Heavy Metal Pathways (adapted from 1995 U.S. Geological Survey Circular)[a]
TCAS has been providing heavy metal consultative and toxicological assessment
services for more than 34 years. Although some heavy metals are necessary ingredients
in a healthy body, heavy metal contamination can cause a wide range of adverse health
effects. Unlike organic pollutants, heavy metal compounds degrade slowly or not
at all after entering the environment. Additionally, heavy metals can bioaccumulate
in the body. These are significant factors with respect to toxicological assessments and
remediation.
The illustration at right (adapted from a 1995 U.S. Geological Survey report)1 illustrates some of the common pathways by which heavy
metals contaminate water. As heavy metals flow into water, some will undergo biological
and chemical reactions. Thus, heavy metal exposure is most often a composite process
involving water consumption, ingestion of fish and wildlife, contact with sediment,
ingestion of dust and/or inhalation of volatile vapors (such as mercury vapors). The expert toxicologist
will establish the various exposure pathways and assess exposure based on a determination
of cumulative dosage by each pathway for each heavy metal and/or heavy metal valence or compound.
Heavy metals can be released from numerous sources including municipal wastewater
treatment plants, manufacturing industries, mining, rural agricultural cultivation,
fertilization and others. Heavy metals are transported as either a dissolved compound
in water or as an integral part of suspended sediments. Heavy metals may be volatilized
to the atmosphere or stored in sediments. Toxic heavy metals can be absorbed by
a variety of organisms. Metals dissolved in water generally have the greatest potential
of causing the most deleterious health effects.1
Toxicological Characteristics of Heavy Metals
Each elemental heavy metal presents a unique set of toxicological characteristics
with respect to exposure mediums (air, soil and water). Toxicological assessment
factors vary widely by substance and can be greatly affected by circumstances. For
example, some heavy metals can exist as both organic and inorganic compounds. The
chemical differences of binding to carbon, hydrogen and other substances can significantly
alter absorption characteristics, including toxicity and other factors.
Similarly, some metals can move up the food chain through an uptake process called
biomagnification in which heavy metal accumulations increase at an accelerating
rate. When microscopic aquatic organisms consume contaminated organic material,
they may be eaten by small fish. These in turn are eaten by larger fish, then by
birds, animals and ultimately by humans. At each stage, the bioaccumulation
rate increases in proportion to the amount and frequency of ingestion. Thus, both
chronic and acute exposure to certain heavy metals can result in toxicologically-significant
cumulative doses. These doses can be determined and applied scientifically to a human
health risk assessment by following the generally-accepted, peer-reviewed toxicological
methods for assessing each exposure pathway.
The following paragraphs outline some of the heavy metals for which TCAS
has performed toxicological assessments, produced written reports and/or provided
expert testimony subsequent to being retained by defendants, plaintiffs, prosecutors,
state attorney generals and/or public defenders' offices. Please
contact our office for information concerning any heavy metals not listed
on this page.
Click on any heavy metal to view toxicological characteristics.
Show all results for "Heavy Metals"
Antimony
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Antimony (element 51) is found primarily in natural ore deposits. It is used in
batteries, pigments, ceramics and glass. The most widely used antimony compound
is antimony trioxide which is used as a flame retardant. Antimony has low toxicity compared
to some other heavy metals and direct exposures are rare. The primary historical
source of contamination is in drinking water. The major sources of antimony in drinking
water are discharge from petroleum refineries, fire retardants, ceramics, electronics
and solder waste.
Long-term ingestion of antimony-contaminated water (in excess of the U.S. EPA 6
PPB maximum contaminant level goal) may enhance the risk of
a drop in blood glucose levels and an increase in blood cholesterol.2 There is limited evidence
of antimony carcinogenicity in humans and insufficient evidence of carcinogenicity
in experimental animals. The International Agency for Research on Cancer (IARC)
classifies antimony trioxide as a "Class 2B" carcinogen (possibly carcinogenic
to humans).3 Antimony is sometimes used to
treat parasite infections. Sensitive or overdosed patients have experienced some
adverse health effects including diarrhea, joint and/or muscle pain, vomiting, anemia
and cardiovascular issues (including altered electrocardiograms).4
Please
contact our office for additional information.
