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ABSTRACT: Exposure to
environmental pollutants
is an important problem
of environmental
toxicology. Heavy
metals are regarded as
toxic to living
organisms because of
their tendency to
accumulate in selected
tissues. Moreover,
their presence is a
causative agent of
various sorts of
disorders, including
neuro-, nephro-, carcino-,
terato-,
and immunological.
Exposures of human to
environmental chemicals
can occur simultaneously
from various
sources. One exposure
route is ingestion of
hazardous chemicals
through contaminated
food and beverages.
Considering
the above-mentioned
menace, efforts should
be focused on the
estimation of dietary
intakes of potential
toxic agents by
consumers. Dietary
exposure assessment to
nonnutrients is usually
performed by combining 2
sets
of data—the
concentration of
elemental contaminants
in various food products
and the consumption data
of these
food items. A variety of
approaches exist for
evaluating exposure to
food chemicals, and the
method chosen is
influenced,
among others, by the
intended goal, the
availability of data,
cost, and time
frame.Moreover, it is
also important
to note how accurate and
detailed the information
concerning toxic
elements intake needs to
be. There are a
number of sources of
food consumption data
currently used in
exposure
assessments,which range
from1 d to habitual
intake. Frequently, the
heavy metals for which
dietary exposure is of
interest are present in
trace and ultra-trace
quantities. Hence, an
analytical technique
with sufficient
sensitivity is required
for the accurate
determination of
these chemicals in food
samples. It is important
to remember that the
accuracy of quantitative
analysis is strongly
dependent on the
sampling and preparation
steps.
Keywords: dietary
exposure, food analysis,
food contamination,
heavy metals, risk
assessment
Introduction
Contamination of food
products by heavy metals
is becoming
an unavoidable problem
these days. Air, soil,
and water pollution
are contributing to the
presence of harmful
elements, such as
cadmium, lead, mercury,
and arsenic in
foodstuff. The
occurrence
of heavy metals-enriched
ecosystem components,
firstly, arise from
rapid industrial growth,
advances in agricultural
chemicalization,
or the urban activities
of human beings. These
agents have led to
metal dispersion in the
environment and,
consequently, impaired
health of the population
by the ingestion of
victuals contaminated
by harmful elements.
One of the most glaring
examples of metal
poisoning by industrial
water pollution
isMinamata disease. This
illness’ occurrences,
which took place at
Minamata City (1956) and
then at Niigata City
(1965) in Japan, were
attributed to the intake
of fish and zoobenthos
contaminated with methyl
mercury (Kudo and others
1998).
The above-mentioned
organic form of mercury
is the most prevalent
and particularly toxic
compound, which is
bioaccumulated
and passed up the food
chain.
As mentioned previously,
metal contamination can
be influenced
by factors ranging from
environmental conditions
during
growth to the
processing, handling,
and storage of food
products
(Morgan 1999). Contact
between food and the
coat metal surface
of packing containers or
the processing equipment
is a significant
source of toxic
contamination in food.
Thus, the presence of
these
elements in crops
originating, for
example, directly from
harvest,
can undergo a change
owing to the effects of
food processing. A
number of papers
presented data about the
influence of some
processing
factors on the content
of metals in foodstuffs
(Kroyer 1995;
Atta and others
1997;Watzke 1998; Ersoy
and others 2006).
The toxicity of heavy
metals for humans is
mainly caused by
their persistence in the
environment and hinged
strongly upon
the chemical form in
which they are ingested.
In this regard,
inorganic
arsenic (As3+) and
organic forms of mercury
(methyl- and
dimethyl mercury) belong
to the most toxic forms
of discussedmetals.
Nonetheless, all other
forms of mercury are
toxic (Gochfeld
2003). As distinct from
acutely toxic trivalent
arsenic trioxide,
elemental
as well as organic
arsenic compounds (with
the exception
of methylarsonic acid
[MMA] and
dimethylarsinic acid
[DMA]) are
considered to be
virtually nontoxic.
According to a report
published
by the U.S. EPA,
alkyl-lead compounds
(especially
tetraethyllead
[TEL] and
tetramethyllead [TML])
have a higher toxicity
than inorganic
forms of lead (USEPA
1999). The lipophilic
nature of alkyllead
and its ability to
permeate
biologicalmembranes
render these
compounds readily
bioavailable. Unlike
particularly toxic forms
of
described metals, all
inorganic forms of
cadmium have the same
toxic endpoints;
however, the toxicity
depends upon whether it
is
ingested or inhaled.
