Transcript Part III: Persistence and Prevention

Environmental Endocrine
Disruptors
Part III: Persistence and Prevention
Hypothalamus
Thyroid
gland
Adrenal
glands
Testicles
(men)
Pineal gland
Pituitary gland
Parathyroid
gland
Michael H. Dong
MPH, DrPA, PhD
Pancreas
Ovaries
(women)
Readings
05/30/2004, Elk Grove, California, USA
Course Objectives
 Learn the terms pertaining to a chemical’s
persistence properties, especially those of an
environmental endocrine disruptor.
 Recognize the science-based criteria and
assessment models for the determination of
persistence potential.
 Appreciate the cumulative effects due to a
disruptor’s persistence properties.
 Review the processes and strategies for
prevention of persistent pollution.
Basics of Persistence (I)
 There are two major aspects or quantities
of persistence for each chemical substance.
 The first is its intrinsic or chemical
persistence and the other, its persistence in
the (overall) environment.
 Environmental persistence is a term used
to imply a constant quantity or availability of
the chemical in the environment(s) at issue;
this type of potential thus has greater public
health importance.
Basics of Persistence (II)
 Chemical persistence is often expressed in
terms of a substance’s degradation half-life.
 The amount of material lost by degradation
in a medium is determined both by the rate
constant specific to the reactivity occurring in
that medium, and by the amount (initially and
subsequently) present in that medium.
 For chemicals in the environment, their
degradation half-lives depend not only on
their physicochemical properties, but also
much more on the environmental conditions.
Basics of Persistence (III)
 Environmental persistence is much more
dynamic, situational, and complex, when
compared to persistence in a single medium.
 Many substances have properties that
allow them to partition or dissolve in certain
media, and even to speciate into a particular,
irreversible form.
 Environmental persistence thus should be
evaluated on the basis of degradation rates,
partitioning, speciation, and availability.
Long-Range Transport
 An important aspect of environmental
persistence is its effects towards long-range
transport (LRT) of the environmental
contaminants.
 Through LRT, persistent pesticides and
other industrial chemicals can be present in
remote regions such as the Arctic, where these
substances have never been used.
 The modes of LRT are not limited to atmospheric, oceanic, or terrestrial, but include
cyclone activities and animal migration.
PBT Endocrine Disruptors
 Pollutants that are persistent, bioaccumulative, and toxic (PBT), of which a good
number are endocrine disruptors, are longlasting and can build up in the food chain.
 PBTs are of great ecological concern
because they can induce severe adverse
health effects, such as endocrine disruption.
 Numerical criteria have been set up to
identify the persistence and bioaccumulation
potentials of toxic environmental pollutants.
Mass Balance Models (I)
 For overall environmental persistence and
persistence within a single medium, both the
measurement and the quantity are typically
based on some mass balance theories.
 Mass balance per se is based on the law of
conversion of mass; it builds on the concept
that physical or chemical changes do not
destroy or create matter.
 In practice, the main task of a mass balance
model is on knowing how much of a material
is left in the form of interest.
Mass Balance Models (II)
 Mass balance models (MBMs) appear to
offer the most convenient means for assessing
a pollutant’s persistence in a single medium or
in a multimedia environment.
 Simple or multimedia MBMs are actually
mathematical constructs designed to gain an
understanding of the environmental behavior
of chemicals.
 In almost all cases, the use of multimedia
or simple MBMs is limited by the availability
of data on degradation half-lives.
Low Levels of PBTs
 For certain types of adverse health effects
such as endocrine disruption, even low levels
of persistent and bioaccumulative toxicants
(PBTs) in the environment collectively are
still of global concern.
 This is because, by comparison, inducibility of environmental endocrine disruption is
not chemical-selective.
 Yet more importantly, inducibilities of
certain multiple PBTs for the same adverse
effect are likely additive, if not synergistic.
Cumulative Effects (I)
 Concurrent or subsequent exposures to
persistent and bioaccumulative toxicants, of
which many are environmental endocrine
disruptors, are of global concern even when
these pollutants are present at very low
concentrations.
 Synergism or additive effects have been
observed between steroid hormones; between
temperature and hormone response; between
weakly estrogenic compounds; and between
pesticides.
Cumulative Effects (II)
 More recently, additive effects have been
observed from the combination of a strong
estrogen 17-estradiol and a weak estrogen
such as bisphenol A, nonylphenol, phenyl
salicylate, butylparaben, or genistein.
 The natural estrogen estradiol is many
thousand times more estrogenic than any of
the weak xenoestrogens tested.
 Additive antiandrogenic effects also have
been observed with the pesticides vinclozolin
and procymidone.
Cumulative Effects (III)
 Additive-like effects have been observed
from multiple thyroid hormone disruptors
given to rats; these disruptors are dioxins,
furans, and dioxin-likes such as PCBs.
 Other thyroid hormone disruptors, such as
aldicarb, atrazine, and nitrate, also have been
found to exert synergistic effects.
 Cumulative effects of this type are highly
critical in that thyroid functions are important
for proper growth, metabolism, reproduction,
mental development, etc.
Cumulative Effects (IV)
 Studies in various species (rats, salmon,
mink, chickens) were conducted, which
confirmed the bioaccumulative effects of
concurrent or subsequent exposures to
pollutants found in the Great Lakes fish.
 So was an epidemiology study showing
babies born with poorer visual recognition
from cumulative maternal exposure to PCBs;
mothers of this cohort consumed 2 to 3 meals
of PCB-contaminated fish monthly for 6 years
prior to and after pregnancy.
Time of Exposure
 While exposure to environmental pollutants
is mainly a function of their availability, time
of exposure is equally a very crucial factor in
the induction of endocrine disruption.
 For example, estrogen levels in humans and
rodents were seen to increase steadily throughout the pregnancy period, primarily due to a
feed-forward mechanism of regulation; that is,
any dose of a xenoestrogen would be additive
with the endogenous level, due to the lack of
feed-back control during this period.
Pollution Prevention (I)
 The potency and toxicity of environmental
endocrine disruptors (EEDs) are less critical.
 The effects of an EED’s persistence and
bioaccumulation are much more dynamic,
more attenuable, and hence more critical.
 One effective method of intervention is to
divert the pollutant to an environment where
its degradation half-life can be shortened
considerably; in addition, chemical bioaccumulation can be intervened by segregation of
relevant predators in the food chain.
Pollution Prevention (II)
 All processes and strategies for prevention
and intervention of environmental pollution
revolve around the principles of use reduction
and source elimination.
 Modern methods tend to rely on integrated
pest management and on chemical treatments
for elimination and reduction of pollutants.
 A great deal of sewage sludge and other
wastes also have been reduced considerably in
the USA through regulatory statues, such as
the Clean Water Act.
Pollution Prevention (III)
 Scientific procedures have been adopted
worldwide for the elimination and the
reduction of persistent and bioaccumulative
toxicants (PBTs).
 Numerical criteria are incorporated into
Canada’s Persistence and Bioaccumulation
Regulations for qualifying pollutants as PBTs.
 Using numerical criteria somewhat more
health conservative, U.S. EPA has continued
updating its PBT Profiler for many chemical
substances.
Pollution Prevention (IV)
 International actions on persistent and bioaccumulative toxicants (PBTs) require a
complex process of global compromise.
 The Stockholm Convention in 2001 is a
global treaty for protection from PBTs. Upon
ratification, it will ban outright the use of 8
persistent pesticides and take initiatives to
reduce the use of 4 non-pesticide PBTs.
 Pollution prevention actually should start
with an increase in public awareness of the
environmental levels and sources of PBTs.