Arsenic
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Arsenic (element 33) has been found in at least 1,149 of the 1,684 "National
Priority List" contamination sites identified by U.S. EPA.5
Arsenic typically presents as two basic compound types. Organic arsenic
(combined with carbon and hydrogen) was historically used as a pesticide in orchards
and cotton fields or as an additive in certain animal feeds (such as "Roxarsone,"
which has been used extensively for swine and poultry) to increase feed efficiency
and stimulate rapid growth. The U.S. Food and Drug Administration has recently banned
three arsenic compounds used in swine and poultry feeds.6
Inorganic arsenic (combined with chromium, copper and oxygen) has been
used primarily to preserve wood in the form of copper chromated arsenate (CCA) which
is used to make pressure-treated lumber in industrial applications (CCA is now banned
for U.S. residential purchase).
Although different arsenic compounds have different potencies and variable adverse
health effects, both organic and inorganic arsenic compounds have been formally
classified by the International Agency for Research on Cancer (IARC) as "Group 1
Human Carcinogens."7 At high levels, inorganic arsenic
can be fatal. Long-term exposure to lower levels can cause disease, discoloration
of the skin and small corns or warts. Toxicological factors required to assess arsenic
exposure in humans include ingestion characteristics, exposure pathways, timeframe(s),
bioavailability and organic structure. TCAS has extensive experience in
toxicological assessments of arsenic exposures and remediation issues in residential,
environmental, agricultural and industrial cases. Please contact our
office for additional information.
Cadmium
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Cadmium (element 48) has been formally classified by the International Agency for
Research on Cancer (IARC) as a Group 1 human carcinogen.7
U.S. EPA rates cadmium as a "probable human carcinogen" (group B1). Cadmium
has been shown to be highly toxic at relatively low concentrations. It can accumulate
in body tissues over long periods of time and is nonessential for human health.
Most cadmium exposures occur occupationally where cadmium products are made or used.
The general population can be exposed from a variety of pathways including cigarette
smoke, consumption of cadmium-contaminated water and foods (most recently in cocoa
beans), etc. The largest source of cadmium exposure in the U.S. (for nonsmoking
adults and children) is through dietary intake. Cadmium has been found in at least
1,014 of the 1,669 National Priority List sites identified by U.S. EPA.8
Cadmium can cause damage to the kidneys, lungs and bones. The documented toxicological
properties of cadmium are similar for the several different cadmium salts and oxides
that have been investigated. However, differences in absorption and distribution
characteristics can lead to different adverse health effect levels.9
TCAS has produced toxicological assessments of cadmium on behalf of both
plaintiffs and defendants and was recently retained to produce a comprehensive
human health risk assessment relating to cadmium in cacao powder used in food products
and candy. Please
contact our office for additional information.
Chromium
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Chromium (element 24) occurs naturally in animals, plants, rocks, soil and in volcanic
dust and gases. Chromium is widely used in manufacturing metal alloys (such as stainless
steel) and in consumer products (such as tanned leather, cookware and others). There
are three primary valence states or "forms" of chromium: Chromium (0),
Chromium (III) and Chromium (VI) (also known as "Hexavalent Chromium."). Chromium (III)
is an essential nutrient required for normal metabolism. Chromium (VI) compounds
have been classified by the International Agency for Research on Cancer (IARC) as
Group 1 human carcinogens.7
There are numerous adverse toxicological endpoints associated with chromium (VI) exposure
including respiratory, gastrointestinal, immunological, hematological, reproductive
and developmental effects. High doses of chromium (VI) compounds can also result
in severe cardiovascular, hematological, hepatic, renal and neurological damage.10 Chromium (VI) has been the subject of some well-publicized
exposure events in the United States, such as the case of the town of Hinkley, CA whose groundwater
was contaminated with hexavalent chromium used as a pumping system
additive.11 TCAS has performed numerous
toxicological risk assessments associated with chromium on behalf of both plaintiffs
and defendants and provided toxicological consultative and assessment services for
more than 34 years. Please
contact our office for additional information.
