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= Arsenic =
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Introduction
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Arsenic is a chemical element; it has symbol As and atomic number 33.
It is a metalloid and one of the pnictogens, and therefore shares many
properties with its group 15 neighbors phosphorus and antimony.
Arsenic is notoriously toxic. It occurs naturally in many minerals,
usually in combination with sulfur and metals, but also as a pure
elemental crystal. It has various allotropes, but only the grey form,
which has a metallic appearance, is important to industry.
The primary use of arsenic is in alloys of lead (for example, in car
batteries and ammunition). Arsenic is also a common n-type dopant in
semiconductor electronic devices, and a component of the III-V
compound semiconductor gallium arsenide. Arsenic and its compounds,
especially the trioxide, are used in the production of pesticides,
treated wood products, herbicides, and insecticides. These
applications are declining with the increasing recognition of the
persistent toxicity of arsenic and its compounds.
Arsenic has been known since ancient times to be poisonous to humans.
However, a few species of bacteria are able to use arsenic compounds
as respiratory metabolites. Trace quantities of arsenic have been
proposed to be an essential dietary element in rats, hamsters, goats,
and chickens. Research has not been conducted to determine whether
small amounts of arsenic may play a role in human metabolism. However,
arsenic poisoning occurs in multicellular life if quantities are
larger than needed. Arsenic contamination of groundwater is a problem
that affects millions of people across the world.
The United States' Environmental Protection Agency states that all
forms of arsenic are a serious risk to human health. The United States
Agency for Toxic Substances and Disease Registry ranked arsenic number
1 in its 2001 prioritized list of hazardous substances at Superfund
sites. Arsenic is classified as a group-A carcinogen.
Physical characteristics
==========================
The three most common arsenic allotropes are grey, yellow, and black
arsenic, with grey being the most common. Grey arsenic (α-As, space
group Rm No. 166) adopts a double-layered structure consisting of many
interlocked, ruffled, six-membered rings. Because of weak bonding
between the layers, grey arsenic is brittle and has a relatively low
Mohs hardness of 3.5. Nearest and next-nearest neighbors form a
distorted octahedral complex, with the three atoms in the same
double-layer being slightly closer than the three atoms in the next.
This relatively close packing leads to a high density of 5.73 g/cm3.
Grey arsenic is a semimetal, but becomes a semiconductor with a
bandgap of 1.2-1.4 eV if amorphized. Grey arsenic is also the most
stable form.
Yellow arsenic is soft and waxy, and somewhat similar to
tetraphosphorus (). Both have four atoms arranged in a tetrahedral
structure in which each atom is bound to each of the other three atoms
by a single bond. This unstable allotrope, being molecular, is the
most volatile, least dense, and most toxic. Solid yellow arsenic is
produced by rapid cooling of arsenic vapor, . It is rapidly
transformed into grey arsenic by light. The yellow form has a density
of 1.97 g/cm3. Black arsenic is similar in structure to black
phosphorus.
Black arsenic can also be formed by cooling vapor at around 100-220 °C
and by crystallization of amorphous arsenic in the presence of mercury
vapors. It is glassy and brittle. Black arsenic is also a poor
electrical conductor.
Arsenic sublimes upon heating at atmospheric pressure, converting
directly to a gaseous form without an intervening liquid state at 887
K. The triple point is at 3.63 MPa and 1090 K.
Isotopes
==========
Arsenic occurs in nature as one stable isotope, 75As, and is therefore
called a monoisotopic element. As of 2024, at least 32 radioisotopes
have also been synthesized, ranging in atomic mass from 64 to 95. The
most stable of these is 73As with a half-life of 80.30 days. The
majority of the other isotopes have half-lives of under one day, with
the exceptions being
: 71As ( 65.30 hours),
: 72As ( 26.0 hours),
: 74As ( 17.77 days),
: 76As ( 26.26 hours),
: 77As ( 38.83 hours).
Isotopes that are lighter than the stable 75As tend to decay by β+
decay, and those that are heavier tend to decay by β− decay, with some
exceptions.
At least 10 nuclear isomers have been described, ranging in atomic
mass from 66 to 84. The most stable of arsenic's isomers is 68mAs with
a half-life of 111 seconds.
Chemistry
===========
Arsenic has a similar electronegativity and ionization energies to its
lighter pnictogen congener phosphorus and therefore readily forms
covalent molecules with most of the nonmetals. Though stable in dry
air, arsenic forms a golden-bronze tarnish upon exposure to humidity
which eventually becomes a black surface layer. When heated in air,
arsenic oxidizes to arsenic trioxide; the fumes from this reaction
have an odor resembling garlic. This odor can be detected on striking
arsenide minerals such as arsenopyrite with a hammer. It burns in
oxygen to form arsenic trioxide and arsenic pentoxide, which have the
same structure as the more well-known phosphorus compounds, and in
fluorine to give arsenic pentafluoride. Arsenic makes arsenic acid
with concentrated nitric acid, arsenous acid with dilute nitric acid,
and arsenic trioxide with concentrated sulfuric acid; however, it does
not react with water, alkalis, or non-oxidising acids. Arsenic reacts
with metals to form arsenides, though these are not ionic compounds
containing the As3− ion as the formation of such an anion would be
highly endothermic and even the group 1 arsenides have properties of
intermetallic compounds. Like germanium, selenium, and bromine, which
like arsenic succeed the 3d transition series, arsenic is much less
stable in the +5 oxidation state than its vertical neighbors
phosphorus and antimony, and hence arsenic pentoxide and arsenic acid
are potent oxidizers.
Compounds
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Compounds of arsenic resemble, in some respects, those of phosphorus,
which occupies the same group (column) of the periodic table. The most
common oxidation states for arsenic are: −3 in the arsenides, which
are alloy-like intermetallic compounds, +3 in the arsenites, and +5 in
the arsenates and most organoarsenic compounds. Arsenic also bonds
readily to itself as seen in the square ions in the mineral
skutterudite. In the +3 oxidation state, arsenic is typically
pyramidal owing to the influence of the lone pair of electrons.
Inorganic compounds
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One of the simplest arsenic compounds is the trihydride, the highly
toxic, flammable, pyrophoric arsine (AsH3). This compound is generally
regarded as stable, since at room temperature it decomposes only
slowly. At temperatures of 250-300 °C decomposition to arsenic and
hydrogen is rapid. Several factors, such as humidity, presence of
light and certain catalysts (namely aluminium) facilitate the rate of
decomposition. It oxidises readily in air to form arsenic trioxide and
water, and analogous reactions take place with sulfur and selenium
instead of oxygen.
