CE Update
Submitted 12.21.10 | Revision Received 3.16.11 | Accepted 3.17.11
Testing for Toxic Elements: A Focus on Arsenic,
Cadmium, Lead, and Mercury
Deborah E. Keil, PhD, DABT,1 Jennifer Berger-Ritchie,2 Gwendolyn A. McMillin, PhD, DABCC(CC, TC)3
(1University of Utah, Department of Pathology, Salt Lake City, 2University of Nevada, Las Vegas, NV, 3University of Utah,
Department of Pathology, ARUP Institute of Clinical and Experimental Pathology, Salt Lake City, UT)
DOI: 10.1309/LMYKGU05BEPE7IAW
Abstract
Toxic elements (“heavy metals”) are common
to the environment and are responsible for
both intentional poisonings and unintentional
exposures that can lead to adverse health
effects and potentially death. Dangerous
exposures can be prevented by recognizing
and minimizing common sources of toxic
elements in our diet, water, workplace, and
homes. Laboratory testing is an important
tool for detecting and managing toxic element
exposure; several analytical methods are
available. However, the clinical value of
elemental testing is dependent upon collecting
an appropriate specimen at an appropriate
time, with consideration of many pre-analytical
After reading this article, readers should be able to describe the indications for toxic element testing, list pre-analytical and analytical variables
associated with elemental testing, and discuss the best specimens to
collect for detecting and monitoring exposures to arsenic, cadmium, lead,
and mercury.
Exposure to toxic elements (“heavy metals”) poses unique
issues for human health. Metals differ from other pollutants in that they are neither created nor destroyed and occur
naturally in the environment. Anthropogenic activity largely
contributes to human exposure because metals are bioconcentrated from the environment through mining activities or
through processes occurring at various industrial settings.
Corresponding Author
Gwendolyn A. McMillin, PhD, DABCC(CC, TC)
[email protected]
Abbreviations
BEI, biological exposure index; ACGIH, American Conference of
Government Industrial Hygienists; ASV, anodic stripping voltammetry; AAS, atomic absorption spectroscopy; ICP-OES, inductively
coupled plasma-optical emission spectroscopy; ICP-MS, inductively
coupled plasma-mass spectrometry; CCA, chromated copper arsenic; EPA, Environmental Protection Agency; MMA, monomethylarsonic acid; DMA, dimethyl arsenic acid; MT, metallothionein; BLL,
blood lead levels; ZPP, zinc protoporphyrin; CNS, central nervous
system; EPO, erythropoietin; NHANES, National Health and Nutrition
Examination Survey
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variables that can compromise testing. In
this review, toxicokinetics and pre-analytical
variables associated with toxic element testing
are discussed, with emphasis on arsenic,
cadmium, lead, and mercury.
Keywords: clinical chemistry, heavy metals,
arsenic, cadmium, lead, mercury
Chemistry exam 21103 questions and corresponding answer form are
located after this CE Update on page 743.
By definition, metals have high reflectivity, high electrical
conductivity, high thermal conductivity, mechanical ductility,
and strength.1 Metals may vary in oxidation state by losing 1
or more electrons to form cations. Metals may also form organometallic compounds. The species and forms of metals can
define the toxicity profile and target organ(s). For instance,
inorganic forms of mercury target the kidney, while methylated (organic) mercury is a potent neurotoxin. Also, hexavalent chromium causes adverse health effects, while trivalent
chromium is an essential trace metal.
Although each metal exhibits unique toxicology, there
are common mechanisms of toxicity to include mimicry,
oxidative damage, and adduct formation with DNA or protein. Non-essential metals may mimic the essential metals
causing a disruption in cellular and enzymatic mechanisms.
Examples include the replacement of essential zinc by cadmium, replacement of potassium by thallium, replacement of
phosphates by arsenate, and mimicry of manganese in place
of iron. Organometallic compounds such as methylmercury
may also mimic biologicals and be transported by amino acid
or organic anion transporters. Further, the generation of reactive oxidative species is often induced by metals in their ionic
form, resulting in oxidative modification of DNA or proteins,
including aberrant gene expression and carcinogenesis.2-4
Health and analytical issues relevant to detection and
management of toxic element exposures are discussed below.
Pre-analytical considerations, methods, and limitations of
result interpretation applicable to elemental testing in general
are presented first, followed by element-specific descriptions
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of common sources and toxicokinetics for arsenic, cadmium,
lead, and mercury.
