) obviously involve more complex
chemistry and biochemistry and are not so readily identified or understood. The growing recognition of
trace-element interactions is one reason why doctors warn against consuming large doses of mineral
supplements containing a few specific elements. Aside from any potential toxic effects of the particular
elements consumed, there is concern that chemical imbalances of other elements could be created, with
serious health consequences.

Distinguishing Risk from Risk Perception

Costs of asbestos abatement over the last decade or so are estimated to have totaled tens of billions of
dollars for commercial buildings alone. Yet much of this expenditure may have addressed a nonexistent
or insignificant risk. It is quite true that studies have documented increased incidence of certain cancers
and other health problems among miners and other workers with high occupational exposure to some
kinds of asbestos. However, it does not follow that all asbestos everywhere is dangerous.

First, there are six different silicate minerals that are called “asbestos,” for the term is used to describe
various minerals with a fibrous habit. By far the most common—accounting for an estimated 95 percent
of asbestos mined and used in the United States—is chrysotile, a hydrous chain silicate (figure 20.16).
Yet there is little or no evidence that chrysotile poses a health risk to the general population; indeed, one
case study of a school in Ambler, Pennsylvania, where for over a century, schoolchildren have been
exposed to a 150,000-ton pile of chrysotile-containing material beside the school, showed not a single
documented case of asbestos-caused illness. Even where occupationally exposed workers had somewhat
elevated incidence of lung disease, the possible effect of asbestos could not be clearly separated from
effects of the workers’ smoking. Some of the rarer asbestos minerals show clearer risk; but asbestos-
abatement legislation does not make mineralogical distinctions among asbestos varieties.

Part of the push for asbestos abatement, too, comes from the unverified “one-fiber” theory—the
assumption that a single asbestos fiber may cause, for instance, lung cancer, or other asbestos-related
disease. But fibrous minerals, like dust, are everywhere: it is estimated that, breathing average outdoor
air, a normal human would inhale nearly four thousand asbestos fibers a day! This is not to say that
occupational exposures for those who work with asbestos materials should not be regulated in the
interest of workers’ health. The risks to the general public from asbestos, however, would seem to be far
lower than commonly perceived, and much of the investment in “abatement” of chrysotile-containing
material, in particular, may be far out of proportion to the risks that it actually presents.

Impacts of Human Activities

Human activities are modifying trace-element concentrations in many ways, some of which were already
considered in this and the previous two chapters. Soil pollution with metals (lead, zinc, copper,
cadmium, arsenic, uranium, and others) is common near smelters and other ore-processing facilities, and
the pollution may lead not only to water pollution but also to contamination of plants growing nearby.
Soils and plants growing near busy highways frequently showed elevated lead levels due to pollution
from car exhaust. Consideration of the possibility of substituting manganese compounds for now-banned
lead in gasoline has, in turn, raised concerns about possible manganese poisoning: Nerve and brain
damage from excess exposure are known to occur in some miners and other workers exposed to high
levels of manganese. Agricultural practices, including modification of the hydrologic cycle (as by
irrigation with attendant redistribution of leachable elements) and application of natural or synthetic
fertilizers that may add key trace elements, modify geochemical cycles by changing chemical fluxes and
residence times. These and other modifications of natural elemental cycles mask the basic geologic
factors influencing health and disease, making the medical geologist’s task more difficult.

And new environmental health problems continue to emerge. One of the newest is the discovery that
certain pesticides can mimic hormones in the bodies of humans and other animals, causing anomalous
development of various kinds. Some appear to suppress the immune system, lowering an organism’s
resistance to disease. Others are believed to cause reproductive problems, or anomalies in the
development of embryos. While over 60 percent of pesticide sales are currently in the U.S., Western
Europe, and Japan, pesticide use is increasing far more rapidly in developing nations, where mortality
rates are already high. Many agencies are now calling for more extensive and detailed studies of the
toxicity of these pesticides.