Cobalt
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Cobalt (element 27) is used in in a variety of consumer products and specialized
applications. These include superalloys in gas turbine aircraft engines, pigments
in glass, ceramics, paints and paint driers, as catalysts in the petroleum industry
and as trace element additives in agriculture and medicine. Cobalt is a constituent
of vitamin B12, an essential body nutrient. Cobalt poisoning is infrequent.
Most cobalt exposures have occupational origins (such as hard metal industry, tool
production, grinding, etc.). However, people living near industrial sites may be
potentially exposed to high levels of cobalt in air, dust and soil.12
Adverse health effects for short-term inhalation exposure to cobalt dust include
decrease in ventilatory function, congestion, pulmonary edema and lung hemorrhage.
Chronic (long-term) effects of exposure by inhalation include respiratory irritation,
wheezing, asthma, pneumonia, fibrosis, cardiac effects, liver and kidney congestion
and immunological effects.13 Although cobalt is not
classified by U.S. EPA as carcinogenic, the National Toxicology Program conducted
a carcinogenicity study in rats and mice. Exposure to cobalt sulfate aerosols for
2 years (concentrations ranging from 0.11 to 1.14 mg/m3)13 resulted in a spectrum of inflammatory, fibrotic
and proliferative lesions in the respiratory tract of male and female rats and mice
without definitive evidence of carcinogenicity.14
TCAS has provided toxicological guidance and cobalt risk and causation
assessments for clients for many years. Please contact our
office for additional information.
Copper
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Copper (element 29) is an essential body nutrient. It can enter the environment
through releases from mining, industrial emissions, waste water, fossil fuel combustion,
wood production, phosphate fertilizers and natural sources. Copper is also sometimes
found in pesticides, fungicides and herbicides (typically in the form of copper sulfate
pentahydrate). Though non-toxic at low levels, high levels of copper can be harmful.
Copper may cause adverse health effects if present in public or private water supplies
in amounts greater than the drinking water standards set by U.S. EPA.15
Inhalation of high levels of copper can cause irritation of the nose, throat and respiratory
tract. Coughing, sneezing, runny nose (rhinitis), pulmonary fibrosis and increased vascularity
of the nasal mucosa have been reported in workers exposed to high levels of copper dust. There
is some evidence from animal studies to suggest that exposure to airborne copper
or high levels of copper in drinking water can damage the immune system.16
Very high doses of copper can cause damage to liver and kidneys while a sufficiently
high dosage can be fatal. Please
contact our office for additional information.
Lead
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Lead (element 87) is rarely found naturally as a metal. It is usually found combined
with two or more other elements to form lead compounds. Lead is used in batteries,
ammunition, metal products and many other types of consumer products. Owing to significant
documented health concerns, use of lead in paints, ceramics, caulking, pipe
solder and other products have all been dramatically reduced in recent years. The
use of lead as an additive to gasoline in the U.S. (primarily as tetraethyl lead)
was diminished in the 1980's and ultimately banned in 1996.
Environmental lead poisoning from exogenous sources has been extensively studied
at low level chronic doses in children as well as higher level acute occupational
exposures. The main target for lead toxicity is the nervous system in both adults
and children. Long-term exposure of adults can result in decreased performance of
the nervous system. It may also cause weakness in fingers, wrists or ankles as well
as increases in blood pressure and anemia. Exposure to high lead levels can severely
damage the brain and kidneys in adults or children and can ultimately be fatal.
High exposure levels may cause miscarriage in pregnant women and damage the organs
responsible for sperm production in men. U.S. EPA has set its maximum contaminant
level goal (MCLG) for lead to zero (0) to demonstrate the fact that there is no
documented safe level or threshold at this time.17
It should also be noted that background lead levels in some regions of industrialized
countries already approach generally-recognized toxic levels.