Pre-analytical Considerations
As with any type of laboratory testing, the value of toxic
element testing depends on alignment of the test and specimen with the purpose of testing. Toxic element testing may
include both routine screening and targeted testing, such as
when environmental risk of exposure is known, to support
investigation of a known exposure (eg, spill, suicide attempt,
fire) to comply with occupational regulations or guidelines
(eg, OSHA) and to confirm clinical suspicions of poisoning.
Exposures to toxic elements can be acute (one time, shortterm) or chronic (many times, long-term). Clinical signs and
symptoms of toxicity are often different for acute vs chronic
exposures but may be non-specific. Due to non-specific signs
and symptoms of toxicity, as well as the fact that the duration
and extent of exposure is often not known, diagnosis of most
toxic element exposures depends on laboratory testing.
In selecting the best specimen for a given application,
one must recognize that toxic elements are similar to drugs
and other exogenous toxicants, wherein each element exhibits
unique absorption, distribution, metabolism, and elimination
kinetics. As such, the predicted toxicokinetics of the individual
element(s) involved must be coordinated with the selection of
specimen, and timing of specimen collection, relative to the time
of exposure. An exposure could be missed entirely if testing is
performed with an inappropriate specimen. For example, an
exposure to methylmercury could be missed if testing is
performed with urine. An exposure could also be missed when
an appropriate specimen is collected at an inappropriate time.
Thus, an exposure to arsenic could be missed if testing is
performed with blood collected a few days after the exposure.
Toxicokinetic highlights for arsenic, cadmium, lead, and
mercury are summarized in Table 1.
In general, toxic element testing is performed with urine
or blood. For arsenic, most forms are detectable in blood for
only a few hours. However, greater than 90% of an arsenic
exposure is recovered in the urine within 6 days, making
urine the specimen of choice for an exposure that occurred
within the previous week.3 Whole blood is the best specimen
for detecting exposure to toxic elements such as lead (>95%
bound to erythrocytes) that are tightly bound to intracellular
proteins. Blood is also the preferred specimen for detecting a
mercury exposure because the most toxic forms of mercury
(ie, organic, such as dimethylmercury) are not usually eliminated in the urine. Urine and blood are useful for detecting
cadmium exposures. While many toxic elements accumulate
in tissue (eg, bone, kidney), testing most tissue types for heavy
metals is reserved primarily for post-mortem investigations.
Hair and/or fingernails, potential routes of elimination
for toxic elements, can be useful specimens for detecting an
exposure that occurred in the month or more prior to specimen collection. Detection of elements in hair and nails is
somewhat correlated to half-life of the elemental form. As
such, the inorganic forms of arsenic and organic forms of
mercury are preferentially incorporated over those forms with
shorter half-lives. The primary mechanism of incorporation of
inorganic arsenic into hair is thought to be through binding
sulfhydryl groups in keratin, which are concentrated in hair
and nails. Toxic element deposition requires approximately
2 weeks after an exposure to appear in hair or nails. In some
poisoning cases, distinct white lines of arsenic can be observed
in the fingernails, known as Mees’ lines.5 Hair testing is particularly useful when hair is long, because segmental analysis provides
information about the time and duration of an exposure. Hair
Table 1_Overview of Laboratory and Toxicokinetic Information for Common Toxic Elements3,4,61
Toxic Element
Plasma t 1/2
Primary Route of
Deposition/Elimination
Reference
Intervals
Biological
Exposure Index
Common
Companion Testing
Confounding Factors
Arsenic (inorganic)
~10 h, Urine Urine <5 µg/L
Urine† 35 µg/L
Fractionation or
Drinking water,
30 h, speciation industrial setting
200 h
Blood <15 µg/L
(tri-phasic)
Arsenic (organic)
~1 h
ND
ND
Diet, primarily seafood
Cadmium
1 month, Kidney
Urine <2 µg/g Urine 5 µg/g
Urine ß2-microglobulin
Smoking, industrial
10-20 yrs creatinine creatinine setting, age, renal
(bi-phasic)
Blood <5 µg/L
Blood 5 µg/L function
Lead
30 days
Bone
Urine <50 µg/g Blood 30 µg/dL
Blood zinc protoporphyrin
Environment, cosmetics,
creatinine (ZPP), CBC, ∂-amino industrial setting
Blood <5 µg/dL levulinic acid
Mercury (inorganic) 3 days, Urine
Urine <5 µg/g Urine* 35 µg/g
ND
Dental amalgam, diet,
20 days creatinine creatinine environment,
(bi-phasic)
Blood <10 µg/L
Blood† 15 µg/L industrial setting
Methylmercury
52 days
Feces
ND
ND
Vaccines, diet, cosmetics
†End of shift at end of work week.