Summary
In a growing number of instances, the geology and geochemistry of the environment have been shown to
bear on the chemistry and health of plants, animals, and people. Many of these effects are independent of
human activities such as those causing pollution. Often, they are recognized initially through geographic
variations in occurrence of a particular disease. The identification of significant natural geochemical or
mineralogic factors is complicated because cultural factors are superimposed on natural geologic and
biologic systems, by complex interactions between trace elements in which the presence or abundance of
one modifies the effect of another, and by the difficulty of isolating the effects of a single chemical
substance from the dozens of geochemical parameters that vary simultaneously with local geology.
Better understanding of the medical importance of naturally occurring trace elements could lead to the
elimination of many instances of regionally chronic excess or deficiency diseases having a geologic
basis. The health hazards of synthetic chemicals in the environment are not always well determined.
There are currently numerous examples of geographic variations in disease occurrence that may arise
from geologic factors but for which the specific cause is speculative or wholly unknown. Conversely,
there are cases of overreaction to a perceived geologic health hazard, as with asbestos abatement.

Terms to Remember
dose-response curve
trace elements

Exercises
Questions for Review

2. What are trace elements? Are the trace elements in humans the same as those in rocks?
Explain.
2. Draw a hypothetical dose-response curve and explain it.
3. Outline the pathways through which trace elements enter the body, noting both natural
processes and human activities that alter the elements’ concentrations along the way.
4. Links between regional soil chemistry and health may nowadays be easier to recognize in
less-developed countries. Why?
 5. An apparent connection between water hardness and the incidence of heart disease is well
documented in many countries. Suggest several possible explanations.
6. Does a correlation in time or space between two factors or events indicate a cause-and-
effect relationship between them? Explain.
7. The concentration of one trace element may alter the effect of another. Describe an
example. How does this complicate geomedicine?

For Further Thought

1. Examine the label of a package of multivitamin-plus-mineral tablets. How many of the

essential trace elements of table 20.2 are included? Are any additional elements included
for which a need has not yet been clearly established?
2. Investigate the possibility of unusual trace-element concentrations in soils in your area.
(The county Extension Service or an agricultural college may have relevant data.) Are any
of the anomalies believed to be particularly beneficial or potentially harmful? If so, what
(if anything) is being done about them?

