APPENDIX
II-AP: G. Wolterink, et al, Risk
Assessment of Chemicals: What About the Children?
This appendix is copied from:
http://rivm.openrepository.com/rivm/bitstream/10029/9260/1/613340005.pdf[
RIVM
report 613340005/2002
Risk Assessment of Chemicals: What About the Children?
G. Wolterink,
A.H. Piersma, J.G.M. van Engelen.
This
investigation has been performed by order and for the account of the Ministry
of
Health, Welfare
and Sports within the framework of project 613340, .Advisering
Bestrijdingsmiddelen..
RIVM,
Abstract
With regard to
risk assessment of chemicals, children are not little adults. Their pattern of
exposure
(quantitatively and qualitatively), as well as both the toxicokinetics and the
toxicodynamics of
a chemical will be different, and therefore it should be carefully examined
whether children
are adequately protected against adverse effects of chemicals.
In this report a
concise overview of the relevant data on the differences between adults and
children with
respect to exposure, kinetics and dynamics of chemicals is presented, and the
adequacy of
currently used toxicological tests for regulatory purposes is discussed. In
addition, in view
of the potentially increased vulnerability of children a number of
recommendations
for further development of risk assessment of chemicals is presented.
Contents
Samenvatting 4
Summary 6
1 Introduction 8
2 Exposure 10
2.1 Dietary
exposure 10
2.2 Other routes
of exposure 10
2.3 Aggregate and
cumulative exposure 11
3 Physiology 12
4 Toxicokinetics 13
4.1 Absorption 13
4.1.1 Oral
absorption 13
4.1.2 Inhalatory
absorption 13
4.1.3 Dermal
absorption 13
4.2 Distribution 14
4.3 Elimination 14
4.3.1 Metabolism 14
4.3.2 Excretion 15
5 Toxicodyhamics 16
5.1 Development
16
5.1.1 Development
of the respiratory tract 17
5.1.2
Developmental immunotoxicity 17
5.1.3
Developmental neurotoxicity 17
5.2 Endocrine
disruption 19
5.3
Carcinogenicity 19
6 General
conclusions 20
6.1 Exposure 20
6.2
Toxicokinetics and toxicodynamics 20
6.3 Adequacy of
toxicological tests 20
6.4 Vulnerability
of children: Is an additional assessment factor necessary? 21
6.5. Gaps in risk
assessment and directions for further research 22
Acknowledgement 24
References 25
Appendix 1 Mailin list
….
Summary
In regulatory
toxicology there is increased awareness and concern that children and adults
may differ in
their susceptibility to xenobiotics. In this report a concise overview of the
relevant data on
the differences between adults and children with respect to the kinetics,
dynamics and
exposure to chemicals is presented and adequacy of currently used
toxicological
tests for regulatory purposes is discussed. In view of the potentially
increased
vulnerability of
children a number of recommendations for further development of risk
assessment of
chemicals are made.
There are three
major areas in which children and adults differ: exposure to and
toxicokinetics
and toxicodynamics of xenobiotics. Especially the situation in which children
are higher
exposed than adults this needs full attention.
Children consume
more food and drink more fluids per kg body weight than adults, and their
dietary pattern
is different and less varied. Moreover, children have a relatively high
inhalatory rate
(which may lead to a higher inhalatory exposure) and a high body surfacebody
weight ratio
(which may result in a higher dermal exposure). Children may also be more
exposed to toxic
substances than adults since children spend more time in the same room or
area, are in
closer contact with a contaminated surface (e.g. by crawling) and display less
hygienic
behaviour (mouthing of hands, objects, surfaces; pica behaviour). The route of
exposure may be
of importance for the potentially toxic effects of a chemical. In most
toxicological
studies for regulatory purposes the oral route of administration is used.
However, the
expected route of exposure of a child should be taken into consideration in the
design of
toxicological studies since the systemic exposure to a chemical may differ due
to
different levels
of absorption and the absence of a first pass effect following dermal and
inhalatory
absorption. A further point of concern, for adults as well as children, is the
aggregate
exposure to a specific chemical from different sources and the cumulative
exposure
to different
substances with the same mechanism of action (e.g. organophosphates and
carbamates).
The physiological
differences between children and adults may affect the kinetics of a
substance in the
body, which may render the child less or more susceptible to toxic effects of
a chemical. For
instance, oral absorption of a substance may be affected by the different
gastric pH,
gastric emptying rate, concentration of digestive enzymes and gut flora. The
high
respiratory
activity and higher body surface-body weight ratio may increase the relative
inhalatory and
dermal absorption of substances. The high body water content, the low plasma
protein binding
capacity and the permeability of the blood brain barrier may affect the
distribution of a
chemical. The immaturity of the metabolic enzymes in the liver and the low
renal blood flow
and glomerular filtration rate may affect the elimination of a xenobiotic.
With respect to
toxicodynamics, i.e. the interaction between a chemical and the body (organs,
tissues) a major
concern is the influence that a chemical may exert on the developing organs
and systems in
young children. Disruption of proliferation, differentiation, migration and
maturation of
cells may have severe and irreversible consequenses. In humans, the
development of
certain organs or systems, e.g. the respiratory tract, the immune and
endocrine systems
and the brain, continues long after birth. The presently used reproduction
toxicity tests
and the newly introduced neurodevelopmental toxicity test mainly focus on
reproductive and
neurotoxic effects and are not designed to detect, for instance, immunotoxic
effects, effects
on lung development or general effects on histopathology or haematology. It
is recommended
that it is ascertained that, in case of exposure through the milk of the dams,
significant
quantities of test substance are excreted in the milk. Moreover, since the
critical
windows in the
development of laboratory animals and children are not necessarily the same,
the exposure
period should be carefully examined.
Since there is a
multitude of processes that may differ between children and adults, and the
net result of
these differences is not clear, at present there is not sufficient information
to give
a general
quantitative statement with respect to differences in vulnerability. For
instance, an
increased
absorption of a substance in a child does not necessarily lead to an increased
risk if
there is no
formation of a toxic metabolite or if there is an increased elimination of the
substance. It
should be noted that in risk assessment, the use of an intraspecies factor of
10
implies that the
.most sensitive child. is about 10x more sensitive than the .average. human,
and thus that
there is a 100-fold variation in susceptibility in the entire human population.
In
addition, the use
of the default assessment factor of 10 for interspecies extrapolation implies
that the average
human is 10 times more sensitive than the most sensitive animal tested. In
general, if a
full set of toxicological data is available, the presently used assessment
factors
(10 x 10) are
considered adequate in safeguarding the human population. However, the use of
an additional
assessment factor in order to protect the sensitive groups in the human
population, among
others children, should always be considered, on a case-by-case basis.
For future
developments in risk assessment of chemicals with respect to children a number
of
recommendations
are made. For adults as well as children more insight into the specific
exposure
scenarios is needed. It is proposed to design a decision tree which indicates
whether
a toxicological
data set and the knowledge on the specific exposure are adequate for risk
assessment for
children. The suitability of young animal toxicity tests should be
investigated.
A comparison of
the dose-response data and the NOAELs of tests in adults and juveniles may
provide insight
in the range of intraspecies variation with respect to age. Since for
pharmaceuticals
human data are available it will be worthwhile to follow the developments in
the field of
kinetics and dynamics of paediatric pharmaceuticals. By using distributions of
the
physiological and
kinetic parameters and PBPK modeling it can be assessed which group of
children is most
at risk for a certain chemical.
