Arsenic Poisoning –
Effects on Human Health
Abstract:
Arsenic has different toxicological properties
dependent upon both its oxidation state for inorganic compounds as well as the
different toxicity levels exhibited for organic arsenic compounds. Some
currently used chromatographic and mass spectrometry applications as well as
sample handling procedure necessary to detect and quantify arsenic in its
various chemical forms.This article describes the essential background and toxicity of arsenic in the environment.
Introduction
Arsenic and its compounds are ubiquitous in nature and
exhibit both metallic and non metallic properties. It is known that arsenic has
different toxicological properties dependent upon both its oxidation state for
inorganic compounds as well as the different toxicity levels exhibited for
organic arsenic compounds. Arsenic appears in nature primarily in the form of
sulfides in association with ores of silver, lead, copper, nickel, antimony,
cobalt and iron. Trace amount of arsenic are found in soils and other
environmental media. Arsenic is released
to the atmosphere from both natural and anthropogenic sources. The principal
source is volcanic activity. Man-made emissions to air arise from the smelting
of metals, the contribution of fuels and use of pesticides. World production of
arsenic kept rising mid-1940s. As arsenic pesticides, especially insecticides,
were gradually replaced by the other preparations, the production of arsenic
declined. Arsenic is still used in production of agricultural chemicals. Arsenic
is an active component of antifungal wood preservatives. It is also used in the
pharmaceutical and glass industries and in the manufacture of poisonous
baits. Arsenicals are used in the
manufacture of pigments while metallic arsenic is used in the manufacture of
alloys. Gallium arsenide and indium arsenide are used in the production of
certain semiconductor devices. This article describes toxicity and some
currently used chromatographic mass spectrometry applications to detect and
quantify arsenic in its various chemical forms.
Forms of Arsenic
The trivalent and pentavalent forms are the most common
oxidation states. From both the biological and the toxicological points of
view, arsenic compounds can be classified into three major groups
·
Inorganic arsenic compounds
·
Organic arsenic compounds
·
Arsenic gas
The most common trivalent inorganic arsenic compounds are
arsenic trioxide, sodium arsenite and arsenic trichloride. Pentavalent
inorganic compounds include arsenic peroxide, arsenic acid and arsenates such
as lead arsenate and calcium arsenate. Common organic arsenic compounds are
arsanilic acid, methylarsonic acid, dimethylarsinic acid (cacodylic acid) and
arsenobetaine. Arsenic trioxide is only slightly soluble in water, in sodium
hydroxide it forms arsenite and with concentrated hydrochloric acid it forms
arsenic trichloride. Sodium arsenite and sodium arsenate are highly soluble in
water. Interchange of valence state may occur in aqueous solutions depending on
the pH and on the presence of other substances which can be reduced or
oxidized. Arsenic is mainly transported in the environment by water. In
oxygenated water arsenic usually occurs as arsenate but under reducing
conditions e.g. in deep well-water, arsenates predominates. In water the methylation
of inorganic arsenic to methyl and dimethyl arsenic acids (DMA) is associated
with biological activity. In oxygenated soil inorganic arsenic is present in
the pentavalent form. Under reducing conditions it is in the trivalent form.
Toxicokinetics
The major routes of arsenic absorption in the general
population are ingestion and inhalation. Human and animal data indicate that
over 90 % of the ingested dose of dissolved inorganic trivalent or pentvalent
arsenic is absorbed from the gastrointestinal tract. Organic arsenic compound
in sea food are also readily absorbed. Factors affecting the extent of
absorption from the lungs includes the chemical form , particles size ad
solubility. Air borne arsenic is usually in the forms of arsenic trioxide. Particles of more than 10 micrometer in
aerodynamic diameter are predominantly deposited in the upper airways
(nasopharynx), particles of between 5 and 10 micrometer are deposited in the
airways cleaned by mucociliary action and particles with diameter of less than
2 micrometer penetrate significantly into the alveoli. The overall absorption
as a proportion of the inhaled dose was about 30-35 % while the amount of
arsenic excluded in urine was about 40-60 % of the estimated inhaled dose.
Blood is the main vehicle for the transport of arsenic
following absorption and arsenic is cleared relatively rapidly from it. In
humans, skin, excretory and storage organs such as nails and hair, have the
highest concentration. Arsenic in the kidneys, liver, bile, brain, skeleton,
skin and blood are 2-2.5 times higher for the trivalent than for the
pentavalent form, and are greatly increased at higher doses.