Suggested Readings/References

• Adriano, D.C. 1986. Trace elements in the terrestrial environment. New York: Springer-Verlag.
• Anderson, B.A. 1971. Calcium-carbonate hardness of public water supplies in the conterminous
United States. In Environmental geochemistry in health and disease, edited by H.L. Cannon and
H.C. Hopps, pp.151–53. Geological Society of America Memoir 123.
• Beeson, K.C., and Matrone, G. 1976. The soil factor in nutrition. New York: Marcel Dekker.
• Brookins, D.G. 1981. Earth resources, energy, and the environment. Columbus, Ohio: Charles E.
Merrill.
• Calabrese, E.J. 1981. Nutrition and environmental health. New York: Wiley-Interscience.
• Cannon, H.L., and Hopps, H.C., eds. 1970. Geochemical environment in relation to health and
disease. Geological Society of America Special Paper 140.
• ———. 1971. Environmental geochemistry in health and disease. Geological Society of America
Memoir 123.
• The citizen’s guide to geologic hazards. 1993. American Institute of Professional Geologists.
(See “Asbestos,” pp.23–28, and “Radon,” pp.29–37.)
• Draggan, S., Cohrssen, J.J., and Morrison, R.E., eds. 1987. Environmental impacts on human
health. New York: Praeger.
• ———. 1987. Geochemical and hydrologic processes and their protection. New York: Praeger.
• Feder, G.L. 1985. Environmental influence of selenium in waters of the western United States.
U.S. Geological Survey 1985 Annual Report, 5–8.
• Fergusson, J.E. 1982. Inorganic materials in the biosphere. Chapter 13 in Inorganic chemistry
and the earth, pp.334–59. New York: Pergamon Press.
• Gunter, M.E. 1994. Asbestos as a metaphor for teaching risk perception. Journal of Geological
Education 42: 17–24.
• Hurley, P.M. 1982. Living with nuclear radiation. Ann Arbor, Mich.: University of Michigan
Press.
 • Lippmann, M., and Schlesinger, R.B. 1979. Chemical contamination in the human environment.
New York: Oxford University Press.
• Masironi, R. 1977. World Health Organization studies in geochemistry and health. In
Geochemistry and the environment, vol. 2. Washington, D.C.: National Research Council.
• McLachlan, J.A., and Arnold, S.F. 1996. Environmental estrogens. American Scientist 84
(September-October): 452–61.
• National Research Council. 1977. The relation of other selected trace elements to health and
disease. In Geochemistry and the environment, vol. 2. Washington, D.C.: National Research
Council.
• Oehme, F.W., ed. 1979. Toxicity of heavy metals in the environment. New York: Marcel Dekker.
• Otton, J.K. 1992. The geology of radon. Washington, D.C.: U.S. Geological Survey.
• Parr, R. 1983. Trace elements in human milk. International Atomic Energy Agency Bulletin 25
(2): 7–15.
• Pier, S.M., and Bang, K.M. 1980. The role of heavy metals in human health. Chapter 11 in
Environment and health, edited by N.M. Trieff, pp.366–409. Ann Arbor, Mich.: Ann Arbor
Science Publishers.
• Raloff, J. 1994. The gender benders. Science News 145 (8 January): 24–28.
• Sauer, H.I., and Brand, F.R. 1971. Geographic patterns in the risk of dying. In Environmental
geochemistry in health and disease, edited by H.L. Cannon and H.C. Hopps, pp.131–50.
Geological Society of America Memoir 123.
• Shacklette, H.I., Sauer, H.I., and Miesch, A.T. 1970. Geochemical environments and
cardiovascular mortality rates in Georgia. U.S. Geological Survey Professional Paper 547C.
• Skinner, H.C.W., and Ross, R. 1994. Minerals and cancer. Geotimes (January): 13–15.
• Speidel, D.H., and Agnew, A.F. 1982. The natural geochemistry of our environment. Boulder,
Colo.: Westview Press.
• Villanueva, W. 1982. The cartography of death. Discover (September): 82–85.
• Webb, J.S. 1971. Regional geochemical reconnaissance in medical geography. In Environmental
geochemistry in health and disease, edited by H.L. Cannon and H.C. Hopps, pp.31–42.
Geological Society of America Memoir 123.

NetNotes
The U.S. Agency for Toxic Substances and Disease Registry is a good source of fact sheets that respond
to frequently asked questions (FAQs) about various health hazards, including geologic ones. Their home
page, through which one can also access a new newsletter, Hazardous Substances and Public Health, is
at http://atsdr1.atsdr.cdc.gov:8080/HEC/hsphhome.html

The FAQ fact sheets are accessible more directly via http://atsdr1.atsdr.cdc.gov:8080/toxfaq.html

The Citizen Information Page of the U.S. Environmental Protection Agency includes information sheets
on potential health hazards such as lead and radon, at http://www.epa.gov/epahome/Citizen.html

EPA’s Citizen Guide to Radon is found at http://www.epa.gov/docs/RadonPubs/citguide.txt.html

The U.S Geological Survey offers a variety of information on radon at
http://sedwww.cr.usgs.gov:8080/radon/radonhome.html

A variety of environmental-medicine case studies are accessible via

http://uhs.bsd.uchicago.edu/case.studies/index.html

Canada’s Environmental Health directorate, a division of Health Canada, can be reached at

http://www.hwc.ca/dataehd/English/opening.htm

Canada is a major supplier of chrysotile asbestos. The Canadian-based Asbestos Institute is at

http://www.asbestos-institute.ca/

Highlights of a World Resources Institute report, “Pesticides and the Immune System,” can be found
online at http://www.wri.org/wri/health/pis-high.html
Table 20.1
Principal Chemical Constituents of the Human Body*