Cadmium is not as toxic
via oral routes as
through inhalation
(Robson 2003).
There are a number of
different factors that
can influence a metals’
toxicity (Table 1), but
generally the poisonous
effect of heavy
metals is a function of
a concentration which is
attained in a target
organ in the human body.
Thus, similar effects
will occur following
long- and
short-termexposure to
low and high cadmium
levels, respectively
(Wang 1984).
Consequently, both acute
and chronic systemic
toxic effectsmay be
induced.
Among possible target
organs of heavy metals,
soft tissues such
as the kidney and liver
and the central nervous
system appear to be
especially sensitive
(Apostoli 2002).
Considering the
above-mentioned problem,
efforts should be
focused
on the evaluation of
dietary intakes of heavy
metals by consumers
in order to assess risk.
Whenever feasible,
monitoring data from
dietary intake studies
should be compared with
the acceptable or
tolerable daily intake
(ADI/TDI) recommended by
the Joint FAO/WHO Expert
Committee
on Food Additives
(JECFA). The ADI refers
to substances that
can be ingested daily
without substantial
health risks, whereas
TDI
is used to emphasize the
importance of limiting
the daily intake
of food contamination
over a period of
lifetime without
appreciable
health risk. Regulation
for food contaminants
with cumulative
properties is distinct
fromADI/TDI guidelines.
In the case of several
heavy metals, such as
As, Pb, Cd, Hg, which
are able to accumulate
Factors
physical and chemical
properties (among others
chemical forms of
element)
element interaction
formation of compounds
or complexes among metal
and other metalloids
interchange of metal
bounds to proteins
sources and sinks,
environmental transport,
and transformation
influence of
concentration and other
exposure variables (for
example: time, route,
pattern of exposure,
bioavailability)
nutritional status
taking drugs such as
alcohol and nicotine
natural chemicals,
unintentionally present
in foodstuff
2. intentionally added,
to modify food
properties (for example,
food additives and
pesticides; Nasreddine
and Parent-Massin
2002)
Considering that food is
a particularly important
source of the
overall metals exposure,
undertaking a risk
assessment appears to
be justified.
Risk assessment, as a
part of risk analysis,
is defined as the
process
of evaluating the
possibility of adverse
health effects that may
occur as a consequence
of exposure to a hazard.
This scientifically
based process consists
of hazard
identification, hazard
characterization,
exposure assessment, and
risk characterization
(Figure 1).
At the beginning of the
risk assessment process,
the sources and
types of potential
hazard should be
considered and the
ensuing appropriate
monitoring ought to be
established. For this
purpose, various
factors, such as:
the threshold amount of
toxic contaminants
(PTWI) that results
in no adverse health
effects,
the relationship between
the exposure level and
frequency of adverse
health effects
(dose–response
assessment),
the severity of the
health effects, by
considering multiple
biological
endpoints (for example,
morbidity, fatalities),
which have an effect on
the health threat,
should be taken into
consideration.
Finally, it is important
to emphasize the
qualitative description
and quantitative
evaluation of the intake
of potential toxic
agents
via food.
Intake measurement
A quantitative
evaluation of exposure
can be realized using:
direct assessment,
including the following
approaches: “point–
of–contact measurement”
(duplicate diet), and
“reconstruction”
(biomarkers),
indirect assessment,
including “scenario
evaluation” (Kroes and
others 2002).
Both of the
above-mentioned
approaches are mutually
distinct.
The indirect assessment
is a modeling approach,
whereas in the
case of direct
assessment, the
measurement of exposure
is done
directly using personal
monitoring. Each
approach relies on
differ-
Risk characterization
Integration of hazard
identification, hazard
characterization and
exposure assessment in
order to provide a risk
estimate
Exposure assessment
Providing an assessment
of the occurrence
and level of toxins in a
specified portion of
food at the time of
consumption
Hazard characterization
Description of the
adverse health
effects that may arise
from the
ingestion of toxic
agents
intake of specific
age-sex groups in the
population. TDS surveys
involve purchasing
samples of food items
representing products
most commonly consumed
in a defined population,
preparing the
foods in accordance with
standard household
procedures, combining
the foods into food
composites or
aggregates, and
analyzing
each food group to
measure the levels of
selected food
contaminants.