*Prior to shift.
ND, not defined.
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grows at a rate of approximately 1 cm per month and is used
to define the history associated with an exposure, possibly
demonstrating a pre-exposure, exposure, and post-exposure
pattern of elemental concentrations, indicative of an acute
poisoning. Hair testing has also been used for post-mortem
evaluations. Of note, hair testing results have been used to implicate lead poisoning in the death of Ludwig van Beethoven,
and arsenic poisoning in the death of Napoleon.6
Once a specimen preference is established, pre-analytical
variables, such as elimination patterns, analyte stability, and
specimen collection procedures, must be considered.7-9 For
urine testing, 24-hour collections are common, to compensate
for elimination patterns varying throughout a day. Results for
24-hour collections may also be used to estimate body burden
(eg, provocation, mobilization procedures) or quantify decontamination efforts for a patient who has received therapeutic
chelation therapy.10 Many laboratories report concentration
of toxic elements as a function of urine volume collected
during a 24-hour period. Normalizing results per gram of
creatinine is also common (eg, µg/mg creatinine), particularly if comparing results to the biological exposure index
(BEI) published by the American Conference of Government
Industrial Hygienists (ACGIH), for relevant elements (Table
1). Random urine collections may not be as comprehensive
as a 24-hour collection, but they are useful for screening and
qualitative detection of exposure to several potentially toxic
elements. Random urine collections are also advantageous
because they minimize the risk of sample contamination
during collection.
The risk of elemental contamination during specimen
collection is a significant pre-analytical concern because of
the inherent risk of a falsely elevated result that may lead to
inaccurate, indefensible results.8,9 For this reason, any elevated
result inconsistent with clinical expectations should be confirmed by testing a second specimen or a second specimen
type. In addition, procedures to minimize the likelihood of
contamination should be strictly enforced. Common sources
of external contamination include patient clothing, skin,
hair, the collection environment (eg, dust, aerosols, antiseptic
wipes), and specimen handling variables (eg, container, lid,
preservatives).6,11
Several aspects of specimen handling and storage can
introduce external contamination as well. For example, urine
samples were historically preserved for elemental testing with
concentrated acid. Either the acid itself or the process used
to add the acid to the urine (eg, pipette tips) could introduce elemental contamination. Most laboratories now advise
against adding preservatives to urine samples.12 Laboratories
may also recommend specific containers (and lids) because
some plastics and plastic dyes have been shown to contain
lead, cadmium, and other elements.13 For blood testing, certified “trace element free” collection tubes are available (royal
blue top, for most elements; tan, for lead). Failure to use
such tubes has been shown to produce falsely elevated results,
which, depending on the specific circumstances of testing,
may be clinically significant.14-16
Concerns surrounding the possibility of external contamination do not end with specimen collection. As a result, elemental testing is frequently performed under environmentally
controlled conditions that may include a certified clean room,
laminar flow hoods or dust covers, Teflon-coated equipment
and components, silicon tubing, and trace element grade
reagents and supplies. Acid washing of pipette tips, containers,
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and other supplies with an acid known to be free from the
element(s) of interest, can further minimize environmental
contamination of specimens.9,15 Laboratories may also need
to exercise precautions to prevent loss of toxic elements due
to in vitro volatilization and metabolism. Mercury and arsenic
are particularly vulnerable to loss or metabolism, respectively,
during sample processing and storage.9,17
Methods
Many analytical methods have been used to detect and
quantify toxic elements. Commonly used commercial methods include the LeadCare (Magellan Biosciences, Chelmsford,
MA) device, available as a CLIA-waived point of care test,
and based on anodic stripping voltammetry (ASV); and
direct mercury analyzers using atomic absorption spectroscopy
(AAS) technology. These methods are convenient but are
considered screening devices due to associated limitations.18-20
High-complexity laboratory developed tests are also available,
primarily through reference laboratories. These methods are
designed to detect and quantify single or multiple elements,
and they employ primarily AAS, inductively coupled plasmaoptical emission spectroscopy (ICP-OES), or inductively
coupled plasma-mass spectrometry (ICP-MS).17,21 Promising
new technologies include sector-field, accurate mass, and time
of flight ICP-MS. Preferences for a specific technology may
be based on ease of use, sample preparation requirements,
detection limits, dynamic range, multi-analyte capability,
speed of analysis, risk of analytical interferences, and capital
expense (Table 2). Not surprisingly, the most complex and
expensive technologies offer the greatest sensitivity and multianalyte capabilities.