1 Introduction
In regulatory
toxicology, recently attention has been focussed on the possible differences
between children
and adults with respect to their susceptibility to xenobiotics. In order to
assess the
adverse health effects of xenobiotics a variety of toxicological tests in
animals has
been developed, among
others reproduction toxicity tests. Whereas these tests are generally
accepted for
hazard identification of prenatal effects, concern has been risen that the
toxicological
tests presently used may not be adequate to detect some specific effects of
perinatal
exposure to xenobiotics [1]. It has been shown that there are differences in
toxicokinetics
and toxicodynamics between the developing animal and the developing child.
In addition, the
exposure pattern and exposure levels to xenobiotics may also differ between
children and
adults. The question has arisen whether the present procedures for risk
assessment do
adequately protect children against adverse effects of chemicals.
In literature the
term .child. may have a dual meaning. On the one hand it may refer to a
human being that
has not yet reached adolescence. On other occasions the term .child. is used
to identify a
specific period in the development of a human being from birth to adulthood.
According to the
classification by Crom [2] a neonate is less than 1 month of age, an infant is
1 to 23 months of
age, a child is 2 to 12 years of age and an adolescent is 13 to 18 years old.
In the present
report the terms .child. and .children. refer to the entire period from birth
up to
and including
adolescence, while the terms neonate and infant refer specifically to the
periods
in life as
described by Crom.
Children may be
exposed to toxic substances for longer periods or to higher external doses
than adults,
because children may spend more time in a room or area in which a toxic
chemical is
present (e.g. bedroom), are in closer contact with a contaminated surface (e.g.
by
crawling),
display more hand-to-mouth behaviour, and display less hygienic behaviour (e.g.
mouthing of
objects/surfaces, pica behaviour). In addition, children have a higher body
surface to body
weight ratio [3] and relatively higher food [1, 4], water [5] and oxygen
intakes [3],
which may increase their exposure on a body weight basis.
It has become
clear from inadvertent exposures that, for certain chemicals, children may be
more vulnerable
to the toxic effects than adults. This increased vulnerability of children may
be the result of
differences in toxicokinetics and toxicodynamics. As compared to adults,
especially in the
early postnatal stages of life marked differences in kinetics and dynamics of
substances may
exist.
Most data on
toxicity of chemicals come from animal studies. Based on the daily dose that
causes no adverse
effects in the animals, i.e. the No-Observed-Adverse-Effect-Level
(NOAEL) an
Acceptable Daily Intake (ADI), expressed as mg/kg bw/day, is calculated for
humans. To
extrapolate the data from the animal studies to the human situation assessment
factors are
applied. A factor of 10 is used to allow for species differences (4 for
toxicokinetics,
2.5 for toxicodynamics) and another factor of 10 is applied to safeguard the
variability in
the human population (3.2 for toxicokinetics, 3.2 for toxicodynamics)[6].
The concern that
children may potentially be at higher risk, due to specific sensitivities and
incomplete data
with respect to toxicity and exposure led to new legislation in the United
States. The Food
Quality Protection Act of 1996 [7] requires that .« …an additional tenfold margin of safety for
the pesticide chemical residue and other sources of exposure shall be applied
for infants and children to take into account potential pre- and postnatal
toxicity and completeness of data with respect to exposure and toxicity to
infants and children…”.
In the present
report a concise overview of the relevant data from public literature on
differences in
exposure, and in susceptibility, due to toxicokinetic and -dynamic differences
between children
and adults is presented. The adequacy of the presently used toxicological
tests is discussed
and directions for further research are indicated.
2 Exposure
Whether children
should be considered as a special sub-population in risk assessment of
chemicals should
in first instance be based on the exposure profile of children and not on the
hazard profile of
a chemical (only). Since children have a different dietary pattern, a different
activity pattern
and behave differently from adults, the exposure pattern and exposure levels
of children may
substantially differ from that of adults.
2.1 Dietary exposure
Infants may be
exposed to xenobiotics through breastfeeding and by consumption of other
liquid and solid
food products. Children have a higher need for nutrients and a higher caloric
demand and hence
consume more food and drink more fluids per kg bodyweight than adults.
Accordingly their
relative exposure to xenobiotics in food is increased. It should be noted
that, although
children consume the same food products as adults, their dietary pattern is
different and
less varied [1]. For instance children consume relatively large quantities of
dairy
products and
fruit.
2.2 Other routes of
exposure
Apart from
exposure to chemicals that are present in food, individuals may be exposed to
chemicals through
inhalation, dermal exposure or non-dietary oral exposure. As a
consequence of
their higher inhalatory rate, children have a higher intake of gaseous or
airborne
chemicals. Also dermal exposure in children may lead to higher systemic levels
due
to a higher body
surface:body weight ratio.
In addition,
children may be more exposed to toxic substances than adults because children
may spend more
time in the same room or area, are in closer contact with a contaminated
surface (e.g. by
crawling), display more hand-to-mouth behaviour, and display less hygienic
behaviour (e.g.
mouthing of objects/surfaces, pica behaviour). At the RIVM a consumer
exposure model
(ConsExpo 3.0) [8] has been developed to estimate exposure levels through
inhalatory,
dermal and non-dietary oral routes in adults as well as children. Exposure
during
application
(primary exposure) as well as after application (secondary exposure) of a
chemical is taken
into account.
Specific
scenarios for children, especially during the post-application phase, are
implemented
in ConsExpo.
These scenarios use default parameters for e.g. crawling (duration of dermal
contact with a
surface etc) and hand-to-mouth behaviour that are based on literature data and
observational
studies.
With respect to
testing for regulatory purposes the intended use of the test substance should
be taken into
account in deciding on the exposure route in the toxicological test. For
instance,
in case a
substance is intended to be vaporised, the developmental effects following
inhalatory
exposure should be evaluated, since the inhalatory uptake may differ from oral
uptake and since
there is no first pass elimination in the liver, this might lead to different
systemic levels.
The same holds true for dermal exposure. Furthermore inhalatory exposure
may uncover local
developmental effects on the lung.
2.3 Aggregate and
cumulative exposure
At present, risk
assessment is generally performed for individual substances coming from one
source. However,
the same substance may be present in a variety of sources, which all
contribute to the
exposure of an individual, the so-called aggregate exposure. Aggregate
exposure is
usually referred to as the total exposure of a defined population to a certain
chemical from all
relevant sources and through all exposure routes [9].
Apart from
aggregate exposure to a specific chemical from different sources, it should be
noted that often
toxic chemicals are part of a certain class of structurally closely related
substances which
all exert their effect through the same mechanism of action (cumulative
exposure).
Typical examples of this are classes of pesticides such as organophosphates,
pyrethroids and
carbamates. Even substances belonging to different classes may affect the
same functional
target. For instance, both organophosphates and carbamates inhibit
cholinesterase
activity. The effects of cumulative exposure of adults and children to
functionally
related substances are largely unknown and, consequently, are not considered in
the present risk
assessments for regulatory purposes. Although aggregate and cumulative
exposure to
chemicals is a matter of concern for the entire human population, in view of
the
differences in
exposure characteristics of children compared to adults, this subject needs
special attention
with respect to children. Especially for substances for which the risk is
considered only
on a product base for a given application (for instance pesticides), exposure
to a given active
substance might be much higher due to simultaneous exposure from other
sources (e.g.
veterinary medicine, biocide, pesticide) that contain the same active
substance.