The U.S. Department of Health and Human Services has determined that lead and lead
compounds are reasonably anticipated to be human carcinogens and U.S. EPA has determined
that lead is a "probable human carcinogen." The International Agency for
Research on Cancer (IARC) has determined that inorganic lead is probably carcinogenic
to humans.18 TCAS has extensive experience
in toxicologically assessing lead contamination and exposures in residential, environmental
and industrial cases in both plaintiff and defendant causation claims and risk assessment
scenarios. Please
contact our office for additional information.
Mercury
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Mercury (element 80) is a liquid metal found in natural ore deposits. Mercury is
the only metallic element found as a liquid at normal temperatures and pressures.
Batteries, fluorescent light bulbs, switches and other control equipment account
for 50% of commercial mercury used.19 Mercury pollution
was common in the mid-20th century from a wide range of sources. However, most mercury
pollution is presently produced by coal-fired power plants and industrial processes.
Mercury combines with other elements (such as chlorine, sulfur or oxygen) to form
inorganic mercury compounds (or "salts"). Mercury also combines with carbon
to make organic mercury compounds. The most common compound, methylmercury, is produced
mainly by microscopic organisms in the water and soil.
The most common exposure pathway for methylmercury is consumption of contaminated
fish and shellfish.20 Consequently, state agencies
and U.S. EPA regularly issue fish consumption advisories21
Methylmercury and metallic mercury vapors are more harmful than other forms because
more mercury in these forms reaches the brain. Exposure to high levels of metallic,
inorganic or organic mercury can permanently damage the brain, kidneys and developing
fetus. Effects on brain functioning may result in irritability, shyness, tremors,
changes in vision or hearing and memory problems. Short-term exposure to high levels
of metallic mercury vapor may cause effects including lung damage, nausea, vomiting,
diarrhea, increases in blood pressure or heart rate, skin rashes and eye irritation.22 TCAS has produced numerous risk assessments
involving mercury contamination and provided expert testimony on behalf of both
plaintiffs and defendants. TCAS has also conducted residential occupancy
safety evaluations prior to re-occupancy by displaced families following mercury
contamination remediation. Please
contact our office for additional information.
Manganese
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Manganese (element 25) occurs naturally at low levels in soil, water, air and food.
Manganese is an essential trace element necessary for optimal health. Manganese
can be found in grains and cereals and in higher amounts in tea. Manganese can combine
with carbon to create organic manganese compounds. Common organic manganese compounds
include pesticides (such as maneb or mancozeb) and methylcyclopentadienyl manganese
tricarbonyl (MMT), a gasoline fuel additive. MMT was banned for use in unleaded
gasoline in the United States in 1977, but the U.S. EPA lifted the ban in 1995 and
MMT has been used freely since that time.23
Toxic exposure to manganese occurs primarily through ingestion of foods containing
manganese, but exposure can also occur through dust inhalation, soil intake or drinking
manganese-contaminated water. At high doses by inhalation, manganese can be very
toxic (as documented in studies of occupational exposures in miners). Manganese toxicity attacks
the nervous system. Common symptoms of toxic exposure include ataxia, dementia,
anxiety, a "mask-like" face and manganism, a syndrome similar to Parkinson's
disease. Manganese generally has low toxicity by oral ingestion and reports of adverse
toxicological effects by this route are rare.24
TCAS was involved in an investigative case in which a potential homicide attempt
resulted in severe dementia and death. Additionally, TCAS was recently retained
to forensically analyze
MMT abundance in layers of LNAPL which were used to characterize gasoline
contributions and thus identify (and disqualify) specific petroleum manufacturers
in remediation litigation. Please
contact our office for additional information.