As indicated previously, most elements exist in several
forms and valence states, also known as “species.” Discerning
the species of an element involved in an exposure can be
clinically relevant because different elemental species exhibit
unique toxicokinetics and toxic potential. However, most
analytical methods available for toxic element testing methods
do not differentiate between elemental forms. Instead, most
methods generate a “total” element concentration. If results
are “normal” or otherwise consistent with expectations, this
limitation is inconsequential. If, however, results are elevated,
additional testing may be required to evaluate the clinical significance of the elevated results. Arsenic provides a good example
of this concept because more than 30 forms of arsenic are
known, with differential toxic potentials. To distinguish between species responsible for an elevated total arsenic result,
the analytical method must incorporate a separation step, either during sample preparation (eg, extraction) or analysis (eg,
chromatography). Several such methods are described but are
not widely available clinically.22,23
Interpretation
Appropriate interpretation of any toxic metal testing
result requires consideration of patient-specific factors, the
scenario surrounding the request for testing, and comparison
of results to reference intervals or other interpretive guidelines. Reference intervals may be somewhat inconsistent,
based on the specific population from which the data was
generated. There may be substantial differences between the
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Table 2_Overview of Common Laboratory Methodologies Used for Toxic Element Testing
Anodic Stripping Voltametry (Lead Only) Atomic Absorption Spectroscopy
Inductively Coupled
Mass Spectrometry
Principle of detection
Current required to strip plated electrode
Absorption of element-specific wavelength
CLIA-waivedYes
No
Approximate capital expense <1x
3-6x
(x ~ $10,000)
Multi-analyte capability
No
Possible, but not common
Detection limit
3.3 µg/dL
Dynamic range
65 µg/dL
Sample preparation
None
YES—dilution and chemical modifiers common
Mass to charge ratio (m/z)
No
15-30x
concentrations considered “normal,” the concentrations
that are “abnormal,” and the concentrations associated with
toxicities. The reference intervals for both lead and mercury
have been significantly lowered in recent years based on reduced
environmental pollution and risk of exposures, published
clinical data correlating clinical effects with blood concentrations, as well as improved analytical techniques providing
more accurate testing results. Example reference intervals are
included in Table 1. It must be emphasized that the actual
toxic threshold for an individual is not equivalent to exceeding the reference interval. Development of heavy metal toxicity
depends on many factors, such as genetic vulnerabilities, as
well as whether the exposure is acute or chronic, age, and
any co-morbidities.
Arsenic
Background and Potential Sources of Exposure
Arsenic is widespread in the environment. Inorganic
arsenic is found in highest concentration in natural groundwater near geothermal activity, primarily in the Southwest,
Northwest, Northeast, and Alaska.24 Drinking water obtained
from wells will typically contain arsenite (arsenic III), or arsenate (arsenic V).25 The EPA has set 10 ppb as the allowable
level for arsenic in drinking water (maximum contaminant
level; EPA 2006). Similarly, the WHO recommends a provisional drinking water guideline of 10 ppb.26 Ingestion of
less harmful organic forms of arsenic such as arsenobetaine
and arsenocholine can occur with seafood, including bivalves
(clams, oysters, scallops, mussels), crustaceans (crabs, lobsters),
and some bottom-feeding fish.24
Domestic production of arsenic was ceased in 1985, yet
the United States is 1 of the world’s largest consumers of arsenic.24 Potential industrial exposures to arsenic may result from
the use of gallium arsenide and arsine gas in semiconductor devices, arsenic as a byproduct for the smelting process,
and chromated copper arsenic (CCA) as a preservative used
in wood. In 2003, CCA accounted for more than 90% of
domestic use of arsenic trioxide.27 Based on health concerns,
the EPA cancelled the registration of CCA-treated wood in
residential applications.24 However, wood treated prior to
December 31, 2003, is still found and represents a continued
exposure concern, such as from the inhalation of wood dust
or fumes from the burning of CCA-treated wood. The permissible exposure limit for arsenic in an occupational setting
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Yes
LOWEST
HIGHEST
YES—dilution with nitric acid common
is 10 µg or less of inorganic arsenic per cubic meter of air,
averaged over any 8-hour period for a 40-hour work week.