In the present
regulatory processes these uses are not considered at the same time.
3 Physiology
There is a number
of differences in physiology between children and adults. These
differences may
affect the level of exposure to a chemical, and the kinetics and dynamics of a
chemical.
Kinetics refers to the processes of absorption, distribution, metabolism of a
chemical in the
body and excretion from the body. Dynamics describes the interaction
between a toxic
chemical and the body (tissues, organs, receptors). The most relevant
differences with
respect to vulnerability and exposure of children to substances are described
in table 1.
Table 1. Differences in
physiology between children and adults.
Parameter Difference ref.
body surface:body
weight ratio
About 2.5 times
higher in neonates than in adults 3
body water
content About 75 % in neonates, 40-60 % in adults. 10
water intake
About 2-5 times higher in infants * 11
caloric demand
Energy requirement and relative food consumption is 3-4 times higher in infants
than
in adults *
4
ventilation rate
Volume of inhaled air/kg bw/min is about 3 times higher in neonates than in
adults 3
gastric pH About
neutral at birth, shortly thereafter falling to pH 2 (adult value) 12, 13
gastric emptying
rate Irregular at birth, approaching adult values by 6-8 months 14, 15
digestive enzyms
Concentration of digestive enzymes is lower at birth and gradually increases
during
the first year of
life
16
gut flora
Bacterial flora in the intestines is established soon after birth but changes
in
composition over
time
17
brain Relatively
large in children and in development, showing a high rate of tissue
proliferation and
differentiation. Blood brain barrier is underdeveloped. Brain blood
flow increased.
Relative high water and low myelin content.
18
liver Metabolic
enzyme system underdeveloped at birth, especially oxidation and
glucuronidation
processes. Substantial increase within 2-6 months. Adult levels
reached within
1-3 years
19
kidney Renal
blood flow and glomerular filtration rate low at birth, reaching adult levels
in
about 3-5 and
7-12 months respectively.
20, 21
plasma protein
(binding)
Plasma protein
concentration and binding capacity are lower during the first year of
life
20, 21
immune system
High level of tissue proliferation and differentiation 22
*
when compared on a body weight basis
An extensive
review on the differences in physiology between adults and children, with
respect to
factors that influence absorption, distribution, metabolism and excretion of
xenobiotics, is
provided by De Zwart et al. [19].
4 Toxicokinetics
It is known that
there are differences between children and adults with respect to the fate of a
toxic substance
in the body. Especially in the early postnatal stages of life marked
differences
in the kinetics
exist. Recently the age-related differences in kinetics of xenobiotics have
been
reviewed at RIVM
[19]. In the present chapter an overview of the differences between
children and
adults, based on that RIVM report, is presented. The significance with respect
to
risk assessment
of toxic chemicals is discussed.
4.1 Absorption
Substances may be
absorbed through ingestion, inhalation or dermal penetration. For most
substances oral
ingestion is the predominant route of exposure, however volatile substances
may be absorbed
mainly by inhalation. For compounds with a logPow between .1 and 4 and a
molecular weight
below 500, the transdermal route of absorption may be of significance.
4.1.1 Oral absorption
Child-specific
factors that may influence the absorption of substances are gastric pH, gastric
emptying rate,
activity of digestive enzymes and the existence and composition of the
bacterial gut
flora, composition of bile and bile flow. At birth the pH of the stomach in the
neonate is about
neutral (pH 6-8) but shortly after birth falls to pH 2, which is comparable to
adult pH [12,
13]. The neutral pH in neonates will result in a different level of ionisation
of
toxins as
compared to adults, which in turn affects absorption from the gut. The gastric
emptying rate is
variable and irregular until 6-8 months after birth [14, 15]. The concentration
of digestive
enzymes is lower at birth and gradually increases during the first year of life
[16]. The
intestinal wall of neonates is more permeable to macromolecules [19]. The
bacterial
flora in the
intestines is established soon after birth, however, its composition gradually
changes over time
[17]. All these factors may increase or decrease the absorption of a toxin
from the gut.
4.1.2 Inhalatory
absorption
Children have a
relatively higher caloric demand than adults [4], and consequently a
relatively
increased respiratory activity [3]. As a consequence, the intake over a certain
period of time of
an airborne toxin, for instance in the form of a gas, aerosol or dust particle
will be increased
in children on a body weight basis. In addition, pulmonary function declines
with age. Further
research is required to establish how the physiological differences in the
respiratory
system between children and adults affect the absorption of substances.
4.1.3 Dermal absorption
The skin in
neonates is immature in comparison to that of adults. For instance, neonates
have
a higher skin
surface pH and skin roughness, a lower hydration of the stratum corneum and
RIVM
report 613340005 page 14 of 29
desquamation of
the epidermis [23]. However, there are no indications that the dermal
absorption of
substances in children is substantially different from adults [24].
4.2 Distribution
From a recent
publication in which the pharmacokinetic parameters of 45 drugs were
compared, it can
be concluded that there is a tendency towards larger volumes of distribution
of these drugs in
children (from neonates up to adults)[25].
The body
composition of newborn children differs from that of adults. For instance, the
body
of neonates
contains a higher percentage of water (75 % in neonates vs 40-60% in adults).
The body water
content is about 63 % in 2 year old infants, and approaches adult values in 12
year old children
[10]. The body fat content increases from about 18 % in neonates to 30 % in
1 year old
infants, followed by a decrease to 17 % in 15 year old boys. In girls body fat
content remains
higher. Adult fat content is about 30% in males and 35% in females [19].
Moreover,
neonates have a relatively underdeveloped muscular system and the head
contributes a
large proportion to the body weight. There are some other differences that may
be of
significance. Firstly, the plasma protein levels and binding capacity are lower
in
neonates and
infants, reaching adult values at about 1 year after birth [20]. This may be of
particular
importance for exposure to substances that, in the adult, are largely bound to
plasma proteins.
Generally speaking chemicals that are neutral or hydrophilic or have a low
molecular weight
are least likely to bind to plasma protein. Since the unbound fraction in the
blood determines
the degree of distribution to other tissues, a reduction in plasma protein
binding may
dramatically increase the toxic potential of a substance. On the other hand,
lower plasma
protein binding may enhance the elimination of a substance.
Secondly, in the
neonate the blood brain barrier is immature, which may lead to higher
exposure of the
brain to hydrophilic xenobiotics, for instance cadmium. The relative size of
the brain is
larger in children. Moreover the composition of the brain of neonates and
children
is different from
adults. There is less myelin and an increased blood flow in the brain. All
these factors may
lead to a higher exposure of the brain to toxic substances. This is of
particular
concern since the brains of neonates and children are still developing and
therefore
more vulnerable
to the toxic effects of substances acting on the nervous system.
4.3 Elimination
A substance may
be eliminated from the body through two different processes, metabolic
conversion and
excretion. Often, excretion of a substance that is rather lipophilic, only
occurs
after it has been
enzymatically converted to more hydrophilic metabolites.
4.3.1 Metabolism
Metabolism of
substances serves two purposes. Firstly, by metabolising a substance it often
loses its toxic
potential, although in certain instances the metabolites are more toxic than
the
parent compound.