Arsenic forms colorless, odorless, crystalline oxides As2O3 ("white
arsenic") and As2O5 which are hygroscopic and readily soluble in water
to form acidic solutions. Arsenic(V) acid is a weak acid and its
salts, known as arsenates, are a major source of arsenic contamination
of groundwater in regions with high levels of naturally-occurring
arsenic minerals. Synthetic arsenates include Scheele's Green (cupric
hydrogen arsenate, acidic copper arsenate), calcium arsenate, and lead
hydrogen arsenate. These three have been used as agricultural
insecticides and poisons.
The protonation steps between the arsenate and arsenic acid are
similar to those between phosphate and phosphoric acid. Unlike
phosphorous acid, arsenous acid is genuinely tribasic, with the
formula As(OH)3.
A broad variety of sulfur compounds of arsenic are known. Orpiment
(As2S3) and realgar (As4S4) are somewhat abundant and were formerly
used as painting pigments. In As4S10, arsenic has a formal oxidation
state of +2 in As4S4 which features As-As bonds so that the total
covalency of As is still 3. Both orpiment and realgar, as well as
As4S3, have selenium analogs; the analogous As2Te3 is known as the
mineral kalgoorlieite,
and the anion As2Te− is known as a ligand in cobalt complexes.
All trihalides of arsenic(III) are well known except the astatide,
which is unknown. Arsenic pentafluoride (AsF5) is the only important
pentahalide, reflecting the lower stability of the +5 oxidation state;
even so, it is a very strong fluorinating and oxidizing agent. (The
pentachloride is stable only below −50 °C, at which temperature it
decomposes to the trichloride, releasing chlorine gas.)
Alloys
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Arsenic is used as the group 5 element in the III-V semiconductors
gallium arsenide, indium arsenide, and aluminium arsenide. The valence
electron count of GaAs is the same as a pair of Si atoms, but the band
structure is completely different which results in distinct bulk
properties. Other arsenic alloys include the II-V semiconductor
cadmium arsenide.
The main use of arsenic is in alloying with lead. Lead components in
car batteries are strengthened by the presence of a very small
percentage of arsenic. Dezincification of brass (a copper-zinc alloy)
is greatly reduced by the addition of arsenic. "Phosphorus Deoxidized
Arsenical Copper" with an arsenic content of 0.3% has an increased
corrosion stability in certain environments. Gallium arsenide is an
important semiconductor material, used in integrated circuits.
Circuits made from GaAs are much faster (but also much more expensive)
than those made from silicon. Unlike silicon, GaAs has a direct
bandgap, and can be used in laser diodes and LEDs to convert
electrical energy directly into light.
Organoarsenic compounds
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A large variety of organoarsenic compounds are known. Several were
developed as chemical warfare agents during World War I, including
vesicants such as lewisite and vomiting agents such as adamsite.
Cacodylic acid, which is of historic and practical interest, arises
from the methylation of arsenic trioxide, a reaction that has no
analogy in phosphorus chemistry. Cacodyl was the first organometallic
compound known (even though arsenic is not a true metal) and was named
from the Greek 'κακωδία' "stink" for its offensive, garlic-like odor;
it is very toxic.
Occurrence and production
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Arsenic is the 53rd most abundant element in the Earth's crust,
comprising about 1.5 parts per million (0.00015%). Typical background
concentrations of arsenic do not exceed 3 ng/m3 in the atmosphere; 100
mg/kg in soil; 400 μg/kg in vegetation; 10 μg/L in freshwater and 1.5
μg/L in seawater. Arsenic is the 22nd most abundant element in
seawater and ranks 41st in abundance in the universe.
Minerals with the formula MAsS and MAs2 (M = Fe, Ni, Co) are the
dominant commercial sources of arsenic, together with realgar (an
arsenic sulfide mineral) and native (elemental) arsenic. An
illustrative mineral is arsenopyrite (FeAsS), which is structurally
related to iron pyrite. Many minor As-containing minerals are known.
Arsenic also occurs in various organic forms in the environment.
In 2014, China was the top producer of white arsenic with almost 70%
world share, followed by Morocco, Russia, and Belgium, according to
the British Geological Survey and the United States Geological Survey.
Most arsenic refinement operations in the US and Europe have closed
over environmental concerns. Arsenic is found in the smelter dust from
copper, gold, and lead smelters, and is recovered primarily from
copper refinement dust. Arsenic is the main impurity found in copper
concentrates to enter copper smelting facilities. There has been an
increase in arsenic in copper concentrates over the years since copper
mining has moved into deep high-impurity ores as shallow, low-arsenic
copper deposits have been progressively depleted.
On roasting arsenopyrite in air, arsenic sublimes as arsenic(III)
oxide leaving iron oxides, while roasting without air results in the
production of gray arsenic. Further purification from sulfur and other
chalcogens is achieved by sublimation in vacuum, in a hydrogen
atmosphere, or by distillation from molten lead-arsenic mixture.
Rank !! Country !! 2014 As2O3 Production
1 25,000 T
2 8,800 T
3 1,500 T
4 1,000 T
5 52 T
6 45 T
| -- || **World Total (rounded)** || **36,400 T**
History
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The word 'arsenic' has its origin in the Syriac word 'zarnika', from
Arabic al-zarnīḵ 'the orpiment', based on Persian zar ("gold") from
the word 'zarnikh', meaning "yellow" (literally "gold-colored") and
hence "(yellow) orpiment". It was adopted into Greek (using folk
etymology) as 'arsenikon' () - a neuter form of the Greek adjective
'arsenikos' (), meaning "male", "virile".
Latin-speakers adopted the Greek term as , which in French ultimately
became , whence the English word "arsenic".
Arsenic sulfides (orpiment, realgar) and oxides have been known and
used since ancient times. Zosimos () describes roasting 'sandarach'
(realgar) to obtain 'cloud of arsenic' (arsenic trioxide), which he
then reduces to gray arsenic. As the symptoms of arsenic poisoning are
not very specific, the substance was frequently used for murder until
the advent in the 1830s of the Marsh test, a sensitive chemical test
for its presence. (Another less sensitive but more general test is the
Reinsch test.) Owing to its use by the ruling class to murder one
another and its potency and discreetness, arsenic has been called the
"poison of kings" and the "king of poisons". Arsenic became known as
"the inheritance powder" due to its use in killing family members in
the Renaissance era.
During the Bronze Age, arsenic was melted with copper to make
arsenical bronze.