Element Percentage of body weight

oxygen 61
carbon 23
hydrogen 10
nitrogen 2.6
calcium 1.4
phosphorus 1.1

Total 99.1

Source: R.M. Parr, “Trace Elements in Human Milk,” International Atomic Energy Agency Bulletin 25, 2 (1983): 8.
*All other elements—led by sulfur, potassium, sodium, and chlorine—make up the other 0.9 percent of the body.
Table 20.2
Essential Trace Elements in Humans

Average body Year recognized Some effects of

Element concentration (ppm) as necessary Function deficiency in humans

iron 60 17th century oxygen transport anemia

iodine 0.2 1850 component of thyroid goiter
hormones
copper 1 1928 interacts with iron; in anemia, bone changes;
some enzymes possible elevated cholesterol
manganese 0.2 1931 involved in metabolism not known
zinc 33 1934 involved in metabolism depressed growth, slow
healing
cobalt 0.02 1935 in vitamin B12 seen as B12 deficiency
molybdenum 0.1 1953 in some enzymes not known
selenium 1957 in enzymes; interacts selenium deficiency heart
with heavy metals disease
chromium 0.03 1959 involved with insulin insulin resistance; lowered
action glucose tolerance
tin* 0.2 1970 not known not known
vanadium* 1971 not known not known
fluorine 37 1971 important to proper tooth increased tooth decay;
and bone growth osteoporosis
silicon 260 1972 calcification; may not known
function in connective
tissue
nickel 0.1 1976 interacts with iron not known
absorption
arsenic* 18 1977 not known not known
cadmium* 0.7 1977 not known not known

Source: R.M. Parr, “Trace Elements in Human Milk,” International Atomic Energy Agency Bulletin 25, 2 (1983): 8.
*Need in humans not directly established; inferred from animal experiments
Table 20.3
Variations in Composition Among Common Rock Types*

Element Average concentration

GRANITE BASALT SANDSTONE SHALE CARBONATE

silicon (%) 32.3 24.0 36.5 27.1 2.4
aluminum (%) 7.7 8.8 3.0 9.7 0.5
iron (%) 2.7 8.6 1.0 4.7 0.4
magnesium (%) 0.6 4.5 0.7 1.5 4.8
calcium (%) 1.6 6.7 3.9 2.2 30.0
sodium (%) 2.8 1.9 0.3 1.0 0.04
potassium (%) 3.3 0.8 1.1 2.7 0.3
titanium (ppm) 2,300 9,000 4,600 1,500 400
phosphorus (ppm) 700 1,400 750 170 400
manganese (ppm) 600 2,000 850 10–100 1,100
chromium (ppm) 25 200 100 35 11
copper (ppm) 20 100 50 1–10 4
lead (ppm) 20 8 20 7 9
nickel (ppm) 8 160 80 2 20
cobalt (ppm) 5 45 20 0.3 0.1

Sources: A. Brownlow, Geochemistry, copyright © 1979 Prentice-Hall, Inc., Englewood Cliffs, NJ; K.B. Krauskopf,
Introduction to Geochemistry, copyright © 1979 McGraw-Hill, Inc., New York, NY; and B. Mason and C.B. Moore,
Principles of Geochemistry, copyright © 1982 John Wiley & Sons, New York, NY.

*Data are presented for major elements and selected minor elements.
Table 20.4
Apparent Relationship Between Age of Rocks and Heart Disease

Age of surface and underlying Mean ischemic heart disease rate,

rocks Countries 1967 (per 100,000 population)

Precambrian Sweden, Finland, Denmark, Scotland 314

(over 600 million years)
Early Paleozoic Norway, England, Northern Ireland 291
(600–300 million years)
Late Paleozoic Ireland, Austria, Hungary, France, 175
(300–180 million years) Germany, Netherlands, Belgium,
Czechoslovakia, Poland, Spain,
Portugal, Romania
Mesozoic or younger Italy, Yugoslavia, Bulgaria, Greece, 159
(under 180 million years) Switzerland

Source: R. Masironi, “World Health Organization Studies in Geochemistry and Health,” Geochemistry and the Environment,
vol. 2, 132–138, National Research Council, 1977.