There is no general air pollution limit for arsenic.28
Arsenic has a long-standing history of use in medicinals. This includes its application in some traditional Asian
medicines and naturopathic remedies.29,30 Fowler’s solution
containing 1% arsenic trioxide was used to treat skin conditions, such as eczema and psoriasis, until the emergence of
skin cancer resulted from this application. Prior to antibiotics,
arsphenamine (Salvarsan), an arsenic compound, was used to
cure syphilis.24,25 Arsenic trioxide (eg, Trisenox) is currently
used to treat patients with acute promyelocytic leukemia.
Toxicity and Toxicokinetics
Arsenic’s toxicity depends largely on the valence state,
solubility, and rate of absorption and elimination. The 3
major groups for arsenic include arsine gas (-3 oxidation
state), inorganic, and organic forms. Arsine gas is the most
toxic arsenical. Inorganic arsenic compounds that also have
high toxic potential include arsenite (trivalent), arsenate
(pentavalent), arsenic oxide, and gallium arsenide. Metabolites
of inorganic arsenic compounds include methyl and phenyl
derivatives of arsenic such as monomethylarsonic acid (MMA)
and dimethyl arsenic acid (DMA) and have moderate toxic
potential. The methylated metabolites may arise from the
environment or by metabolism of inorganic arsenicals.24
Arsenobetaine and arsenocholine are the most common
organic forms, sometimes called “fish arsenic,” and are
relatively nontoxic to humans.
Ingestion of arsenic-containing foods and contaminated
water is a primary route of exposure. Approximately 95% of
an ingested dose of trivalent arsenic compounds is absorbed
from the gastrointestinal tract.25 In the workplace, inhalation
of arsenic trioxide is a common form of airborne arsenic.
It is estimated that 60%-90% of inhaled arsenic trioxide
is absorbed through the lung.31 Following absorption,
arsenic is taken up by RBCs and WBCs, leading to
hematological changes such as macrocytic anemia, elevated
eosinophils, and basophilic stippling of RBCs. When arsenic
is transported to the liver, pentavalent arsenic undergoes
reduction by glutathione to trivalent arsenic. With the aid
of methyltransferases, trivalent arsenic is methylated to
form MMA and DMA, which are eliminated in urine.3,4
Hepatotoxicity may occur during this process.
Arsenic is rapidly cleared from the blood, such that blood
levels may be normal even when urine levels remain markedly
elevated. The “fish arsenic” such as arsenobetaine and
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arsenocholine are cleared in urine within 48 hours. However,
the initial half-life of inorganic arsenic is approximately
10 hours.24,25 Approximately 70% of inorganic arsenic is
secreted in urine, of which 50% of excreted inorganic arsenic
appears as DMA, and 25% as MMA, with the remainder as
inorganic.32 However, these patterns may vary with the dose
and clinical status of the patient.
Cadmium
Background and Potential Sources of Exposure
Cadmium is often found near sites of metal mining and
refining, production and application of phosphate fertilizers,
waste incineration, and disposal.33 Occupational exposure
to cadmium is a serious consideration in the battery, smelting, and electroplating industries. Soluble forms are transported easily by water and accumulate in aquatic organisms.
Cadmium may also be transported as a particle or vapor for
long distances in the atmosphere, depositing on soil and water
surfaces. Cadmium binds strongly to organic matter where it
is immobilized in soil and taken up by plant life and agricultural crops.
Since tobacco leaves accumulate cadmium from the soil,
regular use of tobacco-containing products is a common route
of human cadmium exposure. The U.S. national geometric
mean blood cadmium levels for non-smoking adults is 0.47
µg/L, whereas the mean in smokers is approximately twice as
high, at 1.58 µg/L.34 Smoking is estimated to at least double
the lifetime body burden of cadmium exposure. For nonsmokers, human exposure to cadmium is largely through the
consumption of shellfish, organ meats, lettuce, spinach, potatoes,
grains, peanuts, soybeans, and sunflower seeds.35
Toxicity and Toxicokinetics
In the workplace, inhalation is the primary route of
exposure where 5%-35% of inhaled cadmium is absorbed
into the blood depending on its form, site of deposition, and
particle size. If the cadmium penetrates to the alveoli, it is
estimated that there is 100% absorption into the blood.36
Approximately 1-3 µg cadmium is absorbed per pack of
cigarettes. Following ingestion, it is estimated that 5%-10%
of cadmium is absorbed.1 In diets with low iron, calcium, or
protein, it is possible that more cadmium is absorbed. Dermal
exposure is not a typical human health concern as cadmium
does not penetrate the skin barrier.