The combination of the
toxic concentrations
obtained in the
food products analyzed
and the information on
consumption permit
the estimation of
dietary exposure in
population groups. The
main asset of the total
diet surveys is the
capacity to monitor
trends
of contaminant exposure
without burdening the
participants of
the study. As mentioned
previously, the TDS
approach covers only
a limited number of
foods (representative of
diets) in a studied
population. Therefore,
the important foods for
contaminants with
an unusual distribution
(such as mercury) may
not be included
within the foods tested.
On the other hand, the
levels of these
compounds in the foods
investigated may not be
representative.
Finally, the assessment
of heavy metals intake
may be either underor
overestimated.
Furthermore, total diet
study provides
information
that is understandable
for use by government
surveillance
programs.
Both food supply data
and household surveys
provide only
rough estimates of foods
available.However, they
cannot be used to
determine the intakes of
individuals. The main
advantage of these
approaches is no
requirements for costly
stages connected with
burdensome foodstuffs
sampling and their
subsequent analysis.
Consequently, it
contributes to a lack of
knowledge to the point
of
actual exposure levels.
Food supply studies
(also known as food
balance sheets [FBS])
have been developed
because of the need for
assessing the amount
of food and nutrients
available for human
consumption. FBS data
depict a comprehensive
picture of the pattern
of a country’s food
annually consumed by the
population. Information
on foods quantities
is collected based on a
calculation of the
annual amount of
foodstuffs produced in a
country, changes in
stocks and imports
and exports transactions
during a specified
period. Such
measurements
are valuable due to the
realizability of
assessing the overall
deficiencies or
surpluses in the food
supply in the country.
However,
food balance data do not
indicate any differences
between
food consumption levels
between different
regions, different
occupations,
or at different income
levels.
A food frequency
questionnaire
(FFQ)method is utilized
as a tool
in assessing the
frequency of individual
foods or food group’s
intake
over extended periods of
time (weeks, months, or
years). The
underlying principle of
this approach is the
possibility of a
longterm
evaluation of a diet,
which may provide more
valuable information
for estimating average
exposure to chemicals
than shortterm
methods. An FFQ consists
of a structured list of
foods and a
frequency of its
consumption by
respondents. Moreover,
the questionnaire
may include questions
regarding the amount of
food
consumed, usual food
preparation methods, use
of dietary supplements,
and so forth. In
assessing exposure to
deleterious substances,
the FFQ enables the
obtainment of useful
information on
the consumption of
particular types of
foodstuffs containing a
high
content of toxicants.
Consequently, FFQ
approaches are
increasingly
used to measure dietary
intakes in epidemiologic
studies of
chronic disease (Tucker
2007). Dietary history
methods are designed to
assess an ordinary
individual’s
total food consumption
and meal pattern. The
main purpose
of this approach is to
obtain a picture of
dietary consumption
habits, which seems to
be more related to
slowly developing
diseases
than intake over a short
time, which does not
reflect habitual
nutrition. However,
Hoffman and others
(2002) have reported
that
a repetition of
short-term measurements
(24-h recalls),
performed
on 2 nonconsecutive
sampling days, can be
used to describe the
habitual dietary intake
distribution in food
consumption surveys.
The interviewer’s task
is to elicit detailed
information on the
types of foods and
beverages commonly eaten
over an extended
time period, which is
often a “typical
week.”Moreover, dietary
survey
questionsmay apply to
recall the meals eaten
during the previous
24 h. In some cases,
participants are also
asked to report a 3-d
estimated record
(Committee on Diet and
Health 1989).
Distinct from FFQs, data
of dietary history
possess limitations
in estimating dietary
exposure of the
population. Dwyer and
others
(1987) have demonstrated
that respondents
participating in a
study may both fail to
report foods actually
eaten and recall foods
that were never eaten.
Consequently,
information about kinds
and
amounts of food consumed
can be inaccurate.