Secondly, metabolic processes such as oxidation, glucuronidation,
hydroxylation and
sulphate conjugation increase the hydrophilicity of the molecule and
therefore render
it more susceptible to renal excretion. At birth, the liver has an
underdeveloped
system of metabolic enzymes. Especially oxidation and glucuronic acid
conjugation
processes are immature in neonates [18, 19, 25]. This means that the rate of
detoxification
(or activation) and elimination processes in the liver may be less efficient.
The
various enzyme
systems in the liver mature at different time points. In general, the levels
increase
substantially within 2-6 months after birth. By 1 to 3 years of age the
metabolic
potency of the
liver approaches adult levels. Although the liver is the major organ for
metabolism of
xenobiotics, other organs such as the lung, the small intestine and the kidney
may also
significantly contribute to the metabolic conversion of substances [19]. The
metabolic
activity of the lung is particularly relevant for substances to which an
individual is
exposed through
the inhalatory route. Human and animal studies have shown that important
enzym systems in
the lung that are still developing after birth are the cytochrome P450
monooxygenase
system, glutathione S-transferases, epoxide hydrolases, superoxide
dismutase,
catalase and glutathione peroxidase [26].
4.3.2 Excretion
Substances can be
excreted through renal excretion, biliary excretion, expiration or
perspiration,
often after being metabolised. The renal function in the neonate, in particular
the
renal blood flow
and glomerular filtration rate, is low as compared to adults [20, 21], which
may lead to a
prolonged half-life of the toxin or its metabolites in the blood plasma. In
full
term neonates
renal blood flow and glomerular filtration rate reach adult levels in about 3-5
months and in
7-12 months respectively. Due to the high caloric demand in children their
relative
respiratory activity is increased. On the one hand, this indicates that in
children the
relative intake
of volatile toxins or toxins in aerosols through inhalation is enhanced, on the
other hand the
potential to eliminate toxins or their metabolites through expiration will be
increased. Little
is known about the particularities of biliary excretion of xenobiotics in
children.
Experimental
research into the pharmacokinetics of drugs has demonstrated that in the early
days after birth
there may be a decreased elimination of drugs, resulting in a prolonged half
life. However,
after the neonatal period has passed the elimination of drugs is often enhanced
[18, 27]. The
same probably is true for non-drug chemicals.
5 Toxicodynamics
The main
difference between adults and children with respect to toxicodynamics is the
influence that
toxic chemicals may exert on the developing organs in young children. The
organs in the
foetus, but also in the newborn child, undergo cell proliferation (growth),
cell
differentiation,
cell migration, cell maturation, and development of enzyme or receptor
systems. Disruption
of these processes by toxins may have severe and irreversible
consequences. For
instance, postnatal exposure to lead induces increased anti-social
behaviour and
decrements in cognitive function and intelligence, later in life [28, 29].
However, generally
speaking little is known at present about the influence of the majority of
chemicals on the
development of organ systems. In the following sections, the potential
effects of
chemicals on organ systems in laboratory animals and humans, and the adequacy
of
the presently
used toxicological tests in detecting potential deleterious effects, will be
discussed in more
detail.
5.1 Development
Exposure to
chemicals during the phase that organs develop may have serious effects. Once
the organs have
developed and enter a phase of growth they are, in general, much less
susceptible and
effects induced by exogenous substances are often reversible.
With respect to
risk assessment, the important question is whether the currently used
toxicological
tests using laboratory animals are sufficiently sensitive to predict damaging
effects of a
number of specific toxins on the developing human. The toxicological testing of
chemicals in
laboratory animals is based on the assumption that the damaging effects of
chemicals in the
laboratory animal can be extrapolated to humans, and vice versa. From this
assumption it
follows that most of the development-disrupting effects of chemicals are
adequately
detected by reproductive and teratogenic tests in laboratory animals, and that
these laboratory
tests predict the potentially damaging effects of chemicals on children.
Although in
long-term studies in rats and mice treatment usually already starts when the
animals are 6
weeks of age, this is still after the critical period for the development of
most
organs and organ
systems. At present the toxicity of substances in neonates and young
animals is tested
in reproduction toxicity (OECD guidelines 415 and 416) studies. These
studies include
postnatal exposure up to the weaning age (OECD415 and the F2 in
OECD416) or until
adulthood (F1 in OECD416). Before weaning, exposure occurs largely
via lactation,
although feed consumption increases in the third week before weaning and
dietary exposure
of pups may occur. These tests allow the study of effects postnatal exposure,
especially the F1
generation in OECD416, although at present testing is generally limited to
clinical signs,
food consumption, weight development and sometimes behavioural testing.
Moreover, if
there are differences in toxicokinetics, sensitivity of target organs, or
period of
the development
of organs in the mammalian species used for toxicology tests and humans,
toxic effects of
chemicals that are specific for humans may be overlooked. The brain may be
exemplary for
this. At birth the development of the brain has further progressed in humans
than in rodents.
However, in contrast to rodents, where brain development is largely
completed at
weaning, in the human this organ continues to develop long after birth [30, 31,
32]. Although in
studies on reproductive toxicity animals are exposed during the prenatal and
postnatal period,
the pattern (and level) of exposure in humans may differ, especially in the
critical period
with respect to brain development
Below, the
particularities of the development of certain organ systems in humans, as
compared to other
mammalians, and the consequences for risk assessment are discussed.
5.1.1 Development of
the respiratory tract
The development
of the respiratory tract occurs in several phases. In humans, the
development of
the lungs, in particular of the alveoli, continues well after birth. The
development of
alveoli even extends into adolescence. In this respect humans differ from rats
and mice, in
which formation of alveoli occurs during the first 4 weeks after birth. It has
been
observed that a
number of toxins retard lung maturation, decrease the alveolar number and
surface area, and
impair the function of the lung surfactant system. Little research into the
long-term
consequences of exposure to these toxins has been performed, although there are
indications that
the observed functional disturbances may be permanent [26, 33]. More longterm
studies are
needed, however, to clarify this matter.
5.1.2 Developmental
immunotoxicity
Developmental immunotoxicology
is still in its infancy. The developing immune system goes
through phases of
cell production, cell migration through haematopoietic organs, cell-cell
interactions,
cell differentiation and cell maturation. An overview of the toxic effects of
chemicals on the
developing immune system has been compiled at RIVM [34]. Disruption of
immunodevelopmental
processes may have serious consequences, such as long-term or
permanent
immunosuppression. On the other hand, immune hyper-responsiveness (i.e.
allergies) or
autoimmune reactions could result from disrupted development of the immune
system.
Accordingly, the lack of immunological challenge due to increased hygienic
standards, and
exposure to environmental factors during the development of the immune
system have been
implicated in autoimmune diseases [35] and atopia and asthma [22, 36].
From animal
studies there is evidence that the developing immune system may be not more
sensitive to the
effects of toxins than the mature immune system, but that the changes in
immune function
induced by the toxins may be more persistent [37]. However, it has also
been reported
that perinatal exposure to TCDD reduces vaccination responses and increases
the risk of
otitis media in Dutch school children [38]. At present, the toxicological tests
used
for risk
assessment are not designed to detect immunodevelopmental disturbances or
immunotoxic
effects, since immunological parameters are usually not included. It is
recommended,
however, to amend the current OECD guidelines for developmental and
reproductive
toxicity testing so that the developing immune system is considered as a
potential target
of toxicity during developmental stages [39].