Upon absorption in the blood, cadmium binds to
albumin and is transported to the liver. Cadmium-induced
liver damage increases hepatic enzymes.33 Metallothionein
(MT), a low molecular weight metal-binding protein, binds
cadmium where it is either stored in this conjugated form in
the liver or transported to the kidney. Once filtered through
the renal glomerulus, the cadmium-MT complex is reabsorbed
in the proximal tubules and degraded to release free cadmium.
It is this reactive cadmium ion that contributes to renal
tubular toxicity while accumulating in the cortex of the
kidney.37 Renal tubule damage is a hallmark of cadmium
toxicity and is reflected in increased concentrations of
biomarkers such as β2-microglobulin. Glomerular damage
may follow with corresponding increased levels of albumin
and transferrin.38 The concentration of cadmium in the
kidney is reflected in urinary levels. As such, urine testing
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can be used as a marker of long-term cadmium exposure.
Elimination of cadmium is very slow, as the elimination halflife of cadmium is up to 20 years.
Urinary cadmium concentration of less than 2.5 µg/g
creatinine has been considered to represent safe exposures.39
However, recent studies have identified increased risk to
cancer and mortality following chronic, low-level cadmium
exposure.40 Cancers primarily of the lung, but also prostate,
kidney, and pancreas have been documented. A disease
unique to cadmium exposure, Ouch Ouch disease, is
characterized with severe osteomalacia and osteoporosis from
the long-term consumption of cadmium-contaminated rice
but has implications for aging patients in general who are
diagnosed with osteoporosis.37,41
Lead
Background and Potential Sources of Exposure
Lead is a ubiquitous bluish-gray metal found in the
earth’s crust that has been mobilized in the environment by
recent anthropological activities. Lead is malleable, corrosionresistant, ductile, and is present primarily in its divalent form
(Pb2+). These properties allow for easy smelting and the
addition of lead in the production of batteries, ammunition,
paints, dyes, ceramic glazes, gasoline, and medical equipment.4
In the United States, the use of leaded gasoline and leadbased paint has resulted in airborne lead, contaminated soil,
and leaded dust. In the 1970s, environmental lead was decreased through the removal of tetraethyl lead from gasoline
and reduction of smokestack emissions from smelters. Lead
was also banned from residential paint.42 These actions to reduce environmental lead significantly reduced the blood lead
levels (BLL) of children throughout the U.S. Between 1976
and 1980, young children (1 to 5 years of age) had a median
BLL of 15 µg/dL, which decreased to 1.9 µg/dL in 1999.43
Common sources of exposure today include occupational
settings, residual pollution, or environmental contamination.
Children are at particularly high risk of exposure due to agerelated toxicokinetics and behavioral tendency for oral exploration. Older children or adults participating in hobbies using
lead also increase the risk of adult lead poisoning.
Toxicity and Toxicokinetics
Human lead exposure occurs as the result of
gastrointestinal absorption or pulmonary absorption, with
ingestion being the most significant route of exposure. The
bioavailability of the lead is dependent on the form of lead
(ie, inorganic, organic, or metallic), quantity ingested, age
of the individual, and current dietary status. A diet high
in calcium inhibits the binding of lead absorption through
the intestinal binding sites. A calcium deficiency activates
vitamin D and calbindin-D, a calcium-binding protein in the
intestines enhancing the absorption of calcium. However, if
calcium is not available in a sufficient quantity, lead and other
trace metals will be absorbed in place of the calcium. Adults
absorb approximately 15% of ingested lead, while children
and pregnant women absorb nearly 50% of ingested lead.44
Pulmonary lead exposure is mainly a concern for occupational
exposure.4 The health effects of lead are the same regardless of
the route of exposure.45
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After absorption, 99% of the lead is bound to the hemoglobin portion of erythrocytes and is circulated via the vascular
system to soft tissues (liver and kidney), bone, and hair. Lead
has a half-life in the blood of approximately 30 days, consistent with the half-life of a RBC. As such, BLL indicates relatively recent lead exposure.43 During systemic circulation, lead
interrupts the heme biosynthesis pathway primarily through
inhibition of ∂-amino levulinic acid, an effect observed when
BLL exceeds 5 µg/dL.44 Elevated levels of zinc protoporphyrin
(ZPP) often accompany elevated lead, and as such, ZPP is
routinely included in testing for those who may be at risk for
occupational exposure to lead. Interestingly, an elevated ZPP
may increase risk of lead exposure 6 months later.46
Approximately 96% of the lead entering the body
throughout a person’s life is stored in the bones with a halflife in bone of approximately 32 years. Lead mobilization
from bone is dependent on the rate of biological activity.