Each of the described
methods used for dietary
heavy metal uptake
has some advantages and
limitations (Table 4),
some methods
being better than others
for a specific goal. The
final choice
of the approach depends
to a high degree also on
the population
size study. Research
comprising a relatively
small number of
participants
may use resources of
intensive methods (such
as a duplicate
diet) for measurement of
metal intake. Employment
of the
mentioned approaches is
useful also in the case
of dietary assessment
in collective nutrition
in different sectors,
that is, hospitals,
old people’s home, and
so forth. Nevertheless,
it should be remembered
that in this field of
surveys, dietary
exposure to heavy metals
is representative only
for a specific
population subgroup. For
comprehensive national
nutrition surveys, the
efficiency of dietary
methods in terms of time
and cost was shown to
affect their choice
(Pennington 1998). Among
the methods applied for
large-scale
studies, the most
popular approach was the
total diet study (Berry
and Johnson 1997).
Dietary sources of heavy
metals
Data from the literature
on the dietary metal
intake in various
countries showgreat
variation (Marzec and
Bulinski 1990; Johansen
and others 2000; Schrey
and others 2000; Erzen
and others 2002). It
is obvious that food
choice, like any complex
human behavior, is
influenced by many
interrelated factors,
including various
physiological,
social, and cultural
factors. Nutritional
customs can vary
locally, regionally, and
nationally.
Johansen and others
(2004) evaluated the
dietary intake of
chemical contaminants in
Greenland. It has been
reported that
inhabitants
of Greenland are more
exposed to contaminants
from
their diet than people
in Europe andNorth
America. It was also
recognized
that seal blubber, seal
muscle, seal kidney, and
whale blubber
are the dominant
contributors of
contaminants in this
diet. The
study suggests that
avoiding or limiting the
consumption of specific
diet items will
contribute to a
reduction of the
contaminants’
intake.
Data assembled by
Peterson (1995) pointed
that the major
sources contributing to
dietary cadmium intake,
in almost all countries,
are potatoes and
cereals. In the case of
Sweden, the consumption
of hand-peeled shrimps
with the viscera was
related to high
intakes of cadmium. The
results of the study
presented by Mu˜noz
and others (2005) showed
that fish and shellfish
consumption has
the greatest
contribution in total
cadmium intake.
Currently, the most
common route of exposure
to lead, in countries
where leaded gasoline
has been banned, is
through food (CDC
1999), whereas the
amounts of Pb in
foodstuffs originating
from
plants are found to be
higher than those from
animals (Kr´ıˇzov´a
and others 2004;Mu˜noz
and others 2005). Unlike
lead, themercury
content in food
originates from
animals—fish and
shellfish are the
main sources of this
toxicant in human beings
(Llobet and others
2003; Mu˜noz and others
2005). Available data on
the content of arsenic
in food indicate that,
similar to Hg, fish and
seafood tend to
concentrate
environmental arsenic
(Robberecht and others
2002;
Llobet and others 2003;
Mu˜noz and others 2005).
Nevertheless, the
great majority of
seafood derived arsenic
compounds occur in less
toxic organic forms.
In addition, the
consumption distinctions
between different
consumer groups (for
instance, between vegans
and vegetarians;
infants and adults, and
so forth) have been
observed. Data on the
diversification of metal
intake in different
population groups have
been reported in a
number of countries
(Stanek and others 1998;
Wilhelm and others 2002;
Llobet and others 2003).
Data demonstrated
by the Natl. Food Agency
of Denmark
(Levnedsmiddelstyrelsen
1990) showed that the
daily intake of cadmium
for men
is higher than for
women.
Analytical procedure in
food analysis
Frequently, the heavy
metals for which dietary
exposure is of
interest are present in
trace and ultra-trace
quantities. Hence, an
analytical technique
with sufficient
sensitivity is required
for the
accurate determination
of these chemicals in
food samples. The
major techniques
employed for heavy metal
analysis are flame
atomic absorption
spectrometry (FAAS),
graphite furnace atomic
absorption spectrometry
(GFAAS), cold vapor
atomic absorption
spectrometry (CVAAS),
inductively coupled
plasma atomic emission
spectrometry (ICP-AES),
and inductively coupled
plasma mass
spectrometry (ICP-MS)
(Table 5). Besides those
mentioned previously,
many other techniques
such as differential
pulse cathode
stripping
voltamperometry (DPCSV)
have also been shown
as an excellent tools
for the trace and
ultra-trace analysis
(Inam
and Somer 2000; Szefer
and Nriagu 2007). The
final choice of the
method often depends on
several factors,
including detection
capability,
desired speed, accuracy,
and sensitivity of
assay, simplicity
of use, and cost of
analysis. Thus, although
ICP-MS offers the
advantages over, for
example, FAAS, of lower
detection limits and
possibility of
simultaneous
multi-element analysis,
in case of determination
only one or few
elements, the less
expensivemethod, that
is, flame atomic
absorption spectrometry,
will be more suitable.
