Bioaccumulation, as defined below, may play a part in pd, if toxins truly are
a factor:
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Date: 95-11-03 16:34:17 EST
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BIOACCUMULATION
DEFINING BIOACCUMULATION
An important process through which chemicals can affect living organisms
is bioaccumulation. Bioaccumulation means an increase in the concentration
of
a chemical in a biological organism over time, compared to the chemical's
concentration in the environment. Compounds accumulate in living things any
time they are taken up and stored faster than they are broken down
(metabolized) or excreted. Understanding the dynamic process of
bioaccumulation is very important in protecting human beings and other
organisms from the adverse effects of chemical exposure, and it has become a
critical consideration in the regulation of chemicals.
A number of terms are used in conjunction with bioaccumulation. Uptake
describes the entrance of a chemical into an organism -- such as by
breathing,
swallowing, or absorbing it through the skin -- without regard to its
subsequent storage, metabolism, and excretion by that organism.
Storage, a term sometimes confused with bioaccumulation, means the
temporary deposit of a chemical in body tissue or in an organ. Storage is
just one facet of chemical bioaccumulation. (The term also applies to other
natural processes, such as the storage of fat in hibernating animals or the
storage of starch in seeds.)
Bioconcentration is the specific bioaccumulation process by which the
concentration of a chemical in an organism becomes higher than its
concentration in the air or water around the organism. Although the process
is the same for both natural and manmade chemicals, the term
bio-concentration
usually refers to chemicals foreign to the organism. For fish and other
aquatic animals, bioconcentration after uptake through the gills (or
sometimes
the skin) is usually the most important bioaccumulation process.
Biomagnification describes a process that results in the accumulation
of a chemical in an organism at higher levels than are found in its food. It
occurs when a chemical becomes more and more concentrated as it moves up
through a food chain -- the dietary linkages between single-celled plants and
increasingly larger animal species.
A typical food chain includes algae eaten by the water flea eaten by a
minnow eaten by a trout and finally consumed by an osprey (or human being).
If each step results in increased bioaccumulation, that is, biomagnification,
then an animal at the top of the food chain, through its regular diet, may
accumulate a much greater concentration of chemical than was present in
organisms lower in the food chain.
Biomagnification is illustrated by a study of DDT which showed that
where
soil levels were 10 parts per million (ppm), DDT reached a concentration of
141 ppm in earthworms and 444 ppm in robins. Through biomagnification, the
concentration of a chemical in the animal at the top of the food chain may be
high enough to cause death or adverse effects on behavior, reproduction, or
disease resistance and thus endanger that species, even when levels in the
water, air, or soil are low. Fortunately, bioaccumulation does not always
result in biomagnification.
THE BIOACCUMULATION PROCESS
Bioaccumulation is a normal and essential process for the growth and
nurturing of organisms. All animals, including humans, daily bioaccumulate
many vital nutrients, such as vitamins A,D and K, trace minerals, and
essential fats and amino acids. What concerns toxicologists is the
bioaccumulation of substances to levels in the body that can cause harm.
Because bioaccumulation is the net result of the interaction of uptake,
storage and elimination of a chemical, these parts of the process will be
examined further.
UPTAKE
Bioaccumulation begins when a chemical passes from the environment into
an organism's cells. Uptake is a complex process which is still not fully
understood. Scientists have learned that chemicals tend to move, or diffuse,
passively from a place of high concentration to one of low concentration.
The
force or pressure for diffusion is called the chemical potential, and it
works
to move a chemical from outside to inside an organism.
A number of factors may increase the chemical potential of certain
substances. For example, some chemicals do not mix well with water. They
are
called lipophilic, meaning "fat loving," or hydrophobic, meaning "water
hating." In either case, they tend to move out of water and enter the cells
of an organism, where there are lipophilic microenvironments.
STORAGE
The same factors affecting the uptake of a chemical continue to operate
inside an organism, hindering a chemical's return to the outer environment.
Some chemicals are attracted to certain sites, and by binding to proteins or
dissolving in fats, they are temporarily stored. If uptake slows or is not
continued, or if the chemical is not very tightly bound in the cell, the body
can eventually eliminate the chemical.
One factor important in uptake and storage is water solubility; the
ability of a chemical to dissolve in water. Usually, compounds that are
highly water soluble have a low potential to bioaccumulate and do not leave
water readily to enter the cells of an organism. Once inside, they are
easily
removed unless the cells have a specific mechanism for retaining them.