Jabir ibn Hayyan described the isolation of arsenic before 815 AD.
Albertus Magnus (Albert the Great, 1193-1280) later isolated the
element from a compound in 1250, by heating soap together with arsenic
trisulfide. In 1649, Johann Schröder published two ways of preparing
arsenic. Crystals of elemental (native) arsenic are found in nature,
although rarely.
Cadet's fuming liquid (impure cacodyl), often claimed as the first
synthetic organometallic compound, was synthesized in 1760 by Louis
Claude Cadet de Gassicourt through the reaction of potassium acetate
with arsenic trioxide.
In the Victorian era, women would eat "arsenic" ("white arsenic" or
arsenic trioxide) mixed with vinegar and chalk to improve the
complexion of their faces, making their skin paler (to show they did
not work in the fields). The accidental use of arsenic in the
adulteration of foodstuffs led to the Bradford sweet poisoning in
1858, which resulted in 21 deaths. From the late 18th century
wallpaper production began to use dyes made from arsenic,
which was thought to increase the pigment's brightness. One account of
the illness and 1821 death of Napoleon implicates arsenic poisoning
involving wallpaper.
Two arsenic pigments have been widely used since their discovery -
Paris Green in 1814 and Scheele's Green in 1775. After the toxicity of
arsenic became widely known, these chemicals were used less often as
pigments and more often as insecticides. In the 1860s, an arsenic
byproduct of dye production, London Purple, was widely used. This was
a solid mixture of arsenic trioxide, aniline, lime, and ferrous oxide,
insoluble in water and very toxic by inhalation or ingestion But it
was later replaced with Paris Green, another arsenic-based dye. With
better understanding of the toxicology mechanism, two other compounds
were used starting in the 1890s. Arsenite of lime and arsenate of lead
were used widely as insecticides until the discovery of DDT in 1942.
In small doses, soluble arsenic compounds act as stimulants, and were
once popular as medicine by people in the mid-18th to 19th centuries;
this use was especially prevalent for sport animals such as race
horses or work dogs and continued into the 20th century.
A 2006 study of the remains of the Australian racehorse Phar Lap
determined that its 1932 death was caused by a massive overdose of
arsenic. Sydney veterinarian Percy Sykes stated,
: "In those days, arsenic was quite a common tonic, usually given in
the form of a solution (Fowler's Solution) ... It was so common that
I'd reckon 90 per cent of the horses had arsenic in their system."
Agricultural
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The toxicity of arsenic to insects, bacteria, and fungi led to its use
as a wood preservative. In the 1930s, a process of treating wood with
chromated copper arsenate (also known as CCA or Tanalith) was
invented, and for decades, this treatment was the most extensive
industrial use of arsenic. An increased appreciation of the toxicity
of arsenic led to a ban of CCA in consumer products in 2004, initiated
by the European Union and United States. However, CCA remains in heavy
use in other countries (such as on Malaysian rubber plantations).
Arsenic was also used in various agricultural insecticides and
poisons. For example, lead hydrogen arsenate was a common insecticide
on fruit trees, but contact with the compound sometimes resulted in
brain damage among those working the sprayers. In the second half of
the 20th century, monosodium methyl arsenate (MSMA) and disodium
methyl arsenate (DSMA) - less toxic organic forms of arsenic -
replaced lead arsenate in agriculture. These organic arsenicals were
in turn phased out in the United States by 2013 in all agricultural
activities except cotton farming.
The biogeochemistry of arsenic is complex and includes various
adsorption and desorption processes. The toxicity of arsenic is
connected to its solubility and is affected by pH. Arsenite () is more
soluble than arsenate () and is more toxic; however, at a lower pH,
arsenate becomes more mobile and toxic. It was found that addition of
sulfur, phosphorus, and iron oxides to high-arsenite soils greatly
reduces arsenic phytotoxicity.
Arsenic is used as a feed additive in poultry and swine production, in
particular it was used in the U.S. until 2015 to increase weight gain,
improve feed efficiency, and prevent disease. An example is roxarsone,
which had been used as a broiler starter by about 70% of U.S. broiler
growers. In 2011, Alpharma, a subsidiary of Pfizer Inc., which
produces roxarsone, voluntarily suspended sales of the drug in
response to studies showing elevated levels of inorganic arsenic, a
carcinogen, in treated chickens. A successor to Alpharma, Zoetis,
continued to sell nitarsone until 2015, primarily for use in turkeys.
Medical use
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During the 17th, 18th, and 19th centuries, a number of arsenic
compounds were used as medicines, including arsphenamine (by Paul
Ehrlich) and arsenic trioxide (by Thomas Fowler), for treating
diseases such as cancer or psoriasis. Arsphenamine, as well as
neosalvarsan, was indicated for syphilis, but has been superseded by
modern antibiotics. However, arsenicals such as melarsoprol are still
used for the treatment of trypanosomiasis in spite of their severe
toxicity, since the disease is almost uniformly fatal if untreated. In
2000 the US Food and Drug Administration approved arsenic trioxide for
the treatment of patients with acute promyelocytic leukemia that is
resistant to all-trans retinoic acid.
A 2008 paper reports success in locating tumors using arsenic-74 (a
positron emitter). This isotope produces clearer PET scan images than
the previous radioactive agent, iodine-124, because the body tends to
transport iodine to the thyroid gland producing signal noise.
Nanoparticles of arsenic have shown ability to kill cancer cells with
lesser cytotoxicity than other arsenic formulations.
Military
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After World War I, the United States built a stockpile of 20,000 tons
of weaponized lewisite (ClCH=CHAsCl2), an organoarsenic vesicant
(blister agent) and lung irritant. The stockpile was neutralized with
bleach and dumped into the Gulf of Mexico in the 1950s. Lewisite, the
chemical warfare agent, is known for its acute toxicity to aquatic
organisms. However, studies assessing the environmental impact of
this disposal in the Gulf are lacking. During the Vietnam War, the
United States used Agent Blue, a mixture of sodium cacodylate and its
acid form, as one of the rainbow herbicides to deprive North
Vietnamese soldiers of foliage cover and rice.
Other uses
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* Copper acetoarsenite was used as a green pigment known under many
names, including Paris Green and Emerald Green. It caused numerous
arsenic poisonings. Scheele's Green, a copper arsenate, was used in
the 19th century as a coloring agent in sweets.
* Arsenic is used in bronzing.
* As much as 2% of produced arsenic is used in lead alloys for lead
shot and bullets.