Trivalent inorganic arsenic is oxidized in vivo in animals
and human exposed to arsenite. The reduction of arsenate to arsenite, has been
demonstrated in mice and rabbits. Both arsenite and arsenate are methylated in
the liver. Both methylated species MMA and DMA are considered to be less toxic
and to bind to tissues and are eliminated more rapidly than the unmethylated
form. On average, 20-25% of inorganic arsenic remains unmethylated.
Effects on Human Health
As a consequence of the many different uses of arsenic and
arsenicals, there is wide spectrum of situation in which human may be exposed
to the element. Arsenic in air is present mainly in particulate forms as
inorganic arsenic. Particulate arsenic compounds may be inhaled, deposited in
the respiratory tract and absorbed into the blood. Inhalation of arsenic from
ambient air is usually a minor exposure route for the general population.
Tobacco smoke may contain arsenic especially when the tobacco plants have been
treated with lead arsenate insecticide. The use of arsenic pesticides is now
prohibited in most countries. Occupation exposure to arsenic occurs primarily
among workers in the copper smelting industry, at power plant burning
arsenic-rich coal and using or producing pesticides containing arsenic.
Inhalation exposure to arsenic can also take place during production of gallium
arsenide in the microelectronics industry and metals ore mining.
Drinking water may contribute significantly to oral intake
in regions where there are high arsenic concentrations in well-water or
river-water or mine drainage areas. The concentration in ground water depends
on the arsenic content of the bed-rock.
With the exception of some kind of sea food, most foods contain low
levels of arsenic, normally less than 0.25 mg/kg. Marine organisms may contain
large amount of organo-arsenicals (arsenobetaine). These arsenic derivatives
are not acutely toxic because of their low biological reactivity and their
rapid excretion in urine. Hence, the amount of arsenic ingested daily by humans
via food is greatly influenced by the amount of sea food in the diet.
Recently, the
research done by the Indian Council of Agricultural Research (ICAR) of India
was published showing that 90 % of arsenic-laden water in affected areas (96
districts) was used for irrigation and it was found that some vegetables such
as brinjal, potato, radish, cauliflower had high quantity of arsenic. Arsenic
contamination is spreading fast and entering the food chain through farm
products in the region. As people take contaminated water along with
contaminated food, the chances of damage become greater. The report says total
number of deaths so far is one lakh people, with over three lakh suffering from
arsenic-related disease.
There are three population groups at high exposure risk:
- ·
People drinking water and food products with
abnormally high concentrations of arsenic
- ·
The occupationally exposed people
- ·
Children living in close vicinity of smelters
The clinical picture of chronic poisoning with arsenic
varies widely. It is usually dominated by changes in the skin and mucous
membranes and by neurological, vascular and haematogical lesions. Arsenic and
its inorganic compounds have been known to be neurotoxic. The skin is a common
critical organ in people exposed to inorganic arsenical compounds. Eczematoid
symptoms develop with varying degrees of severity. Hyperkerotosis, warts and melanosis of the
skin are the most commonly observed lesions in chronic exposure. There is
sufficient evidence that inorganic arsenic compounds are skin and lung
carcinogen in human. There are many arsenic compounds both organic and
inorganic in the environment. Inorganic arsenic can have acute, subacute and
chronic effects which may be wither local or systemic. Lung cancer is
considered to be critical effect following inhalation. A WHO working group on
arsenic conducted a quantitative risk assessment for arsenic, assuming a linear
relationship between the cumulative arsenic dose and the relation risk of
developing lung cancer from inorganic arsenic exposure were based on the study
by Pinto et al.
Speciation Analysis
This is the analytical activity of identifying and/or
measuring in a sample the quantity of one or more individual chemical species.
The need to define and measure chemical species of an element lies in the fact
that physiological factors such as toxicity, bioavailability, mobility and
reactivity are dependent on the specific form of an element. Chemical species
can be free ions, inorganic complexes, organometallic compounds, biomolecules
and structures of high molecular mass. The field of arsenic speciation analysis
has grown rapidly in recent years, especially with the utilization of
high-performance liquid chromatography (HPLC) coupled to ICP-MS, a highly
sensitive and robust detector system. Complete characterization of arsenic
compounds is necessary to understand intake, accumulation, transport, storage,
detoxification and activation of this element in the natural environment and
living systems.
Conclusions
As a consequence of the many different uses of arsenic and
arsenicals, there is wide spectrum of situation in which human may be exposed
to the element. The clinical picture of chronic poisoning with arsenic varies
widely. Arsenic has different toxicological properties dependent upon both its
oxidation state for inorganic compounds as well as the different toxicity
levels exhibited for organic arsenic compounds. The field of arsenic speciation
analysis has grown rapidly in recent years, especially with the utilization of
high-performance liquid chromatography (HPLC) coupled to ICP-MS.
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