Some perspectives regarding environmental levels of thallium and
platinum group metals

EPA Region/ORD Emerging Pollutants Workshop

F. R. Anscombe, Industrial Ecologist, USEPA-Chicago;
<anscombe.frank@epa.gov>

> General discussion about intrinsic difficulties in assessing the risks
> of trace exposures to chemicals. Then, two specific cases in point:
> thallium, a neuro-toxic metal akin to lead and mercury, its neighbors
> on the periodic table; and platinum group metals, of increasing
> economic value to society. It has been reported that high consumption
> fish eaters may ingest thallium at four times an EPA reference dose.
> Mercury is routinely monitored, thallium seldom studied. There are
> reports that levels of platinum family metals are rising in the
> environment. This is to be expected, given their new use in catalytic
> converters.

# The first half of this talk is going to offer some comments about the difficulties in evaluating risks from low exposures to any substance. Why venture into this difficult thicket? I am going to discuss several metals that generally do not get talked about. If I am going to point at them, it seems responsible to try to place them in a risk context, acknowledging uncertainties. I will not claim that their existing levels present serious risks, though exposures to thallium bode no known good for living things within our Biosphere. 

# We each hold personal perspectives regarding the risks presented by exposures to chemicals, whether natural ones that comprise the foods on which we subsist, our dominant exposure, or pesticides or pharmaceuticals. Because none of us can know everything, we defer to the judgment of experts and take some things on faith. I do not have a goal of shaking faiths, which would be an unkind intent. 

# Rather evaluation of chemical risk is a public policy matter. Risk is the coin of the realm in environmental policymaking. Consideration of risk, including what we do not know or do not consider, is an inescapable topic. 

# I offer my thoughts. I do not ask you to adopt them. Nor do I have answers to impart, rather questions. I share my quandaries. What follows are my personal views and in at least some are not those of the U.S. Environmental Protection Agency. I have not had chance to seek comments from others; such are always helpful, so this talk is correspondingly the poorer. 

###### Assessing risks from trace chemical exposures

Consider the case of oxygen. Without oxygen, we perish. Carbon monoxide
is dangerous if inhaled, because it displaces oxygen in our blood and
denies us the breath of life. The atmospheric level of oxygen is 21
percent; if we breathe air in which oxygen falls beneath 15 percent, we
suffocate. If oxygen exceeds 25 percent, the air is readily combustible.
An ozone layer high above shields against damaging rays. In various
respects, oxygen is essential and has greatly influenced the development
of the diverse life forms within our Biosphere.

Yet here is a paradox: long-term respiration of oxygen generates free
radical oxidants thought the most important cause of age-related
diseases. So even at normal levels, oxygen is a chronic, long-term
toxicant. We crave it minute-to-minute, in the long run it kills us.
Oxygen represents the circle of life. It seems our alpha and omega.

A high dose of oxygen is also lethal. Oxygen is quite toxic and
reactive. Oxidation can crumble steel to rust. It is a powerful natural
process.

Water is not so toxic. However, if an adult drinks enough of it, say 11
quarts quickly, this will dilute your electrolytes and you die. Water
too is a toxicant.

For those who purchase products labeled non-toxic, caveat emptor.
Pharmacology does not have such a category. The gentlest substances,
like water, are "practically non-toxic." All substances can induce a
toxic effect, at sufficient dose.

If citizens intuitively fear a nearby chemical factory, they would be
given high marks by toxicologists. Factories contain large quantities of
chemicals. If large quantities were to be released, then nearby high
exposures could be deadly. Bhopal is illustrative.

Drinking too much water illustrates that it is not just the toxicity of
a chemical that matters, it is the toxicity of an exposure. A large dose
of a practically safe substance can be just as deadly as a tiny dose of
a virulent poison. We rely on substances like oxygen and water that,
under certain circumstances, are toxic. Toxicants are thus intrinsic to
life itself.

To an analytic chemist, we are made up of chemical substances. Consider
a molecule that contains 2,954 carbon atoms, 4,516 hydrogens, 780
Nitrogens, 806 oxygens, 12 sulfurs, and 4 irons. Such atomic description
can sound off-putting. This chemical compound sounds friendlier by its
name, hemoglobin, which courses through our circulatory system, bearing
oxygen to our tissues, removing carbon-dioxide for exhalation.