Thallium
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Thallium (element 81) can be found in pure form or mixed with other metals in the
form of alloys. It can also be found combined with other substances such as bromine,
chlorine, fluorine and iodine to form salts. Although thallium is highly toxic,
thallium poisoning is uncommon. Studies of thallium toxicity in humans consist mainly
of clinical reports, case studies and medical surveys (thallium poisoning case
reports typically involve accidental ingestion, intentional poisoning attempts or suicide
attempts).
Thallium's acute toxicity is characterized by alopecia (hair loss), severe pain
in extremities, lethargy, ataxia, abdominal pain or vomiting, back pain, abnormal
reflexes, neuropathy, painful paresthesia, muscle weakness, coma, convulsion, mental abnormalities, tremors,
abnormal movements, abnormal vision and death. The average lethal oral dose in adults
has been estimated to range from 10 to 15 mg/kg (Gosselin et al., 1984;
Schoer, 1984). Without treatment, death typically follows in 10-12 days, but death
after 8-10 hours has been documented (IPCS, 1996).25
TCAS was involved in a nationally-publicized case in which Dr. Sawyer determined
through forensic investigation that a hospitalized patient had been re-poisoned with
thallium at the hospital only 12-24 hours before he died (based on his urine/blood
ratio which had been overlooked by both the clinical investigators and the prosecutor).
Dr. Sawyer's toxicological findings, written report and expert testimony ultimately
resulted in the arrest, conviction and double life sentencing of the perpetrator.
Please
contact our office for additional information.
Titanium
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Although titanium (element 22) in pure form has relatively low toxicity, some titanium
compounds (such as titanium dioxide and titanium tetrachloride) have higher toxicity.
Titanium dioxide forms in the human body on the surface of arthroplasty devices
(artificial hips, etc.) and in dental implants. Titanium tetrachloride (used to
make titanium metal and other titanium compounds) is highly irritating to the skin,
eyes and mucous membranes in humans as it reacts with air and rapidly forms hydrochloric
acid if it comes in contact with water. Environmental exposure to titanium tetrachloride
is comparatively rare as it tends to break down rapidly in water.
Neither the IARC nor U.S. EPA have classified titanium tetrachloride with respect to
carcinogenicity.26 IARC classifies titanium dioxide
as a "Class 2B" carcinogen (possibly carcinogenic to humans).3 The primary toxicological exposure pathways are by
inhalation and by dermal contact. Acute (short-term) exposure may result in surface
skin burns and marked congestion and constriction of the upper respiratory tract
in humans. Acute exposure may also damage the eyes. Chronic inhalation exposure
may result in upper respiratory tract irritation, bronchitis, cough, bronchoconstriction,
wheezing, chemical pneumonitis or pulmonary edema. Diseases of the lung (pleural
diseases) have also been associated with long-term occupational exposure of titanium
tetrachloride in titanium metal production workers.27
Please
contact our office for additional information.
Uranium
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Uranium (element 92) is a radioactive element occuring in nearly all rocks and soils.
Although the three uranium isotopes (U-234, U-235 and U-238) behave much the same
chemically, each has different radioactive properties. The primary route of exposure
for the general population is ingestion of food and drinking water. Only about 1%
of ingested uranium will typically enter a person's blood following ingestion. Most
of this is removed by the kidneys and excreted within a few days; however, a small
amount of uranium will be retained and ultimately deposited in the bones. Thus,
bioaccumulation is a significant consideration in any uranium exposure scenario
in which ingestion is a factor, particularly with respect to consumption of contaminated
fish, plants, animals and/or water. Intakes of uranium exceeding U.S. EPA standards
can lead to increased cancer risk, liver/kidney damage or both. Other potential
targets of toxicity include the reproductive system and the developing fetus.
Long-term chronic intakes of uranium isotopes in food, water or air can lead to
internal irradiation and/or chemical toxicity. The toxicity of uranium is mainly
due to chemical damage to the kidneys or respiratory tract. Adverse health effect
levels vary depending on the particular uranium compound and its solubility.28 Absorption of uranium is low by all exposure routes
(inhalation, oral and dermal) but even small quantities of a radioactive substance
can produce adverse health effects. The deposition of inhalable uranium dust particles
in the lungs depends on the particle size and absorption depends on its solubility
in biological fluids (ICRP 1994a, 1996). A final toxicological profile for uranium
was produced by ATSDR in April, 2013, in which minimum risk levels (MRLs) were established
for uranium exposure by both oral and respiratory routes.29
Please
contact our office for additional information.