Trabecular bones are a significant source of endogenous lead,
due to greater level of biological activity, surface area, and
volume of blood flow in comparison to cortical bones.47 A
particular demographic of concern is pregnant women due to
the mobilization of lead during pregnancy as bone is catabolized to assist in the creation of the fetal skeleton.48 Evidence
supports that teeth and bone share similar qualities, such as a
high affinity for metals and similar accumulation rates. The
appearance of a “lead line” also known as a “Burtonian blue”
line, at the gum line, is indicative of chronic lead poisoning.49
The blue line is a common manifestation occurring in individuals with poor dental hygiene and is best described as the
deposition of lead between collagen fibers, around blood vessels, and within cells.50
Acute exposures often manifest as central nervous system
(CNS) and gastrointestinal symptoms. Central nervous system
symptoms include encephalopathy, convulsion, and stupor.
Colic, a gastrointestinal symptom, is a consistent symptom of
lead poisoning characterized by abdominal pain, cramps, and
nausea. Adults have exhibited lead-induced colic at BLL as
low as 40 µg/dL.45 Chronic exposure differs from acute exposure in that chronic symptoms manifest as general malaise,
anorexia, constipation, wrist drop, hematuria, and anemia.
Although not specific to lead poisoning, basophilic stippling
may be seen in erythrocytes due to changes in ribosomes.
Additionally, lead targets the proximal tubules of the kidneys
and is capable of inducing nephrotoxicity in the form of
proximal tubular nephropathy, glomerular sclerosis, and interstitial fibrosis. A decreased glomerular filtration rate and the
direct inhibition of the biosynthesis of erythropoietin (EPO)
production by lead may contribute to “lead-induced anemia,”
which is present at BLLs of ≥20 µg/dL.45
Childhood Exposure to Lead
Young children, less than 6 years old, are a population at
high risk for lead poisoning because they are dispro-portionately exposed to environmental contaminants and generally
exhibit more severe health effects than adults. This is attributed to age-related behaviors, such as hand-to-mouth activities, pica, and playing on or near the ground.42,51 Children
are commonly exposed to lead through the ingestion of contaminated soil or dust and paint chips from deteriorating lead
paint. In addition to common environmental sources of lead
exposure, the emergence of global free trade and increased
international travel have resulted in atypical sources of lead
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exposure, such as folk remedies, imported condiments, imported
candies, glazed ceramics, and toys. Furthermore, children
absorb a greater proportion of lead compared to adults and
are smaller in size, leading to a larger overall dose per unit
of weight (eg, dose per kilogram) of lead exposure. Lead
exposure of pregnant women is a concern as well because lead
readily passes the placental barrier and is found in the breast
milk. Exposure during pregnancy may result in impaired fetal
and infant development.
The BLL of concern established by the Centers for Disease Control and Prevention in 1997 is 10 µg/dL. However,
many states have lowered the upper limit of normal to 5 µg/
dL because children with BLL less than 10 µg/dL can still suffer from permanent IQ and hearing deficits.44,52 In addition,
epidemiologic studies suggest that a BLL less than 5 µg/dL is
achievable.48 However, there is currently no known threshold
(ie, safe level of exposure) for the permanent health effects of
lead poisoning.
Mercury
Background and Potential Sources of Exposure
Mercury exists as metallic mercury, inorganic mercuric
salt, and organic mercury. Metallic mercury is a liquid at
room temperature. Mercury binds to chlorine, sulfur, or oxygen to form inorganic mercurous (Hg1+) or mercuric (Hg2+)
salts. The primary organometallic forms include methylmercury (MeHg) and ethylmercury.53
Mercury has a long history of use in industrial settings
and in many readily available consumer products, such as thermometers, cosmetics, vaccines, and dental amalgams. Dental
amalgams contain approximately 50% mercury combined
with other metals such as silver and copper. The source of
mercury found in vaccines is ethylmercury, from the preservative thimerosal. Although vaccinations have been linked to the
increasing cases of autism in children, this association is highly
controversial, and the pathogenesis of autism is complex.