We are the physical manifestations of chemicals. All chemicals can be at
least somewhat toxic. By the transitive property: toxicants R US. Salt
is essential to the human diet, too much may harden arteries. Aspirin
has saved countless lives and alleviated suffering, yet an excess dose
is a common method of suicide.

As humans, we take comfort in clarity. We like movies in which bad guys
wear black hats, whereas the Lone Ranger drove a palomino (named
Trigger). It is comforting to think of certain chemicals as dangerous.
This is not wrong: some are indeed virulent poisons, devoid of value to
living things. It is comforting to think other chemicals are safe. The
truth on this point seems murkier.

A lot of us rely on coffee as a life-sustaining substance. Coffee is a
complex suite of natural chemical components. Nearly 1,000 have been
analytically distinguished. A handful of these have been tested on
rodents and found to be carcinogens. Should there be warning labels on
cups of coffee? If so, labels would be warranted on many other foods,
for the same reason: the abundance of natural chemicals within foods
that could be classified as carcinogens. A National Research Council
study affirmed it plausible that our exposure to carcinogens naturally
contained within foods dwarfs cancer risk presented by residues of
pesticides on those foods. While there may be programs to prevent
synthetic carcinogens from entering the food supply, there is no
equivalent program to guard us against carcinogenic chemicals naturally
constituent in our foods.

How to distinguish human carcinogens, without mega-dosing humans? Twenty
years ago, a statistician with Pfizer tested 17 known human carcinogens
and found that less than one half induced cancer in rodents. He
concluded: "The lifetime feeding study of mice and rats appears to have
less than a 50 percent probability of finding known human carcinogens.
On the basis of probability theory, we would have been better off to
toss a coin....The definitive bioassay for carcinogenesis ... has never
been subjected to proper validation."[^1] Some question the premise of
chemical dosing of rats and mice on ethical grounds that this injures
animals without yielding persuasive informational benefits for judgments
about risks to humans.

In testing the toxic properties of chemicals in many ways, it might
(possibly) be helpful if there were clear decisionary criterion. Perhaps
there are, since this is a topic area that I know little about. My guess
is that there are not, because these would be hard to set, since we do
not test chemicals on people, and because toxicity tests are not
equivalent to a risk assessment. On the other hand, perhaps it would not
be helpful to have chemical decisionary criteria, because these could be
made too narrow and inflexible and not envision all of society's
economic needs.

For illustration, lead is an intrinsically harmful substance, but much
valued by society in batteries; within this narrow context of use,
humans are not exposed to lead. Would it make sense to ban use of lead
simply on grounds of its toxicity, despite absence of exposure?
Avoidance of lead would entail less release during its refinement and
disposal, but it would also require less effective alternatives in
batteries. The world in which we live is one that currently relies on
lead in certain types of batteries. This may change in future, owing to
technological innovations and market forces. Yet, for now, society makes
use of lead and that is current reality. Similarly, many dentists use
mercury in an alloy for fillings. It is their medical judgment that this
confers health value to the patients who they serve and there are
reasons for thinking this: though it may be counter-intuitive, elemental
mercury is not absorbed by an intact human digestive tract; most dental
mercury is bound up in the alloy; and alternative fillings are costly
and present different chemical exposures.

Decades ago, there was a public policy assumption that synthetic
substances, when indirectly inferred to be human carcinogens, were a
large cause of cancer in humans. In the years since, there is much
greater medical insight regarding cancers, such that this premise now
seems unlikely. Yes, synthetic chemicals can have carcinogenic
properties, but so do natural chemicals in our foods and these provide
the dominating dose. In addition, the total exposure from both foods and
from trace contaminants does not seem a strong factor in explaining the
real-world incidence of cancer.

In recent years, we have popularly heard that chemicals present other
threats as well, not simply cancer. For instance, they may impact our
hormonal system. This seems entirely plausible. Yet, it also seems
likely that the risk would not be large, because the dominant dose
should again be from the chemicals that comprise foods. Theo Colborn's
book, *Our Stolen Future*, alludes to an incident of impaired
reproduction among flocks of sheep in western Australia. The flocks
subsisted on clover that contained a natural compound, formononetin,
with estrogenic properties.[^2] It makes sense that a chemical naturally
intrinsic to a food source could provide a harmful dose if a species is
a heavy consumer. Generally, food is beneficial, but an overdose or
over-reliance, such as sheep subsisting on clover containing an
estrogenic natural ingredient, may yield harm. Humanity's overall
success in reproducing and in nourishment suggests that consuming a
diversified diet minimizes this risk.