Notes and References
- U.S. Geological Survey, "Circular 1133", Reston, Virginia,
1995.
- U.S. Environmental Protection Agency, "Basic Information
about Antimony in Drinking Water," September, 2013.
- IARC, "Monographs on the Evaluation of
Carcinogenic Risks to Humans," Agents Classified by the IARC Monographs,
Volumes 1-135, 2024.
- U.S. Environmental Protection Agency, "Toxicological Review
of Antimony and Compounds," September, 1992.
- ATSDR, Division of Toxicology and Environmental Medicine, "Arsenic,"
CAS #7440-38-2, August 2007.
- New York Times, "F.D.A. Bans
Three Arsenic Drugs Used in Poultry and Pig Feeds," October, 2013.
- IARC, "Monographs on the Evaluation of
Carcinogenic Risks to Humans," Agents Classified by the IARC Monographs,
Volumes 1-135, 2024.
- ATSDR, ToxFaqs, "CAS#: 7440-43-9, Cadmium," October, 2012.
- ATSDR, Division of Toxicology and Environmental Medicine, "Toxicological
Profile for Cadmium," September, 2012.
- ATSDR, Division of Toxicology and Environmental Medicine, "Toxicological
Profile for Chromium," September, 2012.
- Pacific Gas & Electric, "Chromium Remediation: Environmental Restoration Activities at Compressor Station Properties," 2016
- ATSDR, Division of Toxicology and Environmental Medicine, "Toxicological
Profile for Cobalt," April, 2004.
- U.S. Environmental Protection Agency, "Cobalt Compounds," October, 2012.
- National Toxicology Program, "Toxicology Studies of Cobalt Metal,"
CAS No. 7440-48-4, October, 2013.
- U.S. Environmental Protection Agency, "Basic Information
about Copper in Drinking Water," December, 2013.
- ATSDR, Division of Toxicology and Environmental Medicine, "Toxicological
Profile for Copper," September, 2004.
- U.S. Environmental Protection Agency, "Basic Information
about Lead in Drinking Water," February, 2014.
- ATSDR, Division of Toxicology and Environmental Medicine, "Toxicological
Profile for Lead," August, 2007.
- U.S. Environmental Protection Agency, "Basic
Information about Mercury (inorganic) in Drinking Water," February, 2014.
- U.S. Environmental Protection Agency, "Human
Health Criteria - Methylmercury Fish Tissue Criterion," October, 2012.
- U.S. Environmental Protection Agency, "Fish Consumption Advisories,"
March, 2014.
- ATSDR, Division of Toxicology and Environmental Medicine, "Toxicological
Profile for Mercury," March, 1999.
- ATSDR, Division of Toxicology and Environmental Medicine, "Toxicological
Profile for Manganese," September, 2012.
- U.S. Environmental Protection Agency, "Health Effects Support
Document for Manganese," February, 2003.
- U.S. Environmental Protection Agency, "Toxicological Review
of Thallium and Compounds," September, 2009.
- U.S. Environmental Protection Agency, "Titanium Tetrachloride Hazard
Summary," January, 2000.
- ATSDR, Division of Toxicology and Environmental Medicine, "Toxicological
Profile for Titanium Tetrachloride," September, 1997.
- U.S. Environmental Protection Agency, "Uranium," October, 2012.
- ATSDR, Division of Toxicology and Environmental Medicine, "Toxicological
Profile for Uranium," February, 2013.
Images
- Image adapted from U.S. Geological Survey Circular 1133, "Sources and Sinks of Heavy Metals," Reston, Virginia, 1995.
- Multiple atomic structure images adapted from public domain Creative Commons Attribution.