Neutron activation
analysis (NAA) is a less
frequently used
technique
in the determination of
metals in food samples
because of
the necessity of
accessing the reactor.
Furthermore, NAA cannot
be used to determine
some elements, for
example, Pb. Despite the
mentioned negative
features, neutron
activation analysis
possesses
a number of specific
advantages and
possibilities. Among
these are
lack of necessity of
chemical destruction (in
the case of instrumental
neutron activation
analysis [INAA]),
appropriate accuracy,
possibility
of simultaneous
quantification of many
elements both in
very small samples (a
few milligrams) and very
large samples (up
to several kilograms).
As distinct from INAA,
in the spectroscopic
methods, like
in most determinative
techniques for elements,
food samples
must be prepared Both
sampling and preparation
are usually the most
important
steps of any analytical
procedure (Namie´snik
2001, 2002). Errors
committed at these
stages influence the
quality of the final
results
and cannot be corrected
during further analysis.
Directions
for proper sample
collection have been
widely described in many
studies (Horwitz 1990;
Keith 1991; Markert
1995; Melcher and others
1996; Stoeppler 1997).
There is a plethora of
sampling tools, which
depend greatly on
the samples to be
analyzed and expected
concentrations of
analytes
to be determined in
food. Considering the
character of the
targeted
compounds, the
collection of food
samples for heavy metal
determination has to be
carried out by applying
carefully selected
and cleaned vessels. A
suitable choice of
vessels is important to
minimize the potential
risks of contamination,
especially in most
cases, where the
investigated analytes
are present in very low
concentration
levels in food samples.
The basic requirement is
maintaining
the representative
character of the
collected food sample to
be analyzed. The
following factors may
affect the contents of
the
analytes in collected
food sample:
contamination derived
from handling of
foodstuffs,
adsorption of metal ions
on the vessels wall and
instruments
used for analysis,
desorption of harmful
chemicals to be analyzed
from contaminated
walls of containers as
well as from the tools
and devices
used during laboratory
analysis,
contamination of the
sample with compounds
from reagents
used during different
steps of the analytical
procedure.
To counteract chemical
composition changes in
samples, several
basic principles should
be followed (Boutron
1990):
use of a clean
laboratory, together
with a very careful
choice of
labware, ultra-pure
water, and reagents of
high purity,
all standard solutions
should be acidified and
storage in a
refrigerator
or freezer,
prudent choice of
laboratory materials
used during the various
stages of the analytical
procedure,
cleaning of all the
containers and the
labware using acid
cleaning
baths,
storage of food samples
for a short time in a
refrigerator, and for
longer periods in a
freezer.
One of the most critical
steps in the analytical
process that can
affect heavy metal
content in food samples
is sample preparation.
In most cases, treatment
stage includes several
operations, such
as drying (most often by
freezing),
homogenization,
grinding, subsampling,
digestion, and
dissolution. Each of
these stagesmay be a
potential source of
contamination; thus a
great attention should
be
paid to prevent the
original chemical
constitution of the
sample.
The sources of major
errors are most often
related to the materials
from which the vessels
and labware are made as
well the purity
of reagents used during
sample treatment.
Buldini and others
(2002) have reported the
average content of
some elements in the
materials frequently
used for storage vessels
and other equipments.
According to these data,
laboratory tools
made from isostatic
molded
polytetrafluoroethylene
(PTFE) and
ultra-pure quartz appear
to be best suited for
metal determinations.
The least proper seems
to be a pyrex glass,
which, compared
to PTFE and quartz,
contains a considerably
higher level of metals.