* Arsenic is added in small quantities to alpha-brass to make it
dezincification-resistant. This grade of brass is used in plumbing
fittings and other wet environments.
* Arsenic is also used for taxonomic sample preservation. It was also
used in embalming fluids historically.
* Arsenic was used in the taxidermy process up until the 1980s.
* Arsenic was used as an opacifier in ceramics, creating white glazes.
* Until recently, arsenic was used in optical glass. Modern glass
manufacturers have ceased using both arsenic and lead.
Bacteria
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Some species of bacteria obtain their energy in the absence of oxygen
by oxidizing various fuels while reducing arsenate to arsenite. Under
oxidative environmental conditions some bacteria use arsenite as fuel,
which they oxidize to arsenate. The enzymes involved are known as
arsenate reductases (Arr).
In 2008, bacteria were discovered that employ a version of
photosynthesis in the absence of oxygen with arsenites as electron
donors, producing arsenates (just as ordinary photosynthesis uses
water as electron donor, producing molecular oxygen). Researchers
conjecture that, over the course of history, these photosynthesizing
organisms produced the arsenates that allowed the arsenate-reducing
bacteria to thrive. One strain, PHS-1, has been isolated and is
related to the gammaproteobacterium 'Ectothiorhodospira
shaposhnikovii'. The mechanism is unknown, but an encoded Arr enzyme
may function in reverse to its known homologues.
:
In 2010, researchers reported the discovery of a strain of the
bacterium 'Halomonas' (designated GFAJ-1) that was allegedly capable
of substituting arsenic for phosphorus in its biomolecules, including
DNA, when grown in an arsenic-rich, phosphate-limited environment.
This claim, published in 'Science', suggested that arsenic could
potentially serve as a building block of life in place of phosphorus,
challenging long-standing assumptions about biochemical requirements
for life on Earth.
The claim was met with widespread skepticism. Subsequent studies
provided evidence contradicting the initial findings. One follow-up
study published in 'Science' in 2011 demonstrated that GFAJ-1 still
requires phosphate to grow and does not incorporate arsenate into its
DNA in any biologically significant way. Another independent
investigation in 2012 used more sensitive techniques to purify and
analyze the DNA of GFAJ-1 and found no detectable arsenate
incorporated into the DNA backbone. The authors concluded that the
original observations were likely due to experimental contamination or
insufficient purification methods. Together, these studies reaffirmed
phosphorus as an essential element for all known forms of life.
Potential role in higher animals
==================================
Arsenic may be an essential trace mineral in birds, involved in the
synthesis of methionine metabolites. However, the role of arsenic in
bird nutrition is disputed, as other authors state that arsenic is
toxic in small amounts.
Some evidence indicates that arsenic is an essential trace mineral in
mammals.
:
Experimental studies in rodents and livestock have shown that arsenic
deprivation can lead to impaired growth, reduced reproductive
performance, and abnormal glucose metabolism, suggesting it may play a
role in essential metabolic processes. Arsenic has been proposed to
participate in methylation reactions, possibly influencing gene
regulation and detoxification pathways. However, because the threshold
between beneficial and toxic exposure is extremely narrow, arsenic is
not currently classified as an essential element for humans, and its
physiological role in higher animals remains uncertain.
Heredity
==========
Arsenic has been linked to epigenetic changes, heritable changes in
gene expression that occur without changes in DNA sequence. These
include DNA methylation, histone modification, and RNA interference.
Toxic levels of arsenic cause significant DNA hypermethylation of
tumor suppressor genes p16 and p53, thus increasing risk of
carcinogenesis. These epigenetic events have been studied 'in vitro'
using human kidney cells and 'in vivo' using rat liver cells and
peripheral blood leukocytes in humans. Inductively coupled plasma mass
spectrometry (ICP-MS) is used to detect precise levels of
intracellular arsenic and other arsenic bases involved in epigenetic
modification of DNA. Studies investigating arsenic as an epigenetic
factor can be used to develop precise biomarkers of exposure and
susceptibility.
The Chinese brake fern ('Pteris vittata') hyperaccumulates arsenic
from the soil into its leaves and has a proposed use in
phytoremediation.
Biomethylation
================
Inorganic arsenic and its compounds, upon entering the food chain, are
progressively metabolized through a process of methylation. For
example, the mold 'Scopulariopsis brevicaulis' produces
trimethylarsine if inorganic arsenic is present. The organic compound
arsenobetaine is found in some marine foods such as fish and algae,
and also in mushrooms in larger concentrations. The average person's
intake is about 10-50 μg/day. Values about 1000 μg are not unusual
following consumption of fish or mushrooms, but there is little danger
in eating fish because this arsenic compound is nearly non-toxic.
Exposure
==========
Naturally occurring sources of human exposure include volcanic ash,
weathering of minerals and ores, and mineralized groundwater. Arsenic
is also found in food, water, soil, and air. Arsenic is absorbed by
all plants, but is more concentrated in leafy vegetables, rice, apple
and grape juice, and seafood. An additional route of exposure is
inhalation of atmospheric gases and dusts.
During the Victorian era, arsenic was widely used in home decor,
especially wallpapers. In Europe, an analysis based on 20,000 soil
samples across all 28 countries show that 98% of sampled soils have
concentrations less than 20 mg/kg. In addition, the arsenic hotspots
are related to both frequent fertilization and close distance to
mining activities. Chronic exposure to arsenic, particularly through
contaminated drinking water and food, has also been linked to
long-term impacts on cognitive function, including reduced verbal IQ
and memory.
Occurrence in drinking water
==============================
Extensive arsenic contamination of groundwater has led to widespread
arsenic poisoning in Bangladesh and neighboring countries. It is
estimated that approximately 57 million people in the Bengal basin are
drinking groundwater with arsenic concentrations elevated above the
World Health Organization's standard of 10 parts per billion (ppb).
However, a study of cancer rates in Taiwan suggested that significant
increases in cancer mortality appear only at levels above 150 ppb. The
arsenic in the groundwater is of natural origin, and is released from
the sediment into the groundwater, caused by the anoxic conditions of
the subsurface. This groundwater was used after local and western NGOs
and the Bangladeshi government undertook a massive shallow tube well
drinking-water program in the late twentieth century. This program was
designed to prevent drinking of bacteria-contaminated surface waters,
but failed to test for arsenic in the groundwater. Many other
countries and districts in Southeast Asia, such as Vietnam and
Cambodia, have geological environments that produce groundwater with a
high arsenic content. Arsenicosis was reported in Nakhon Si Thammarat,
Thailand, in 1987, and the Chao Phraya River probably contains high
levels of naturally occurring dissolved arsenic without being a public
health problem because much of the public uses bottled water. In
Pakistan, more than 60 million people are exposed to arsenic polluted
drinking water indicated by a 2017 report in 'Science'. Podgorski's
team investigated more than 1200 samples and more than 66% exceeded
the WHO contamination limits of 10 micrograms per liter.