In view of the
lack in knowledge, further clinical and epidemiological studies are necessary
to give more
insight in this matter.
5.1.3 Developmental
neurotoxicity
The brain, in
particular the human brain, is an immensely complex organ, of which the
functioning is
still poorly understood. It is known that relatively subtle changes in the
human
brain may alter
the mental capabilities or the personality of an individual. A number of
psychiatric
illnesses, e.g. mental retardation, autism, cerebral palsy, ADHD and
schizophrenia,
probably have their origin in a disturbed pre-or perinatal brain development.
For some of these
disturbances the involvement of chemicals has been suggested [40].
In humans, the
development of the brain is a process that starts during early pregnancy and
continues long
after the child is born. Initially the anatomical differentiation of the brain
is
the most striking
feature, but even when all the anatomical structures of the brain are present,
functional
development still proceeds. Disruption of the anatomical as well as the
functional
developmental
processes may have major consequences.
The brain passes
through several stages during development. First there is neurogenesis, i.e.
the stage when
neurons are formed. This is followed by processes of neuronal migration,
outgrowth of
dendrites and axons, formation of synaptic connections, development of
neurotransmitter
systems and receptors, and myelinization.
Although in
humans the anatomical development of the brain predominantly occurs before
birth, processes
such as axon-, dendrite- and synapse formation continue long after birth;
some of these
processes are not finished before adolescence [31, 32]. The fine-tuning of the
interactions
between the various brain systems also continues for a long period after birth.
In humans the
blood brain barrier is only fully developed from the middle of the first year
of
life [41]. This
means that a chemical such as cadmium, to which the adult blood-brain barrier
is impermeable,
may well enter the brain in infants. In the rat, it has been reported that
exposure to
certain pesticides during the development of the blood brain barrier may alter
the
permeability of
this barrier, even for a prolonged time after cessation of the exposure to
these
pesticides [42].
It is known from
animal studies that the developing brain is more susceptible to certain
chemicals than
the adult brain [3]. It should be noted that substances that are capable of
causing
disturbances in the developing nervous system are not necessarily neurotoxic in
adults.
Furthermore, even short periods of exposure to toxic chemicals, such as PCBs,
PBDEs
and pesticides,
may induce long-lasting behavioural disturbances [43, 44].
The risk
assessment of neonatal exposure to toxic substances, based on studies using
laboratory
animals, is hampered by the fact that in certain instances adequate animal
models
are lacking.
Gross abnormalities as a result of neurotoxic damage will probably be detected
by studies in
animals. However, more subtle, but clinically relevant effects will be
difficult to
uncover. For
instance, learning and memory tasks in animals are probably only sensitive
enough to detect
relatively large effects on cognitive functioning. Relatively small decreases
in cognitive
abilities in human individuals, as a result of neurodevelopmental disturbances
following
exposure to neurotoxins, will be difficult to detect. The possible involvement
of
neurotoxins in
the development of psychiatric disorders will be even harder to detect in
animal
experiments since many manifestations of these disorders are typical for the
human
species and are
also influenced by life-style and other exogenous factors.
In rats, the
major species used for toxicological testing, the brain growth spurt and the
anatomical and
functional development of the central nervous system occurs for a large part
after birth. In
the reproduction toxicity tests the dams are treated until weaning. In case the
test substance is
excreted in substantial amounts in the milk, this test may provide a reliable
indication on the
neurodevelopmental effects of a substance. However, for substances which
are poorly
excreted in milk, the reproduction toxicity studies cannot be used to establish
neurodevelopmental
properties of a substance.
US-EPA has
implemented a guideline for developmental neurotoxicity [45] and the draft for
a new OECD guideline
(426) for the testing of neurodevelopmental toxicity of substances
probably will be
implemented soon. The developmental neurotoxicity test is aimed at
detecting
anatomical, histological and functional disturbances induced by interaction of
a
substance with
the developing brain. In principle the guideline proposes to administer the
test
compound during
pregnancy and lactation in the food, however, other means of
administration
may also be used. In the guideline it is, however, not required to assess the
levels of
compound in the milk in order to determine whether the pups are actually
exposed
after oral
administration of the test compound to the mother. Further, if the substance
undergoes a high
rate of biotransformation, the exposure of the infant through lactation may
differ than when
directly exposed to the parent compound itself.
In case the level
of exposure is too low, the test compound should be administered directly to
the pups,
provided that stress does not preclude this possibility. The intended use of
the test
substance may
prompt for a different route of exposure. For instance, in case a substance is
intended to be
vaporised, the developmental effects following inhalatory exposure should be
evaluated, since
the uptake rate and amount that is absorbed may differ from oral uptake.
Since following
inhalatory exposure there is no first pass elimination in the liver this may
lead to different
systemic levels. Furthermore inhalatory exposure may uncover local
developmental
effects on the lung. For dermal exposure the same reasoning holds true.
5.2 Endocrine
disruption
Endocrine
disruption is not a specific end point of toxicity but rather refers to health
effects
that may be
mediated by mechanisms affecting hormone homeostasis. Children may be
especially
vulnerable in this respect as their homeostatic mechanisms are immature. In
animal
studies, effects
on morphologic development of the urogenital system may indicate endocrine
mechanisms. For
example, the uterotrophic assay carried out in immature female rodents
assesses weight
effects and histological effects on the uterus of substances during a
developmental
phase when the juvenile uterus is responsive to estrogenic compounds,
although at the
same time the endogenous production of estrogens is very low. Such a system
is more sensitive
than the adult uterus, which is already stimulated by endogenous estrogen.
However, as yet
it is unclear as to when a uterotrophic effect in the juvenile uterus should be
considered
adverse. At present, various new test systems using young animals are being
developed for
endocrine disruption, but their role in the risk assessment process has yet to
be
established. The
possible specific relevance of the endocrine disruption issue for children and
their development
is however beyond dispute.
5.3 Carcinogenicity
Little is known
about the specific vulnerability of children to the genotoxic and carcinogenic
effects of
chemicals. Charnley and Putzrath [46] summarized the results of studies on 30
chemicals in
which the effects of age on chemically induced carcinogenesis in rodents were
evaluated.
Overall, the percentage of studies indicating that young animals were more
susceptible than
adults (37 %) was about equal to the percentage of studies indicating that
young animals were
less susceptible than adults (53 %). However, as Landrigan et al. [47]
pointed out in a
comment, the chemicals included in the studies represent less than 0.2 % of
the high
production volume chemicals. Moreover, most studies used only one dose level,
making it
difficult to draw conclusions on differences in sensitivities. Evidently more
research is
needed in order to gain insight in the relative vulnerability of children to
the
carcinogenic
effects of chemicals.
6 General Conclusions
6.1 Exposure
Exposure of
children to toxic chemicals may differ substantially from that of adults. As a
consequence of
their increased metabolic needs, their increased body surface to body weight
ratio and their
behaviour (e.g. crawling, hand-to-mouth behaviour) the relative exposure (per
kg bw) of
children to xenobiotics is likely to be higher than that of adults in case of
similar
external
exposure. So, if children are likely to be exposed, it is necessary in the risk
assessment for
chemicals to perform a separate exposure assessment for children.
6.2 Toxicokinetics and
toxicodynamics
It is evident
that there are differences between children and adults in the kinetics and
dynamics of toxic
substances, which may render the child less or more susceptible to the
toxic effects of
a chemical.