We often learn of chemical risks to humans owing to high dose tragedies.
Yet it is still hard to assess what risks may be presented by low doses
of these same chemicals, given a myriad of possible biological outcomes
without acute symptoms. Consider methylmercury. There have been two
major epi studies during the past decade evaluating fish eating
populations for modest neurological impairments attributed to
methylmercury. One found no impairments, the other found some. Yet fish
contain trace levels of thousands of substances. EPI studies are
wonderfully valuable, but they study populations subject to a great
diversity of chemicals. The world outside is not a controlled
experiment. Even with careful consideration of confounding factors, can
an effect be attributed with certitude to methyl-mercury alone?
Assumptions have to be made. They may be profoundly reasonable, yet some
uncertainty persists.

With sufficiently sensitive and selective analytic technique, with
available reference standards, and if there were money from a generous
sponsor, then an analytic chemist could distinguish thousands of
synthetic chemicals at trace levels, in the waters of Lake Michigan or
in us.

What risks do these trace exposures present, individually or
cumulatively? How can we even begin to think about this complex reality
and set logical priorities? How can we evaluate this thoughtfully and
prudently, ranking one risk against another? Low analytical detection
capabilities present a truly frustrating problem. Chemists can detect a
myriad of chemicals at levels lower than our capacity to estimate
precisely what risks might be entailed. We can find a part per billion,
but does it threaten us? We can make estimates, but they will
necessarily be predicated on assumptions.

Should we fully inform and disclose to the public that they are exposed
to a myriad of trace chemicals? Or should we narrowly inform the public
of just one low chemical exposure at a time?

Generally, the latter choice seems to be made. This is a simpler message
for us to understand and to explain. Simplicity is a mighty virtue in
messages to the public. Yet is it to some degree misleading not to
mention the full range of chemical exposures? Is this ethical?

If the honest situation is that we genuinely do not know how to sum or
compare all the risks presented by a host of trace chemical exposures,
both natural and synthetic, would acknowledgement of this be worthwhile?
Is honesty a good thing? Or would too much detail spark public panic and
thus be irresponsible? Has anyone tested a focus group of citizens to
find out their reactions regarding full disclosure?

How does society advance? Are we best served to wear blinders and avert
our eyes from complexity, from our many trace exposures? Or should we
acknowledge reality, however inconveniently perplexing and troubling?
Should environmental agencies dwell on a handful of "bad" chemicals now
largely gone? This is bureaucratically safe. Or should it be EPA's
mission to be watchful for undetected risks presented by current use
substances? My own answer would be contingent on the degree of wisdom
with which EPA evaluates chemical exposures.

An intrinsic mission of EPA is to be skeptical about chemical exposures,
especially ones involuntarily presented via food, water, or breathing.
It is to safeguard innocent people against unsafe chemical exposures. It
seems a responsible idea to have this workshop, regardless of profound
uncertainties.

Perhaps the worst chemical poisoning in history has been heart-rendingly
unfolding in Bangladesh and India, owing to long term exposure to
arsenic naturally present in underground drinking water. Tens of
thousands have died of cancer. Long-term, high dose exposure to the
inorganic arsenic trioxide, the chemical species found in Bangladesh
groundwater, is a killer. Skepticism of chemical exposures is a deeply
legitimate public health mission. A million people may be paying with
their lives to illustrate this.

At the same time, arsenic compounds in their totality are naturally one
of the most prevalent substances in the oceans and in us, in around
14^th^ place in abundance. Organic arsenic compounds tend to be of low
toxicity, in stark contrast to some organic mercury compounds. Much of
the toxicology data regarding metals has been developed through the
years via analytic methods that aggregate all compounds of a metal.
However, in fact, individual metallic compounds vary greatly in
toxicity. Analytic advances that discriminate different compounds of
metals is ushering in an emerging era of more enlightened understanding
of the risks that metals present.