Since the 1980s, residents of the Ba Men region of Inner Mongolia,
China have been chronically exposed to arsenic through drinking water
from contaminated wells. A 2009 research study observed an elevated
presence of skin lesions among residents with well water arsenic
concentrations between 5 and 10 μg/L, suggesting that arsenic-induced
toxicity may occur at relatively low concentrations with chronic
exposure. Overall, 20 of China's 34 provinces have high arsenic
concentrations in the groundwater supply, potentially exposing 19
million people to hazardous drinking water.
A study by IIT Kharagpur found high levels of Arsenic in groundwater
of 20% of India's land, exposing more than 250 million people. States
such as Punjab, Bihar, West Bengal, Assam, Haryana, Uttar Pradesh, and
Gujarat have highest land area exposed to arsenic.
In the United States, arsenic is most commonly found in the ground
waters of the southwest. Parts of New England, Michigan, Wisconsin,
Minnesota and the Dakotas are also known to have significant
concentrations of arsenic in ground water. Increased levels of skin
cancer have been associated with arsenic exposure in Wisconsin, even
at levels below the 10 ppb drinking water standard. According to a
recent film funded by the US Superfund, millions of private wells have
unknown arsenic levels, and in some areas of the US, more than 20% of
the wells may contain levels that exceed established limits.
Low-level exposure to arsenic at concentrations of 100 ppb (i.e.,
above the 10 ppb drinking water standard) compromises the initial
immune response to H1N1 or swine flu infection according to
NIEHS-supported scientists. The study, conducted in laboratory mice,
suggests that people exposed to arsenic in their drinking water may be
at increased risk for more serious illness or death from the virus.
Some Canadians are drinking water that contains inorganic arsenic.
Private-dug-well waters are most at risk for containing inorganic
arsenic. Preliminary well water analysis typically does not test for
arsenic. Researchers at the Geological Survey of Canada have modeled
relative variation in natural arsenic hazard potential for the
province of New Brunswick. This study has important implications for
potable water and health concerns relating to inorganic arsenic.
Epidemiological evidence from Chile shows a dose-dependent connection
between chronic arsenic exposure and various forms of cancer, in
particular when other risk factors, such as cigarette smoking, are
present. These effects have been demonstrated at contaminations less
than 50 ppb. Arsenic is itself a constituent of tobacco smoke.
Analyzing multiple epidemiological studies on inorganic arsenic
exposure suggests a small but measurable increase in risk for bladder
cancer at 10 ppb. According to Peter Ravenscroft of the Department of
Geography at the University of Cambridge, roughly 80 million people
worldwide consume between 10 and 50 ppb arsenic in their drinking
water. If they all consumed exactly 10 ppb arsenic in their drinking
water, the previously cited multiple epidemiological study analysis
would predict an additional 2,000 cases of bladder cancer alone. This
represents a clear underestimate of the overall impact, since it does
not include lung or skin cancer, and explicitly underestimates the
exposure. Those exposed to levels of arsenic above the current WHO
standard should weigh the costs and benefits of arsenic remediation.
Early (1973) evaluations of the processes for removing dissolved
arsenic from drinking water demonstrated the efficacy of
co-precipitation with either iron or aluminium oxides. In particular,
iron as a coagulant was found to remove arsenic with an efficacy
exceeding 90%. Several adsorptive media systems have been approved for
use at point-of-service in a study funded by the United States
Environmental Protection Agency (US EPA) and the National Science
Foundation (NSF). A team of European and Indian scientists and
engineers have set up six arsenic treatment plants in West Bengal
based on in-situ remediation method (SAR Technology). This technology
does not use any chemicals and arsenic is left in an insoluble form
(+5 state) in the subterranean zone by recharging aerated water into
the aquifer and developing an oxidation zone that supports arsenic
oxidizing micro-organisms. This process does not produce any waste
stream or sludge and is relatively cheap.
Another effective and inexpensive method to avoid arsenic
contamination is to sink wells 500 feet or deeper to reach purer
waters. A recent 2011 study funded by the US National Institute of
Environmental Health Sciences' Superfund Research Program shows that
deep sediments can remove arsenic and take it out of circulation. In
this process, called 'adsorption', arsenic sticks to the surfaces of
deep sediment particles and is naturally removed from the ground
water.
Magnetic separations of arsenic at very low magnetic field gradients
with high-surface-area and monodisperse magnetite (Fe3O4) nanocrystals
have been demonstrated in point-of-use water purification. Using the
high specific surface area of Fe3O4 nanocrystals, the mass of waste
associated with arsenic removal from water has been dramatically
reduced.
Epidemiological studies have suggested a correlation between chronic
consumption of drinking water contaminated with arsenic and the
incidence of all leading causes of mortality. The literature indicates
that arsenic exposure is causative in the pathogenesis of diabetes.
Chaff-based filters have recently been shown to reduce the arsenic
content of water to 3 μg/L. This may find applications in areas where
the potable water is extracted from underground aquifers.
San Pedro de Atacama
======================
For several centuries, the people of San Pedro de Atacama in Chile
have been drinking water that is contaminated with arsenic, and some
evidence suggests they have developed some immunity. Genetic studies
indicate that certain populations in this region have undergone
natural selection for gene variants that enhance arsenic metabolism
and detoxification. This adaptation is considered one of the few
documented cases of human evolution in response to chronic
environmental arsenic exposure.
Hazard maps for contaminated groundwater
==========================================
Around one-third of the world's population drinks water from
groundwater resources. Of this, about 10 percent, approximately 300
million people, obtains water from groundwater resources that are
contaminated with unhealthy levels of arsenic or fluoride. These trace
elements derive mainly from minerals and ions in the ground.
Redox transformation of arsenic in natural waters
===================================================
Arsenic is unique among the trace metalloids and oxyanion-forming
trace metals (e.g. As, Se, Sb, Mo, V, Cr, U, Re). It is sensitive to
mobilization at pH values typical of natural waters (pH 6.5-8.5) under
both oxidizing and reducing conditions. Arsenic can occur in the
environment in several oxidation states (−3, 0, +3 and +5), but in
natural waters it is mostly found in inorganic forms as oxyanions of
trivalent arsenite [As(III)] or pentavalent arsenate [As(V)]. Organic
forms of arsenic are produced by biological activity, mostly in
surface waters, but are rarely quantitatively important. Organic
arsenic compounds may, however, occur where waters are significantly
impacted by industrial pollution.