Therefore, in
cases where children are likely to be exposed, in addition to the assessment of
the specific
exposure pattern, also the toxicological profile of a xenobiotic should be
carefully
examined.
With respect to
toxicokinetics, the present data indicate that there are a lot of physiological
differences
between adults and children, that might result in a different internal dose in
children compared
to adults. However, it is difficult to predict the overall result of the
different
processes (for instance: a substance may be more extensively absorbed, but less
metabolised to a
toxic metabolite, or cleared more rapidly). Therefore, more insight is needed
in the
differences in toxicokinetic processes between adults and children. One way to
study
these differences
is by using generic PBPK models (see paragraph 6.5).
With respect to
toxicodynamics there is evidence that the differences between children and
adults may be
rather large, especially during early development. The anatomical and
functional
development of organ systems may render children more sensitive to toxic
effects
of xenobiotics
than adults, which are fully developed. In principal, these effects will be
detected in the
presently used reproduction and developmental neurotoxicity studies.
It must be kept
in mind, however, that the critical windows in the development of laboratory
animals and man
are not necessarily the same, and therefore, the exposure period should be
carefully
examined. In addition, special attention should be paid to the route and level
of
exposure, with
regard to extrapolation of results in animal studies to the human situation.
At present, a lot
of research with regard to sensitivity of children is going on in the field of
pharmaceuticals.
Since for the risk-assessment of chemicals in general only information on
toxicokinetics
and toxicodynamics in laboratory animals is available, it is important to
closely follow
the developments in the paediatric pharmacology area.
6.3 Adequacy of
toxicological tests
It is generally
assumed that most of the effects of chemicals on postnatal development are
adequately
detected by studies on reproductive toxicity and developmental toxicity in
laboratory
animals, and that these laboratory tests are also predictive for the toxicity
of
chemicals in
children. It should be noted, however, that the presently used toxicity tests
focus
on developmental,
reproductive and neurotoxic effects, while other parameters (e.g.
histopathology,
haematology, immunology) in pups are not investigated.
The newly
introduced neurodevelopmental toxicity test (OECD 426) may detect effects of
substances on the
anatomy and function of the brain. However, relatively subtle
neurodevelopmental
disturbances caused by the test substance may be overlooked, and the
usefulness of
this test still has to be proven. Also substance-induced neurodevelopmental
disturbances
underlying typically human psychopathology may not be detected by tests in
animals. It
should also be noted that the treatment regimen in the test animal might not
adequately
represent the exposure of the young child. For instance, postnatal exposure of
the
pup through
feeding of the mother will be low for substances that are poorly excreted in
milk.
Infants and
children receive, apart from milk, other solid and liquid foods and may thus be
exposed to
chemicals via other routes than breast milk. Thus, for substances for which
excretion in milk
is low the test substance should be administered directly to the pups,
providing that
stress does not preclude this possibility. For substances to which children are
exposed dermally
or by inhalation, toxicological tests should use the same administration
routes.
Moreover, the
presently used protocols for toxicological testing are not adequate for
detecting
effects of early
exposure to substances on carcinogenicity and certain developmental
disturbances. For
instance, substance-induced disturbances in the immune and endocrine
systems may not
be uncovered by toxicological tests since no specific tests aimed at detecting
disturbances in
the functioning of these systems are used. Guidelines to test for
(developmental)
effects on the immune system are in preparation. The investigation of a
broader set of
toxicological parameters in the multi-generation toxicity test, especially the
parameters that
are affected in (sub-)chronic toxicity studies, may provide at least part of
the
information that
is lacking at present.
6.4 Vulnerability of
children: Is an additional assessment
factor necessary?
From a
theoretical point of view it is evident that, due to the complex process of
development
of organs and
organ systems, children may be more vulnerable than adults to the toxic effects
of certain
chemicals. A typical historical example is the detrimental effect of lead. However,
there are not
many indications that the current risk assessment procedures do not adequately
protect the
sensitive groups, such as children, in the human population. There may be
several
explanations for
this:
- The data from
the present toxicological tests may be indeed adequate for extrapolation to
the human
situation, using the present assessment factors.
- An increase in
vulnerability due to one factor may be compensated by a reduction in
vulnerability due
to another factor (for instance, a high absorption of a chemical may be
compensated by a
high elimination).
- Adverse effects
have been identified in epidemiological studies, but an adequate relation
to exposure to
chemicals may be not clear or proven.
- Adverse effects
caused by exposure to chemicals are not identified in epidemiological
studies.
At present there
is not sufficient information, however, to give a general quantitative
statement whether
the presently used default assessment factor of 10 for intraspecies variation
is adequate to
protect children in risk assessment of chemicals. However, in general, the use
of an
intraspecies factor of 10 implies that the .most sensitive child. is about 10x
more
sensitive than
the .average. human, and thus that there is a 100-fold variation in susceptibility
in the entire
human population. In addition, the use of the default assessment factor of 10
for
interspecies
extrapolation implies that the average human is 10 times more sensitive than
the
most sensitive
animal tested.
In general, if a
full set of toxicological data is available, the presently used assessment
factors
(10 x 10) are
considered adequate in safeguarding the human population. However, the use of
an additional
assessment factor in order to protect the sensitive groups in the human
population, among
others children, should always be considered, on a case-by-case basis.
6.5 Gaps in risk
assessment and directions for further research
Exposure
With respect to
exposure of children more insight in specific exposure scenarios is needed to
get a better
estimation of the actual exposure level, especially when it is expected that
children might be
higher exposed than adults.
- The deviant
dietary pattern of children should be taken into account in the exposure
assessment. For
pesticides this is already common practice, in other frameworks this
aspect needs more
attention.
- In the exposure
assessment, not only point estimates, but also distributions of intake
levels should be
taken into account in the exposure assessment.
- For the
establishment of Maximum Residue Levels the deviant dietary pattern of
children should
always be taken into account.
- The
contribution of the oral, dermal and inhalatory routes of exposure to the total
dietary and
non-dietary exposure of children should be considered. Because of the
different
behaviour and physiology of children, both the absolute and the relative
contribution of
the various routes to the actual exposure will be different compared to
adults.
- What are the
effects/risks of aggregate exposure to a substance, or the cumulative
exposure to
substances, either structurally related or from a different chemical class,
with a common
mechanism of action. Based on the scientific data models for
aggregate and
cumulative risk assessment should be developed for the population
(and thus also
for children).
Criteria for the
adequacy of the data set: Decision tree.
As mentioned
before, the presently used reproduction and developmental neurotoxicity tests
will probably
detect many of the effects of chemicals on the developing laboratory animal.
On the other
hand, in the present report a number of situations have been described in which
humans differ
from laboratory animals and in which toxicological tests in animals are not
adequate in
detecting adverse effects of substances, such as developmental neurotoxicity or
immunotoxicity,
on the human child. Thus, it may be concluded that, in view of the
potentially
increased vulnerability of children, risk assessment of chemicals should be
made
on a case by case
base. We propose that in the near future a decision tree is designed which
can be used to
assess whether the available toxicological data set is adequate for risk
assessment of
children.
Suitability of
young-animal toxicity tests
The suitability
of young-animal toxicity models for extrapolation to the human situation
should be
investigated. Furthermore, it is interesting to compare the dose-response data
and
NOAELs of adult
test animals to those of juvenile test animals, to gain more insight in the
range of
intraspecies variation, with respect to age, in a test animal population.