Most human exposure to mercury comes from the diet,
and, in particular, through consumption of contaminated
fish. Mercury accumulates in fish due to contamination of
their marine environment and diet. As such, fish living in
contaminated waters, or predatory fish living a long time, are
more likely to be contaminated with mercury. According to
data available through the FDA, the commercial fish with
the highest mercury content (mean ppm mercury) are tile
fish (1.45 ppm), shark (0.988 ppm), swordfish (0.976 ppm),
and mackerel (0.730). The highest concentration observed
was 4.540 ppm, seen in shark samples.54 It is recommended
by the U.S. FDA that children and pregnant women avoid
fish with high mercury content and limit intake of fish with
low mercury content to 12 ounces per week. Because canned
albacore tuna contains more mercury than other varieties, this
source should be limited to 6 ounces per week. It is advised
to also limit non-commercial fish intake unless the waters and
fish are tested to ensure safety.
Toxicity and Toxicokinetics
The risk of mercury toxicity depends very much on the
form of mercury and route of exposure. Metallic mercury
exposure can occur by inhalation of mercury vapor. Mercury
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CE Update
vapor in the atmosphere is typically low and not considered a major route of exposure.4,53 However, mercury vapor
is a potential occupational exposure in gold mining where
mercury is used to form an amalgam with gold during
its extraction, in dentistry for tooth restoration, and in the
manufacture of scientific instruments and electrical control
devices. Amalgam fillings can provide an exposure to mercury
vapor, and it is estimated that 10 amalgam surfaces would
raise urinary mercury concentrations by 1 μg/L. Typically,
exposures to mercury in the oral cavity have been considered
relatively low enough to not warrant concern. However, during cases of excessive gum chewing in the presence of these
amalgams, urine levels have been measured in excess of 20 μg
mercury per gram creatinine to approach occupational health
safe limits.55
Mercury vapor can demonstrate substantial toxicity in
this form as it easily penetrates the blood-brain and placental
barriers to cause neurotoxicity, developmental toxicity, and
at higher levels, mortality. Acute mercury poisoning occurs
in 3 phases. In the first 1-3 days, flu-like symptoms appear,
followed by severe pulmonary toxicity and then gingivostomatitis, tremor, memory loss, emotional lability, depression,
insomnia, and shyness.56
Mercuric chloride (HgCl2) is a potent nephrotoxin, altering renal glutathione levels, increasing urinary albumin and
β2-microglobulin. Mercuric chloride exposure may also cause
systemic autoimmune disease, setting the stage for nephrotic
syndrome.57 Fortunately, HgCl2 is poorly absorbed through
the gastrointestinal tract, thereby limiting its exposure via
ingestion. Little is known to date regarding the disposition
of ethylmercury within the body. Its pattern is thought to be
similar to methylmercury, however, the pharmacokinetics
remain to be fully characterized.
For most, fish consumption is the major route of exposure
of MeHg. Unlike some other contaminants, such as polychlorinated diphenyls, cooking does not alter the levels found
in this food. Health risks from prenatal exposure to MeHg
have been well studied in the data collected from human
exposures in Minamata, Japan, and the 1971-1972 outbreak
in Iraq.58 Based on these data, Clarkson and colleagues55
defined threshold toxicologic levels associated with severe
adverse effects to the fetus as low as 10 μg/g in maternal
hair. Based on data collected from the National Health and
Nutrition Examination Survey (NHANES), the consumption of fish by women 1-2 times per month resulted in a total
mercury mean concentration of 0.20 μg/g in hair and 1.05
µg/L in blood; >3 times per month was 0.38 μg/g in hair
and 1.94 μg/L in blood; and no fish resulted in total mercury background levels of 0.11 μg/g in hair and 0.51 μg/L in
blood. Based on a comprehensive review by Koren and Bend,
neurodevelopmental abnormalities occurred in children after a
range of gestational exposures from maternal consumption of
highly contaminated fish at the following levels: maternal hair
0.3 to 12.7 μg/g, cord blood 0.75 to 25.7 μg/L, and maternal
blood 3.8 μg/L.59
Methylmercury poisoning is insidious as there is a latency
period before symptoms appear, and a very small exposure can
be deadly. A famous case involving a respected scientist exemplifies the fact that symptoms may not progress for several
weeks to months after the exposure. In this case, specific
neuronal cell loss occurred, leading to paresthesia, with progression to cerebellar ataxia, constriction of the visual fields,
and loss of hearing.60
labmedicine.com
Conclusion
Laboratory testing is critical to the detection of heavy
metal pollution in the environment and to the detection of
human exposures. Most of the concepts presented here for
arsenic, cadmium, lead, and mercury can be fundamentally
applied to other potentially toxic elements, such as aluminum,
beryllium, chromium, cobalt, and thallium. Consult the many
useful references cited here and available elsewhere, as well as
your local laboratory, for more details. LM
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