Arsenic may be solubilized by various processes. When pH is high,
arsenic may be released from surface binding sites that lose their
positive charge. When water level drops and sulfide minerals are
exposed to air, arsenic trapped in sulfide minerals can be released
into water. When organic carbon is present in water, bacteria are fed
by directly reducing As(V) to As(III) or by reducing the element at
the binding site, releasing inorganic arsenic.
The aquatic transformations of arsenic are affected by pH,
reduction-oxidation potential, organic matter concentration and the
concentrations and forms of other elements, especially iron and
manganese. The main factors are pH and the redox potential. Generally,
the main forms of arsenic under oxic conditions are , , , and at pH
2, 2-7, 7-11 and 11, respectively. Under reducing conditions, is
predominant at pH 2-9.
Oxidation and reduction affects the migration of arsenic in subsurface
environments. Arsenite is the most stable soluble form of arsenic in
reducing environments and arsenate, which is less mobile than
arsenite, is dominant in oxidizing environments at neutral pH.
Therefore, arsenic may be more mobile under reducing conditions. The
reducing environment is also rich in organic matter which may enhance
the solubility of arsenic compounds. As a result, the adsorption of
arsenic is reduced and dissolved arsenic accumulates in groundwater.
That is why the arsenic content is higher in reducing environments
than in oxidizing environments.
The presence of sulfur is another factor that affects the
transformation of arsenic in natural water. Arsenic can precipitate
when metal sulfides form. In this way, arsenic is removed from the
water and its mobility decreases. When oxygen is present, bacteria
oxidize reduced sulfur to generate energy, potentially releasing bound
arsenic.
Redox reactions involving Fe also appear to be essential factors in
the fate of arsenic in aquatic systems. The reduction of iron
oxyhydroxides plays a key role in the release of arsenic to water. So
arsenic can be enriched in water with elevated Fe concentrations.
Under oxidizing conditions, arsenic can be mobilized from pyrite or
iron oxides especially at elevated pH. Under reducing conditions,
arsenic can be mobilized by reductive desorption or dissolution when
associated with iron oxides. The reductive desorption occurs under two
circumstances. One is when arsenate is reduced to arsenite which
adsorbs to iron oxides less strongly. The other results from a change
in the charge on the mineral surface which leads to the desorption of
bound arsenic.
Some species of bacteria catalyze redox transformations of arsenic.
Dissimilatory arsenate-respiring prokaryotes (DARP) speed up the
reduction of As(V) to As(III). DARP use As(V) as the electron acceptor
of anaerobic respiration and obtain energy to survive. Other organic
and inorganic substances can be oxidized in this process.
Chemoautotrophic arsenite oxidizers (CAO) and heterotrophic arsenite
oxidizers (HAO) convert As(III) into As(V). CAO combine the oxidation
of As(III) with the reduction of oxygen or nitrate. They use obtained
energy to fix produce organic carbon from CO2. HAO cannot obtain
energy from As(III) oxidation. This process may be an arsenic
detoxification mechanism for the bacteria.
Equilibrium thermodynamic calculations predict that As(V)
concentrations should be greater than As(III) concentrations in all
but strongly reducing conditions, i.e. where sulfate reduction is
occurring. However, abiotic redox reactions of arsenic are slow.
Oxidation of As(III) by dissolved O2 is a particularly slow reaction.
For example, Johnson and Pilson (1975) gave half-lives for the
oxygenation of As(III) in seawater ranging from several months to a
year. In other studies, As(V)/As(III) ratios were stable over periods
of days or weeks during water sampling when no particular care was
taken to prevent oxidation, again suggesting relatively slow oxidation
rates. Cherry found from experimental studies that the As(V)/As(III)
ratios were stable in anoxic solutions for up to 3 weeks but that
gradual changes occurred over longer timescales. Sterile water samples
have been observed to be less susceptible to speciation changes than
non-sterile samples. Oremland found that the reduction of As(V) to
As(III) in Mono Lake was rapidly catalyzed by bacteria with rate
constants ranging from 0.02 to 0.3-day−1.
Wood preservation in the US
=============================
As of 2002, US-based industries consumed 19,600 metric tons of
arsenic. Ninety percent of this was used for treatment of wood with
chromated copper arsenate (CCA). In 2007, 50% of the 5,280 metric tons
of consumption was still used for this purpose. In the United States,
the voluntary phasing-out of arsenic in production of consumer
products and residential and general consumer construction products
began on 31 December 2003, and alternative chemicals are now used,
such as Alkaline Copper Quaternary, borates, copper azole,
cyproconazole, and propiconazole.
Although discontinued, this application is also one of the most
concerning to the general public. The vast majority of older
pressure-treated wood was treated with CCA. CCA lumber is still in
widespread use in many countries, and was heavily used during the
latter half of the 20th century as a structural and outdoor building
material. Although the use of CCA lumber was banned in many areas
after studies showed that arsenic could leach out of the wood into the
surrounding soil (from playground equipment, for instance), a risk is
also presented by the burning of older CCA timber. The direct or
indirect ingestion of wood ash from burnt CCA lumber has caused
fatalities in animals and serious poisonings in humans; the lethal
human dose is approximately 20 grams of ash. Scrap CCA lumber from
construction and demolition sites may be inadvertently used in
commercial and domestic fires. Protocols for safe disposal of CCA
lumber are not consistent throughout the world. Widespread landfill
disposal of such timber raises some concern, but other studies have
shown no arsenic contamination in the groundwater.
Mapping of industrial releases in the US
==========================================
One tool that maps the location (and other information) of arsenic
releases in the United States is TOXMAP. TOXMAP is a Geographic
Information System (GIS) from the Division of Specialized Information
Services of the United States National Library of Medicine (NLM)
funded by the US Federal Government. With marked-up maps of the United
States, TOXMAP enables users to visually explore data from the United
States Environmental Protection Agency's (EPA) Toxics Release
Inventory and Superfund Basic Research Programs. TOXMAP's chemical and
environmental health information is taken from NLM's Toxicology Data
Network (TOXNET), PubMed, and from other authoritative sources.