However,
young animal
models will only be relevant in case developing systems are targets for a
compound or for
its metabolites. Regarding studies in animals this would imply that the
choice of an animal
model can be better substantiated, leading to a refinement of experiments
and a reduction
in the number of animals required for testing.
PBPK modeling
It is obvious
that the physiology and toxicokinetics in children are different from that in
adults. However,
it is difficult to get insight in the overall result of the differences,
especially
in a quantitative
sense. A valuable tool in this respect is PBPK- modeling based on the
physiology of
children. By using distributions of the physiological and kinetic parameters
and
PBPK modeling it
can be assessed which group of children is most at risk for a certain
chemical (e.g.
lean or fat children).
Acknowledgement
The authors would
like to thank ir. P.M.J. Bos, dr. H. van Loveren, dr. M.N. Pieters, dr.
M.T.M. van Raaij,
and dr. A. J.A.M. Sips for their valuable comments.
References
1 NRC (National
Research Council) (1993) Pesticides in the diets of infants and children.
2 Crom W.R.
(1994) Pharmacokinetics in the child. Environmental Health Perspectives 102
(Suppl
11):111-118.
3 Guzelian, P.,
Henry, C., Olin, S. (eds.) Similarities and differences between children and
adults:
Implications of risk assessment. International Life Sciences Press,
1992.
4 Department of
Health (1991) Report on health and social subjects N0. 41. Dietary
reference values
for food energy and nutrients for the
5
intake of food
chemicals. Food Addit. Contamin. 15 (Suppl.), 75-81.
6 WHO. Assessing
human health risks of chemicals: derivation of guidance values for
health based
exposure limits. Envir. Health Criteria. 170 (World Health Organization,
7 Food Quality
Protection Act of 1996. Public Law 104-170, 1996.
8 Van Veen, M.P.
CONSEXPO 3.0. Consumer exposure and uptake models. RIVM report
612810 011. RIVM,
Bilthoven, The
9 Aggregate
exposure assessment. (1998) An ILSI Risk Science Institute workshop report.
ILSI Press,
10 Friis-Hansen,
B. (1971) Body composition during growth: in vivo measurements and
biochemical data
correlated to differential anatomical growth. Pediatrics 47 (Suppl): 264-
274.
11
intake of food
chemicals. Food Addit. Contamin. 15 (Suppl.), 75-81.
12 Avery, G.B.,
Pediatrics 37,
pp. 1005-1007.
13 Nelson W.E.,
Behrman R.E., Kliegman R.M., Arvin A.M. (1996) Nelson Textbook of
Pediatrics. WB
Saunders company, Philadelphia, Pennsylvania.
14 Siegner E.,
Fridrich R. (1975) Gastric emptying in newborns and young infants. Acta
Paediatr. Scand.
64; 525-530.
15 Heimann G.
(1980) Enteral absorption and bioavailability in children in relation to age.
European Journal
of Clinical Pharmacology 18:43-50.
16 Hamosh, M
(1996) Digestion in the newborn. Clin. Perinatology 23: 191-209.
17 Grönlund
M.-M., Salminen S., Mykkänen H., Kero P., Lehtonen O.-P. (1999)
Development of
intestinal bacterial enzymes in infants . relationship to mode of delivery
and type of
feeding. APMIS, 107: 655-660.
18 Renwick, A.G.,
Lazarus, N.R. (1998) Human variability and noncancer risk assessment.
An analysis of
the default uncertainty factor. Regul. Toxicol. Pharmacol. 27, pp.3-20.
19 De Zwart,
L.L., Haenen, H.E.G.M., Versantvoort, C.H.M., Sips, A.J.A.M..
Pharmacokinetics
of ingested xenobiotics in children. RIVM report 623860011. RIVM,
Bilthoven, The
Netherlands, june 2002.
20 Kearns, G.L.
& Reed, M.D. (1989) Clinical pharmacokinetics in infants and children. A
reappraisal.
Clinic. Pharmacokinetics 17 (Suppl. 1) 29-67.
21 Morselli, P.L.
(1989) Clinical pharmacology of the perinatal period and early infancy.
Clinic.
Pharmacokinetics 17 (Suppl. 1) 13-28.
22 Dietert, R.R.,
Etzel, R.A., Chen, D., Halonen, M., Holladay, S.D., Jabarek, A.M.,
Landreth, K.,
Peden, D.B., Pinkerton, K., Smialowitz, R.J., Zoetis, T. (2000) Workshop to
identify critical
windows of exposure for children’s health: immune and respiratory
systems work
group summary. Environ Health Perspect. 108 Suppl 3, pp. 483-490.
23 Hoeger, P.H.,
Enzmann, C.C. (2002) Skin physiology of the neonate and young infant: a
prospective study
of the functional skin parameters during early infancy. Pediatr.
Dermatol. 19, pp.
256-262.
24 Dermal
Exposure Assessment: Principles and Applications. Section 2.3.1.2 Age of the
skin.
EPA/600/9/91/011B. Interim report. 1992.
25 Ginsberg, G.,
Hattis, D., Sonawane, B., Russ, A., Banati, P., Kozlak, M., Smolenski, S,
Goble, R. (2002)
Evaluation of child/adult pharmacokinetic differences from a database
derived from the
therapeutic drug literature. Toxicol. Sci. 66, pp. 185-200.
26 Pinkerton,
K.E., Joad, J.P. (2000) The mammalian respiratory system and critical
windows of
exposure for children.s health. Environ. Health Perspect. 108, suppl 3, pp
457-462.
27 Renwick, A.G.,
Lazarus, N.R. Human variability and noncancer risk assessment. An
analysis of the
default uncertainty factor. Regul. Toxicol. Pharmacol. 27, pp.3-20, 1998.
28 Dietrich,
K.N., Ris, M.D., Succop, P.A., Berger, O.G., Bornschein, R.L. (2001) Early
exposure to lead
and juvenile delinquency. Neurotoxicology and Teratology 23, pp. 511-
518.
29 Morgan, R.E.,
Garavan, H., Smith, E.G., Driscoll, L.L., Levitsky, D.A., Strupp, B.J.
(2001) Early lead
exposure produces lasting changes in sustained attention, response
initiation, and
reactivity to errors. Neurotoxicology and Teratology 23, pp. 519-531.
30 Bayer, S.A.,
Altman, J. Russo, R.J., Zhang, X. (1993) Timetables of neurogenesis in the
human brain based
on experimentally determined patterns in the rat. Neurotoxicology 14,
pp.83-144.
31 Rodier, P.M.
(1995) Developing brain as a target of toxicity. Environ. Health Perspect.
103 (suppl. 6)
pp. 73-76.
32 Uylings, H.B.,
Van Eden, C.G. (1990) Qualitative and quantitative comparison of the
prefrontal cortex
in rat and in primates, including humans. Prog. Brain res. 85, pp. 31-62.
33 Cunningham,
J., Dockery, D.W., Speizer, F.E. (1994) Maternal smoking during pregnancy
as a predictor of
lung function in children. Am. J. Epidemiol. 139, pp. 1139-1152.
34 Boschker,
B.G.C. (2001) Is het zich ontwikkelende immuunsysteem gevoeliger voor
toxische effecten
dan het volwassen immuunsysteem? Stagerapport RIVM.