Chemical risk judgments are difficult. They intrinsically require the
exercise of judgment, since there is not an easy formula for determining
risks presented by low exposures. Risk assessment is an art that
interprets usually incomplete scientific evidence amidst uncertainties.
The poet William Butler Yeats wrote of strife in his native Ireland:
"the worst are full of passionate intensity, the best lack all
conviction." As human beings, we yearn for conviction and clear answers.
Who can provide these in relation to low-level chemical exposures?

Years ago, some esteemed folks within EPA developed an opinion that
exposure to dioxin like substances was approaching an effect threshold.
This sounded ominous. Yet, the experts were unaware that levels of
dioxins and PCBs had fallen 95 percent since the 1970s and were
continuing to ebb. Thus the only way a threshold was being approached
was downwards. This seems unsurprising and quite human. The more time an
expert invests in learning about one thing, the less time available to
learn other, more general things. This underscores the need for
environmental policymaking to take place in transparent ways. No one has
all the answers. We each of us peer through a glass, darkly.

There are educated, conscientious people at EPA. At various times, I
have heard some advocate fish consumption advisories owing to various
trace contaminants. Others might like to issue warnings about barbequing
meat owing to poly-aromatic hydrocarbon exposures. Some other people may
think that EPA should warn against consumption of dairy products owing
to dioxin. Others may worry about fluorides or chlorinated byproducts in
drinking water. If all such warnings were issued, would this imply that
jello and marshmallow fluff are the perfect diet for humans?

In fact, I do not make fun of what I regard as actually legitimate and
serious issues. Barbequing may present some additive carcinogenic risk.
Floridation may confer health benefits, but too high a dose would not.
Chlorine disinfects against the enormously lethal risk of water-born
disease, like cholera, but few would like to drink water with a pungent
chlorine odor. An optimal safety balance is sought between the various
factors of disease, odor, and chemically safe water supply ingredients.

A perplexing aspect of some advisories is that they some may not be
based on holistic medical advice regarding diet. Holistic advice would
also factor the health benefits of the chemicals that comprise food. As
best I understand, EPA seems to focus on whether there is sufficient
safety margin in relation to one chemical contaminant. This
simplification assumes away factoring the reality of the chemicals that
naturally comprise food and all other trace contaminants. This is
powerfully practical, but is also not holistic. If this assumption is
so, should it be expressly acknowledged? (Maybe it is, within the fine
print of fish consumption advisories. I do not know.)

There are thoughtful people who doubt that environmental policies should
seek a balance between benefits and costs. They may have doubts that are
predicated on perceptions that there are practical asymmetries in
available information between costs of regulations as readily estimated
by industry versus environmental benefits that are defy monetization.
This seems a genuine concern that fairly deserves acknowledgement. Cost
benefit analysis can be informative, but should not be expected to yield
precise mathematical right answers, given many factors that defy
monetization, including justice issues associated with externalities.
Many Americans do not think there should be a right to pollute.

Yet what of advisories that do not consider the health benefits of the
predominant dose of chemicals that comprise food, factoring instead only
calculated risks associated with a trace dose of one chemical? Does such
narrow scientific framing beget advice that is defensible on medically
holistic grounds? Do such advisories depend on a premise that
alternative healthy foods are readily available for all? Is this true
for fish-reliant individuals or groups? If this premise does exist, does
it deserve to be acknowledged?

### Thallium

On the periodic table, thallium lies nestled between mercury and lead.
Owing to observed symptoms, thallium was the first suspect for the
neurologic poisoning at Minamata during the 1950s; later methyl-mercury
was revealed to be the cause. Before alkyl lead was selected as an
additive to gasoline, thallium was considered. Neither is kind to living
things if spewed into the Biosphere via combustion.

Within our bodies, thallium mimics an essential metal, potassium. It
affects potassium-activated enzymes in the brain, muscles, and skin.
Symptoms of thallium poisoning include slurred speech, numbness, hair
loss. A fatal dose for an adult is about one gram. This qualifies as
extremely toxic. During the first half of the 20^th^ century, doses of
one-half a gram (50 percent of a fatal dose) were proscribed for
ringworm. Thallium made hair fall out, so that the skin condition was
more accessible for treatment.

Thallium's abundance in the earth's crust is 59^th^ among elements, ten
times more abundant than silver, for contrast.