Bioremediation
================
Physical, chemical, and biological methods have been used to remediate
arsenic contaminated water. Bioremediation is said to be
cost-effective and environmentally friendly. Bioremediation of ground
water contaminated with arsenic aims to convert arsenite, the toxic
form of arsenic to humans, to arsenate. Arsenate (+5 oxidation state)
is the dominant form of arsenic in surface water, while arsenite (+3
oxidation state) is the dominant form in hypoxic to anoxic
environments. Arsenite is more soluble and mobile than arsenate. Many
species of bacteria can transform arsenite to arsenate in anoxic
conditions by using arsenite as an electron donor. This is a useful
method in ground water remediation. Another bioremediation strategy is
to use plants that accumulate arsenic in their tissues via
phytoremediation but the disposal of contaminated plant material needs
to be considered.
Bioremediation requires careful evaluation and design in accordance
with existing conditions. Some sites may require the addition of an
electron acceptor while others require microbe supplementation
(bioaugmentation). Regardless of the method used, only constant
monitoring can prevent future contamination.
Arsenic removal
=================
Coagulation and flocculation are closely related processes common in
arsenate removal from water. Due to the net negative charge carried by
arsenate ions, they settle slowly or not at all due to charge
repulsion. In coagulation, a positively charged coagulent such as iron
and aluminum (commonly used salts: FeCl3, Fe2(SO4)3, Al2(SO4)3)
neutralize the negatively charged arsenate, enable it to settle.
Flocculation follows where a flocculant bridges smaller particles and
allows the aggregate to precipitate out from water. However, such
methods may not be efficient on arsenite as As(III) exists in
uncharged arsenious acid, H3AsO3, at near-neutral pH.
The major drawbacks of coagulation and flocculation are the costly
disposal of arsenate-concentrated sludge, and possible secondary
contamination of environment. Moreover, coagulents such as iron may
produce ion contamination that exceeds safety levels.
Toxicity and precautions
======================================================================
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Arsenic and many of its compounds are especially potent poisons (e.g.
arsine). Small amount of arsenic can be detected by pharmacopoial
methods which includes reduction of arsenic to arsenious with help of
zinc and can be confirmed with mercuric chloride paper.
Classification
================
Elemental arsenic and arsenic sulfate and trioxide compounds are
classified as "toxic" and "dangerous for the environment" in the
European Union under directive 67/548/EEC.
The International Agency for Research on Cancer (IARC) recognizes
arsenic and inorganic arsenic compounds as group 1 carcinogens, and
the EU lists arsenic trioxide, arsenic pentoxide, and arsenate salts
as category 1 carcinogens.
Arsenic is known to cause arsenicosis when present in drinking water,
"the most common species being arsenate [; As(V)] and arsenite [;
As(III)]".
Legal limits, food, and drink
===============================
In the United States since 2006, the maximum concentration in drinking
water allowed by the Environmental Protection Agency (EPA) is 10 ppb
and the FDA set the same standard in 2005 for bottled water. The
Department of Environmental Protection for New Jersey set a drinking
water limit of 5 ppb in 2006. The IDLH (immediately dangerous to life
and health) value for arsenic metal and inorganic arsenic compounds is
5 mg/m3 (5 ppb). The Occupational Safety and Health Administration has
set the permissible exposure limit (PEL) to a time-weighted average
(TWA) of 0.01 mg/m3 (0.01 ppb), and the National Institute for
Occupational Safety and Health (NIOSH) has set the recommended
exposure limit (REL) to a 15-minute constant exposure of 0.002 mg/m3
(0.002 ppb). The PEL for organic arsenic compounds is a TWA of 0.5
mg/m3. (0.5 ppb).
In 2008, based on its ongoing testing of a wide variety of American
foods for toxic chemicals, the U.S. Food and Drug Administration set
the "level of concern" for inorganic arsenic in apple and pear juices
at 23 ppb, based on non-carcinogenic effects, and began blocking
importation of products in excess of this level; it also required
recalls for non-conforming domestic products. In 2011, the national
'Dr. Oz' television show broadcast a program highlighting tests
performed by an independent lab hired by the producers. Though the
methodology was disputed (it did not distinguish between organic and
inorganic arsenic) the tests showed levels of arsenic up to 36 ppb. In
response, the FDA tested the worst brand from the 'Dr.' 'Oz' show and
found much lower levels. Ongoing testing found 95% of the apple juice
samples were below the level of concern. Later testing by Consumer
Reports showed inorganic arsenic at levels slightly above 10 ppb, and
the organization urged parents to reduce consumption. In July 2013, on
consideration of consumption by children, chronic exposure, and
carcinogenic effect, the FDA established an "action level" of 10 ppb
for apple juice, the same as the drinking water standard.
Concern about arsenic in rice in Bangladesh was raised in 2002, but at
the time only Australia had a legal limit for food (one milligram per
kilogram, or 1000 ppb). Concern was raised about people who were
eating U.S. rice exceeding WHO standards for personal arsenic intake
in 2005. In 2011, the People's Republic of China set a food standard
of 150 ppb for arsenic.
In the United States in 2012, testing by separate groups of
researchers at the Children's Environmental Health and Disease
Prevention Research Center at Dartmouth College (early in the year,
focusing on urinary levels in children) and Consumer Reports (in
November) found levels of arsenic in rice that resulted in calls for
the FDA to set limits. The FDA released some testing results in
September 2012, and as of July 2013, is still collecting data in
support of a new potential regulation. It has not recommended any
changes in consumer behavior.
Consumer Reports recommended:
# That the EPA and FDA eliminate arsenic-containing fertilizer, drugs,
and pesticides in food production;
# That the FDA establish a legal limit for food;
# That industry change production practices to lower arsenic levels,
especially in food for children; and
# That consumers test home water supplies, eat a varied diet, and cook
rice with excess water, then draining it off (reducing inorganic
arsenic by about one third along with a slight reduction in vitamin
content).
# Evidence-based public health advocates also recommend that, given
the lack of regulation or labeling for arsenic in the U.S., children
should eat no more than 1.5 servings per week of rice and should not
drink rice milk as part of their daily diet before age 5. They also
offer recommendations for adults and infants on how to limit arsenic
exposure from rice, drinking water, and fruit juice.
A 2014 World Health Organization advisory conference was scheduled to
consider limits of 200-300 ppb for rice.
Reducing arsenic content in rice
==================================
In 2020, scientists assessed multiple preparation procedures of rice
for their capacity to reduce arsenic content and preserve nutrients,
recommending a procedure involving parboiling and water-absorption.