35 Bigazzi, P.E.
(1997) Autoimmunity caused by xenobiotics. Toxicology 119, pp. 1-21.
36 Yazdanbakhsh
M, Kremsner PG, Van Ree R. (2002) Allergy, parasites, and the hygiene
hypothesis.
Science 296, pp.490-494.
37
system:
Differential effects of immunotoxicants depend on time of exposure, Environ.
Health Perspect
108 (suppl 3) pp. 463-473.
38
Weisglas_Kuperus, N. , Patandin, S., Berbers, G.A., Sas, T.C., Mulder, P.G.,
Sauer, P.J.,
Hooijkaas, H.
(2000) Immunologic effects of background exposure to polychlorinated
biphenyls and
dioxins in Dutch preschool children. Environm. Health Perspect. 108, 1203-
1207.
39 Van Loveren,
H., Vos, J., Putman, E. and Piersma, A. Immunotoxicological consequences
of perinatal
chemical exposures: a plea for inclusion of immune parameters in
reproduction
studies. Toxicology (in press).
40 Goldman, L.R.,
Koduru, S. (2000) Chemicals in the environment and developmental
toxicity to
children: A public health and policy perspective. Environ. Health Perspect. 108
(suppl. 3)
442-448.
41 Cornford E.M.,
developing
animals. Fed Proc. 45, pp. 2065-2072.
42 Gupta, A.,
Agarwal, R., Shukla, GS. (1999) Functional impairment of blood-brain barrier
following
pesticide exposure during early development in rats. Hum. Exp. Toxicol. 18,
pp.174-179.
43 Eriksson, P.
(1996) Developmental neurotoxicology in the neonate--effects of pesticides
and
polychlorinated organic substances. Arch. Toxicol. Suppl. 18, pp. 81-88.
44 Eriksson P,
Viberg H, Jakobsson E, Orn U, Fredriksson A. (2002) A Brominated Flame
Retardant,
2,2’,4,4’,5-Pentabromodiphenyl Ether: Uptake, Retention, and Induction of
Neurobehavioral
Alterations in Mice during a Critical Phase of Neonatal Brain
Development.
Toxicol. Sci.67, pp. 98-103.
45
neurotoxicity,
Pesticide assessment guidelines. Subdivision F. Hazard evaluation: Human
and domestic
animals. Addendum 10 Neurotoxicity. EPA 540/09-91-123. pp. 32-48.
National
Technical Information Service,
46 Charnley, G.,
Putzrath, R.M. (2001) Children.s health, susceptibility, and regulatory
approaches to
reducing risks from chemical carcinogens. Env. Health Persp. 109, pp. 187-
192.
47 Landrigan,
P.J., Mattison, D.R., Boardman, B., Bruckner, J.V.,
Krewski, D.,
Weil, W.B. (2001) Comments on .Children.s health, susceptibility, and
regulatory
approaches to reducing risks from chemical carcinogens.. Env. Health Persp.
109, V5, pp. 5-6.
Appendix 1 Mailing list
1. Drs. J. Dornseiffen, VWS, VGB i.o.
2. Dr. ir. J. de Stoppelaar, VWS, Directie
VGB i.o.
3. Mr. J.A.M. Whyte, VWS, GZB
4. Dr. H. Jeuring, KvW
5. Directie RIVM
6. Dr. ir. D. Kromhout, sectordirecteur VCV
7. Dr.ir. M.N. Pieters, CRV
8. Dr. M.T.M. van Raaij, CRV
9. Dr. A.J.A.M. Sips, LBV
10. Dr. A.J. Baars, CRV
11. Dr. F.X.L. van Leeuwen, CRV
12. Dr. P.H. van Hoeven-Arentzen, CRV
13. Drs. T.G. Vermeire, CSR
14. Dr. G.J.A. Speijers, CRV
15. Drs. A.G.A.C. Knaap, CRV
16. Dr. C. Rompelberg, LBV
17. Drs. J. van Eijkeren, LBV
18. Dr. L.L. de Zwart, LBV
19. Dr. ir. M. J. Zeilmaker, LBV
20. Dr. ir. P. Bos, CRV
21. Dr. W.H. Könemann, CSR
22. Dr. H. van Loveren, LPI
23. Dr. H.E.M.G. Haenen, LGM
24. Dr. J.W. Van der Laan, LGM
25. Dr. ir. H. Derks, LGO
26. Dr. J. Bessems, TNO-Voeding, Zeist
27. Dr. C. Groen, Kinesis, Breda
28. Dr. M.C. Lans, CTB
29. Dr. L.P.A. Steenbekkers, Universiteit
Wageningen
30. Dr. H.F.G. van Dijk, Gezondheidsraad
31. Dr. D. Kloet, RIKILT, Wageningen
32. Dr. S. Olin, ILSI Risk Science Institute,
Washington, USA
33. Dr. G. Charnley, Health Risk Strategies,
Washington, USA
34. Dr. N. Freeman Rutgers University, New
Jersey, USA
35. Dr. G. Heinemeyer,Federal Institute for
Health Protection of Consumers and
Veterinary
Medicine, Berlin, Germany
36. Dr. W. Snodgrass, University of Texas, Galveston,
USA
37. Dr. J. Hughes, University of Leicester,
Leicester
38. Dr. I. Kraul, Danish EPA, Copenhage
39. Dr. M. Maroni, Università di Milano,
Milan.
40. Dr. K. Thoran, National Chemicals
Inspectorate, Solna, Sweden
41. Dr. J. Herrman, WHO, Geneva, Switzerland
42. Dr. A. Moretto, University of Padova,
Italy
43. Dr. A. Boobis, Imperial College School of
Medicine, London, UK
44. Dr. L.P. Davies, Therapeutic Goods
Administration, Barton, Australia
45. Dr. V.L. Dellarco, Office of Pesticide
Programs, US-EPA, Washington DC, USA
46. Dr. J. Borzelleca,
47. Dr. H. Häkansson, Karolinska Institute,
48. Dr. S. Page, WHO,
49. Dr. M. Tasheva,
50. Dr. R. Solecki, Federal Institute for
Health Protection of Consumers and Veterinary
51. Dr. I. Dewhurst, Pesticide Safety
Directorate,
52. Dr. S. Logan, Therapeutic Goods
Administration,
53. Dr. F.R. Puga, Instituto Biologico,
54. Dr. W. Phang, Office of Pesticide
Programs, US-EPA,
55. Dr. K.L. Hamernik, Office of Pesticide
Programs, US-EPA,
56. Dr. J.J. Larsen,
57. Dr. T. Marrs, Food Standard Agency,
58. Dr. C. Vleminckx, Scientific
59. Dr. D.B. McGregor, Toxicology Evaluation
Consultants,
60. Dr. E. Mendez, Office of Pesticide
Programs, US-EPA,
61. Dr. A.W. Tejada, FAO,
62. Dr. D.J. Hamilton, Dept. of Primary
Industries,
63. Dr. L.T. Haber, Toxicology Excellence for
Risk Assessment,
64. Depot Nederlandse Publikaties en
Nederlandse Bibliografie
65. Bureau Rapporten Registratie
66. Bibliotheek RIVM
67. SBC/Communicatie
68-77 Bureau Rapportenbeheer
78-80 Auteurs
80-100
Reserveexemplaren