For example, the content
of As in ultra-pure
quartz averages about
0.1 μg/kg whereas in
pyrex glass ranges
from500 to 20000 μg
As/kg.
Because of heterogeneous
food samples collected
for analysis,
many operations have to
be carried out to obtain
a more representative
subsample. Dependent on
the type of samples,
analytical
portions of foods are
drying either before the
homogenization
process or after this
step. Removing water
from the food sample
allows the obtainment of
a material that is
easier comminuted
by suitable equipment.
Unfortunately the drying
process can contribute
to removal of more
compounds than just
water from of the
fresh food sample and
consequently alter the
initial chemical
composition
of the sample.
Homogenization apparatus
may also be a
source of contamination.
The impact of
comminution on changing
the chemical composition
of the original sample
is often underestimated.
The most known reasons
are the abrasion of the
surface
of the instrument and
application of
unsuitable equipment’s
material
(Cubadda and others
2001). Among others,
plastic and equipment
with titanium blades
instead of stainless
steel are the most
appropriate materials
for homogenization
apparatus, which has a
contact with the food
sample.
Most commonly used
techniques to
quantitative analysis of
heavy metal in food
samples require the
complete digestion of
the
material. It is evident
that there is a range of
methods that may be
used to destroy the
composite organic
matrix. Generally,
sample
matrix decomposition can
be carried out by the
dry ashing or wet
digestion procedures.
Comparison of methods
for mineralization
of food samples is
presented in Table 6.
Regardless of the type
of mineralization, all
manner of different
decompositionmethods can
contribute to losses of
some elements,
for example, by
volatilization.
The problem of
volatilization of some
elements of interest,
often
encountered during dry
ashing (these
principally concern
arsenic
and mercury [Hoenig and
others 1998; Vassileva
and others 2001]),
may be avoided by
addition of ashing aids.
Typically a variety of
ashing aids, for
example, magnesium
nitrate (Mg(NO3)2 and
magnesium
oxide (MgO), are
used.Moreover, data
reported in the
literature
showthat the attainment
of ashing temperature by
a slowgradient
allows avoiding losses
of analytes. Another
parameter, such
as ashing vessels, has
to be also taken into
account. The most
commonly
used vessel materials
for dry ashing are
quartz, porcelain,
and platinum, which
resist all acids.
Decomposition of the
sample strongly depends
on the type
of analytes to be
determined. Hence, an
analysis of particularly
volatile elements force
necessitates the use of
procedures based
on wet digestion
methods. The oxidizing
power of a wet digestion
method is based on the
use of a wide range of
chemical
reagents (acids and
oxidants) of which the
most commonly used
are nitric, sulfuric,
and perchloric acids,
and hydrogen peroxide
(Gorsuch 1970; Skurikhin
1993), and their
combination. Moreover,
the large number of
different types of
dissolution apparatus
tomineralization
using acid digestion
causes this approach to
be increasingly
applied in sample
treatment.
Conclusions
Among other routes, food
is one of the main
sources of consumer
exposure to heavy
metals. Since increased
dietary metals
intake may contribute to
the development of
various disorders,
there is a necessity for
monitoring of these
substances in the human
diet. To estimate the
dietary intake of food
contaminants, the
different approaches can
be utilized. The
exposure estimation is a
complex process and no
single approach is
suited to all
circumstances.
The final choice of
method to estimate heavy
metal intake
depends, among other
things, on the intended
goal, the availability
of data, the nature of
the chemicals, and so
forth. In addition, it
is
important to note how
detailed and precise the
information concerning
toxic elements intake
needs to be. Considering
that heavy
metals are present in
low concentrations in
food samples, sensitive
analytical techniques
are required to measure
their concentrations
with appropriate
accuracy. In most
inorganic laboratories,
spectrophotometric
techniques are usually
employed for the
quantification
of metals. Nevertheless,
the application of many
other techniques
have also been shown as
an excellent tools for
the trace and
ultra-trace analysis. It
is important to remember
that sampling and
preparation steps are
the most critical parts
of any analytical
procedure
because they are
responsible for the
largest source of
errors.
Therefore, a great
attention should be paid
to prevent the original
chemical constitution of
the sample by the
suitable choice of,
among others, vessels
using ultra-pure water
and reagents of high
purity.
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