Use of thallium sulfate as a poison was popularized in a mystery story
by Agatha Christie, then used by agents of Saddam Hussein to murder
adversaries. (Apparently they found Agatha Christie inspiring.)

30 tons of thallium is produced for commerce each year. This comes from
zinc and lead smelting, and sulfuric acid production from pyrites.
Thallium is used in specialty glass, crystals for infrared detection,
diagnoses of heart disease.

Thallium sulfate is still used in developing countries as a poison
against rats, cockroaches, and ants. EPA cancelled all pesticide uses of
thallium in 1972, partly owing to a survey that found that 24% of sick
or dead bald eagles were poisoned by thallium.[^3] Thallium explains the
decline in bald eagle populations more persuasively than DDT.

Global air emissions of thallium from coal combustion have been
estimated at 650 to 1,600 tons per year. Cement production may yield
much more: 2,700 to 5,300 tons per year. Thallium pervades the Biosphere
at low levels. Plants absorb thallium by their roots. Cabbages and
grapes near a cement works in Germany had up to 45 ppm, 500 times normal
values.

Why is awareness of mercury and lead so much greater than awareness of
thallium? Lead was once added to gasoline and paints. Mercury has been
used in medicine, gold mining, hat making, chemical manufacturing.
Thallium has been economically minor in comparison.

There is also the Marvin Gaye contribution:

> "Oil wasted on the ocean and upon our seas,
>
> fish full of mercury
>
> Ah oh mercy, mercy me"

What if Mr. Gaye had sung of thallium? Surely we would be more aware.

Thallium has long been undetected by conventional analytic means. This
too seems an important factor in explaining our ignorance. Happily, a
study team from Canada's Burlington research institute developed a
sensitive analytic method. In a 1995 paper, they reported the first
thallium water concentrations in the Great Lakes, ranging from 1.2 ng/L
in Lake Superior to 9.4 in Lake Erie, and 25.7 in Hamilton Harbor, a
steel-making center.[^4]

From the University of Michigan, Lin and Nriagu found that the average
thallium value in 37 Lake Michigan trout collected in 1994 was 140 ng/g.
The bioaccumulation factor accounting for these fish levels was 10,000.
In 1992, EPA suggested a reference dose for thallium (5 ug/70 kg/day) to
support a drinking water criterion. Lin and Nriagu report that thallium
intake by high fish consumers is around 4 times EPA's suggested
reference dose.[^5]

Another study team from Ryerson Polytech in Toronto recent analyzed
forms of thallium within Great Lakes water.[^6] It was discovered that
thallium(III) comprised 74 percent of total dissolved thallium. A test
of thallium(III) on a plankton species found it 43,000 times more toxic
than cadmium, 50,000 times more toxic that thallium 1, which is the
thermodynamically stable form that had been expected to predominate.
Apparently thallium(I) is converted by bacteria to thallium(III), which
is enormously more toxic to the phytoplankton communities that underpin
the Great Lakes food web. This brings to mind the biotic genesis of
methyl-mercury.

Thallium is as old as our planet. What is "emerging" is new knowledge of
long-standing exposures. Inspired scientists are discovering ancient
facts. Since thallium III and methyl-mercury are both extremely toxic
and can induce similar symptoms, and since thallium is seldom analyzed,
it may contribute to effects presently attributed to methyl-mercury
alone, in epi studies.

In any event, the margin of safety for thallium in fish is modest, as
for methyl-mercury. There seems as much reason to consider thallium in
the environment as methyl-mercury. The two together may pose additive
risk greater than either alone. Recent advances in analytic methods can
empower more understanding of thallium exposures.

#### Platinum Group Metals

Platinum, Rhodium, and Palladium have been used in catalytic converters
serving U.S. automobiles since 1975. These convert NOx, hydrocarbons,
and CO to CO2, nitrogen, and water, improving air quality. These metals
are in 75^th^, 76^th^ and 79^th^ place in abundance among the earth's
elements. Each is rare.

This rareness brings with it two implications. Use of these metals in a
hot gas stream produces new vapor exposures. Accordingly, ambient levels
near roads have risen, viewed from the perspective of ultra-low natural
background. The second implication of rareness is that there is not a
lot of knowledge about inhaling platinum group metals.