Occupational exposure limits
==============================
!Country !Limit
|Argentina |Confirmed human carcinogen
|Australia |TWA 0.05 mg/m3 - Carcinogen
|Belgium |TWA 0.1 mg/m3 - Carcinogen
|Bulgaria |Confirmed human carcinogen
|Canada |TWA 0.01 mg/m3
|Colombia |Confirmed human carcinogen
|Denmark |TWA 0.01 mg/m3
|Finland |Carcinogen
|Egypt |TWA 0.2 mg/m3
|Hungary |Ceiling concentration 0.01 mg/m3 - Skin, carcinogen
|India |TWA 0.2 mg/m3
|Japan |Group 1 carcinogen
|Jordan |Confirmed human carcinogen
|Mexico |TWA 0.2 mg/m3
|New Zealand |TWA 0.05 mg/m3 - Carcinogen
|Norway |TWA 0.02 mg/m3
|Philippines |TWA 0.5 mg/m3
|Poland |TWA 0.01 mg/m3
|Singapore |Confirmed human carcinogen
|South Korea |TWA 0.01 mg/m3
|Sweden |TWA 0.01 mg/m3
|Thailand |TWA 0.5 mg/m3
|Turkey |TWA 0.5 mg/m3
|United Kingdom |TWA 0.1 mg/m3
|United States |TWA 0.01 mg/m3
|Vietnam |Confirmed human carcinogen
Ecotoxicity
=============
Arsenic is bioaccumulative in many organisms, marine species in
particular, but it does not appear to biomagnify significantly in food
webs. In polluted areas, plant growth may be affected by root uptake
of arsenate, which is a phosphate analog and therefore readily
transported in plant tissues and cells. In polluted areas, uptake of
the more toxic arsenite ion (found more particularly in reducing
conditions) is likely in poorly-drained soils.
Toxicity in animals
=====================
!Compound !Animal !LD50 !Route
|Arsenic |Rat |763 mg/kg |oral
|Arsenic |Mouse |145 mg/kg |oral
|Calcium arsenate |Rat |20 mg/kg |oral
|Calcium arsenate |Mouse |794 mg/kg |oral
|Calcium arsenate |Rabbit |50 mg/kg |oral
|Calcium arsenate |Dog |38 mg/kg |oral
|Lead arsenate |Rabbit |75 mg/kg |oral
!Compound !Animal !LD50 !Route
|Arsenic trioxide (As(III)) |Mouse |26 mg/kg |oral
|Arsenite (As(III)) |Mouse |8 mg/kg |im
|Arsenate (As(V)) |Mouse |21 mg/kg |im
|MMA (As(III)) |Hamster |2 mg/kg |ip
|MMA (As(V)) |Mouse |916 mg/kg |oral
|DMA (As(V)) |Mouse |648 mg/kg |oral
colspan="4" |im = injected intramuscularly ip = administered
intraperitoneally
Biological mechanism
======================
Arsenic's toxicity comes from the affinity of arsenic(III) oxides for
thiols. Thiols, in the form of cysteine residues and cofactors such as
lipoic acid and coenzyme A, are situated at the active sites of many
important enzymes.
Arsenic disrupts ATP production through several mechanisms. At the
level of the citric acid cycle, arsenic inhibits lipoic acid, which is
a cofactor for pyruvate dehydrogenase. By competing with phosphate,
arsenate uncouples oxidative phosphorylation, thus inhibiting
energy-linked reduction of NAD+, mitochondrial respiration and ATP
synthesis. Hydrogen peroxide production is also increased, which, it
is speculated, has potential to form reactive oxygen species and
oxidative stress. These metabolic interferences lead to death from
multi-system organ failure. The organ failure is presumed to be from
necrotic cell death, not apoptosis, since energy reserves have been
too depleted for apoptosis to occur.
Exposure risks and remediation
================================
Occupational exposure and arsenic poisoning may occur in people
working in industries involving the use of inorganic arsenic and its
compounds, such as wood preservation, glass production, nonferrous
metal alloys, and electronic semiconductor manufacturing. Inorganic
arsenic is also found in coke oven emissions associated with the
smelter industry.
The conversion between As(III) and As(V) is a large factor in arsenic
environmental contamination. According to Croal, Gralnick, Malasarn
and Newman, "[the] understanding [of] what stimulates As(III)
oxidation and/or limits As(V) reduction is relevant for bioremediation
of contaminated sites (Croal). The study of chemolithoautotrophic
As(III) oxidizers and the heterotrophic As(V) reducers can help the
understanding of the oxidation and/or reduction of arsenic.
Treatment
===========
Treatment of chronic arsenic poisoning is possible. British
anti-lewisite (dimercaprol) is prescribed in doses of 5 mg/kg up to
300 mg every 4 hours for the first day, then every 6 hours for the
second day, and finally every 8 hours for 8 additional days. However
the USA's Agency for Toxic Substances and Disease Registry (ATSDR)
states that the long-term effects of arsenic exposure cannot be
predicted. Blood, urine, hair, and nails may be tested for arsenic;
however, these tests cannot foresee possible health outcomes from the
exposure. Long-term exposure and consequent excretion through urine
has been linked to bladder and kidney cancer in addition to cancer of
the liver, prostate, skin, lungs, and nasal cavity.
See also
======================================================================
* Aqua Tofana
* 'Arsenic and Old Lace'
* Grainger challenge
* Hypothetical types of biochemistry
External links
======================================================================
* [
https://www.who.int/news-room/fact-sheets/detail/arsenic WHO fact
sheet on arsenic]
*
[
https://www.cancer.gov/about-cancer/causes-prevention/risk/substances/arsenic
Arsenic] Cancer Causing Substances, U.S. National Cancer Institute.
* [
http://ctdbase.org/detail.go?type=chem&acc=D001151 CTD's
Arsenic page] and
[
http://ctdbase.org/detail.go?type=chem&acc=D001152 CTD's
Arsenicals page] from the Comparative Toxicogenomics Database
*
[
http://www.clu-in.org/contaminantfocus/default.focus/sec/arsenic/cat/Overview/
Contaminant Focus: Arsenic] by the EPA.
* [
http://www.inchem.org/documents/ehc/ehc/ehc224.htm Environmental
Health Criteria for Arsenic and Arsenic Compounds, 2001] by the WHO.
* [
https://www.cdc.gov/niosh/topics/arsenic/ National Institute for
Occupational Safety and Health - Arsenic Page]
License
=========
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License URL:
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Original Article:
http://en.wikipedia.org/wiki/Arsenic