The good news is that the three platinum group metals are generally of
low toxicity. Yet, a study of vegetation and soils beside roads in South
Bend, Indiana reports abundance is comparable to many European studies
and approaching levels that would be economically viable to recover.[^7]
(I live near S. Bend. If you drive along Interstate-80 and someday see
someone who resembles me harvesting highway vegetation, you are
forewarned.)

A study from Rome showed a six fold increase in platinum levels in
topsoils between 1992 and 2001.[^8] A sediment core from Boston Bay and
ice cores in the Arctic show increases. This does not sound great.

However, platinum air concentrations are still three orders of magnitude
below levels for which occupational health limits have been set. The
known health risk from platinum exposure is sensitization of the airways
by soluble platinum compounds, known as platinum salt hypersensitivity.
This has been suffered by platinum workers, who develop runny noses and
asthma. Given that present ambient exposures offer a 1000-fold safety
margin in relation to this condition, inhalation of traffic related
platinum group metals does not seem to pose health risk to the general
population, based on existing understanding. This is important to bear
in mind.

Yet neither should we assume that we understand all human or ecosystem
outcomes for these metals. Our experience with them is not extensive.
And I have tried to make the general case to you that all substances, in
sufficient dose, can create harm. We should always be wary of chemical
exposures.

##### Closing

I have discussed one extremely toxic metal, thallium. As an
anthropologist of environmental bureaucracy, I find thallium oddly
overlooked. Recent work has recognized that the dissolved form in the
Great Lakes water column is very toxic and that high fish consumers
exceed an EPA reference dose for thallium. I have no idea if thallium
exposures impact phytoplankton health or pose unacceptable risk to human
consumers of fish. Such questions deserve consideration. (To spur
interest, perhaps I can enlist a musician to compose a snappy couplet
that rhymes thallium, like mercury and mercy me.)

I have tried to suggest that we human beings are made up of many
chemicals and we are exposed to many more, both natural and man-made.
When you next hear of a synthetic chemical being banned, you may wonder:
what is the public health gain, in a holistic sense? Does anyone know of
the totality of the risks that will be presented by its replacement? How
does the risk of this one chemical exposure compare to the many others?

In writing these remarks, I chanced on a paper regarding caffeine in
some lakes in Switzerland.[^9] Caffeine is contained in coffee, tea,
soft drinks, chocolate, and pharmaceuticals, from cough syrups to No
Doz. Caffeine is likely the most consumed drug in the world. It is found
in municipal wastewater effluents, and thereafter in surface and
groundwater worldwide.

I looked at the National Toxic Pollutant repository of the National
Institutes of Health. Under caffeine, I read "at higher doses, symptoms
include convulsions, Cheyne-Stokes respiration, apnea of preterm
infants, arrhythmias....Overdoses may cause death,
convulsions...Continued excessive use may lead to depressed mental
states. Symptoms may include pulmonary edema, myocardial
infarction."[^10]

A report from the Massachusetts Poison Center by a doctor at the Boston
Children's Hospital: "The issue of fetal toxicity of caffeine remains
controversial. The drug has been demonstrated to induce chromosomal
mutations in vitro, but not in vivo. Clinical trials in humans have
shown heavy coffee drinkers to have an increased incidence of
miscarriages and premature birth, however these differences lose
significance when adjusted for other factors such as smoking. Further
study is needed."[^11]

The paper from Switzerland offers mass balance flows for caffeine
pollution in lakes. It talks about biodegradation, sedimentation,
volatilization. When there is rain, caffeine levels in a lake rise,
perhaps owing to bypassing of wastewater treatment.

I wonder what would happen if it were reported that one chemical, yet
only one, deemed toxic by the respected National Institutes of Health
were to be reported to be present in our drinking water supplies?
Caffeine, many people have to have. Yet what if there were another
chemical and it had an unappealing name? What if someone invokes the
precautionary principle? I started by saying that I offer questions, not
answers.

There is some evidence to suggest that salt levels in the Great Lakes
may be slowly rising. This seems a serious chemical question. It would
seem a pity if the world's largest surface fresh waters became another
inlet of the oceans. This is by no means imminent, but it deserves to be
watched. Perhaps humble salt is another chemical of which to be wary for
fresh waters. Perhaps this is a higher priority than thallium.

In summation:

-   Thallium is intrinsically unhealthful. The same seems true for lead,
    mercury, cadmium, beryllium. When I say this, I offer an
    evolutionary perspective that living things have not incorporated
    and made use of these metals, to the best of my knowledge. This
    natural avoidance seems a useful insight about them. Many other
    metals are used gainfully by the human body.

-   I do not claim that present thallium exposures constitute a problem
    worthy of action. Yet there are indications that this could be so.
    More studies seem warranted.

-   As regards all chemicals, dose (including pathway, quantity,
    chemical forms, timing, and the vulnerability of an exposed person)
    determines harm.

-   I am skeptical of all chemical exposures, from a holistic
    perspective and bearing in mind uncertainties.

-   The world abounds in risks from chemical exposures. It seems
    reasonable to focus on ones that seem most important for a given
    context. Chlorides may be a long-term threat to the Great Lakes, for
    instance.

-   I am made up of chemicals. All chemicals can be toxic. It would be
    sensible for me to be wary of myself. We and our planet contain
    ingredients that can be regarded as good and ill, depending on dose.
    Health consists of striking the best balance.

\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\--

**Acknowledgments**: Elizabeth Walsh of EPA conscientiously shared a
great deal of literature regarding platinum group metals. I am indebted
to Professor Jerome O. Nriagu for studies of thallium in the Great Lakes
and for an entire book on thallium, which he inspired and edited.
Unintended mistakes contained in this presentation redound only upon me.

**Bibliographic Note:**

## [Chemical Risks]{.underline}

-   Nick Lane. 2003. *Oxygen*. Oxford University Press, Oxford

-   John Ensley. 2001. *Nature's Building Blocks*. Oxford Univ. Press,
    Oxford.

-   Joseph Rodricks. 1992. *Calculated Risks: the toxicity and human
    health risk of chemicals in our environment.* Cambridge Univ. Press,
    Cambridge.

-   Aaron Wildalsky. *But is it True? A citizens guide to environmental
    health and safety issues.* Harvard University Press, Cambridge, MA.

-   William H. Baarshers. *EcoFacts and EcoFiction: understanding the
    environmental debate.* Routledge, London.

-   John F. Ross. 1999. *Living Dangerously: Navigating the Risks of
    Everyday Life.* Perseus, Cambridge, MA

-   Roald Hoffman. 1995. *The Same and Not the Same*. Columbia Univ.
    Press, New York.

## [Thallium]{.underline}

*Thallium in the Environment*. J. Nriagu, editor. 1998. John Wiley and
Sons, New York.

[^1]: David Salsburg. 1983. *Fundamental and Applied Toxicology*, 3,
    63-67

[^2]: Theo Colborn et al. 1996. *Our Stolen Future*. Dutton, New York.
    Page 75.

[^3]: Venghout Cheam et al. 1995. "Dissolved and total thallium in Great
    Lakes Waters." *J. Great Lakes Res*. , 21, 3, 384-394

[^4]: Cheam et al 1995

[^5]: T. S. Lin et al. 2001. "Thallium Concentration in Lake Trout from
    Lake Michigan." *Bull. Environ. Contam. Toxicology*, 67, 921-925

[^6]: Benjamin S. Twining et al. 2003. "Oxidation of Thallium by
    Freshwater Plankton Communities." *Environ. Sci. Technol*., 37,
    2720-2726

[^7]: James C. Ely et al. 2001. "Implications of Platinum-Group Element
    Accumulation along U.S. Roads from Catalytic-Converter Attrition."
    *Environ. Sci. Technol*., 35, 3816-3822

[^8]: D. Cinti et al. 2002. "Platinum levels in natural and urban soils
    from Rome and Latium (italy): significance for pollution by
    automobile catalytic converter." *Sci. of the Total Env.,* 293,
    47-57

[^9]: Ignza J. Buerge et al. 2003. "Caffeine, an anthropogenic marker of
    wastewater contamination of surface waters." *Environ. Sci.
    Technol*., 37, 691-700

[^10]: Available at:
    <http://ntp-server.niehs.nig.gov/htdocs/Chem_H&S/NTP_Chem5/Radian58-08-2.html.>

[^11]: Available at:
    <http://www.maripoinsoncenter.com/ctr/9411caffeine.html>.