Heavy metals like mercury and certain other water-soluble chemicals are
such an exception, because they bind tightly to specific sites within the
body. When binding occurs, even highly water-soluble chemicals can
accumulate. This is illustrated by cobalt, which binds very tightly and
specifically to sites in the liver and accumulate there despite its water
solubility. Similar accumulation processes occur for mercury, copper,
cadmium, and lead.
Many fat-loving (lipophilic) chemicals pass into organism's cells
through
the fatty layer of cell membranes more easily than water-soluble chemicals.
Once inside the organism, these chemicals may move through numerous membranes
until they are stored in fatty tissues and begin to accumulate.
The storage of toxic chemicals in fat reserves serves to detoxify the
chemical, or at least removes it from harms way. However, when fat reserves
are called upon to provide energy for an organism the materials stored in the
fat may be remobilized within the organism and may again be potentially
toxic.
If appreciable amounts of a toxin are stored in fat and fat reserves are
quickly used, significant toxic effects may be seen from the remobilization
of
the chemical.
ELIMINATION
Another factor affecting bioaccumulation is whether an organism can
break
down and/or excrete a chemical. The biological breakdown of chemicals is
termed metabolism. This ability varies among individual organisms and species
and also depends on characteristics of the chemical itself.
Chemicals that dissolve readily in fat but not in water tend to be more
slowly eliminated by the body and thus have a greater potential to
accumulate.
Many metabolic reactions change a chemical into more water soluble forms
called metabolites, that are readily excreted.
There are exceptions, however. Natural pyrethrins, insecticides that
are
derived from the chrysanthemum plant, are highly fat-soluble pesticides, but
they are easily degraded and do not accumulate. The insecticide
chlorpyrifos,
which is less fat-soluble but more poorly degraded, tends to bioaccumulate.
Factors affecting metabolism often determine whether a chemical achieves its
bioaccumulation potential in a given organism.
BIOACCUMULATION: A STATE OF DYNAMIC EQUILIBRIUM
When a chemical enters the cells of an organism, it is distributed and
then excreted, stored or metabolized. Excretion, storage, and metabolism
decrease the concentration of the chemical inside the organism, increasing
the
potential of the chemical in the outer environment to move into the organism.
During constant environmental exposure to a chemical, the amount of a
chemical
accumulated inside the organism, and the amount leaving, reach a state of
dynamic equilibrium.
To understand this concept of dynamic equilibrium, imagine a tub filling
with water from a faucet at the top and draining out through a pipe of
smaller
size at the bottom. When the water level in the tub is low, little pressure
is exerted on the outflow at the bottom of the tub. As the water level
rises,
the pressure on the outflow increases. Eventually, the amount of the water
flowing out will equal the amount flowing in, and the level of the tub will
not change. If the input or outflow is changed, the water in the tub adjusts
to a different level.
It is the same concept with living organisms. An environmental chemical
will at first move into an organism more rapidly than it is stored, degraded,
and excreted. With constant exposure, its concentration inside the organism
gradually increases. Eventually, the concentration of the chemical inside
the
organism will reach an equilibrium with the concentration of the chemical
outside the organism, and the amount of chemical entering the organism will
be
the same as the amount leaving. Although the amount inside the organism
remains constant, the chemical continues to be taken up, stored, degraded,
and
excreted.
If the environmental concentration of the chemical increases, the amount
inside the organism will increase until it reaches a new equilibrium.
Exposure to large amounts of a chemical for a long period of time, however,
may overwhelm the equilibrium (for example, overflowing the tub) potentially
causing harmful effects.
Likewise, if the concentration in the environment decreases, the amount
inside the organism will also decline. Should the organism move to a clean
environment, so that exposure ceases, then the chemical eventually will be
eliminated from the body.
FACTORS AFFECTING BIOACCUMULATION
This simplified explanation does not take into account all of the many
factors that affect the ability of chemicals to be bioaccumulated. Some
chemicals bind to specific sites in the body, prolonging their stay, whereas
others move freely in and out. The time between uptake and eventual
elimination of a chemical directly affects bioaccumulation. Chemicals that
are immediately eliminated, for example, do not bioaccumulate.
Similarly, the duration of exposure is also a factor in bioaccumulation.
Most exposures to chemicals in the environment vary continually in
concentration and duration, sometimes including periods of no exposure. In
these cases, an equilibrium is never achieved and the accumulation is less
than expected.
Bioaccumulation varies between individual organisms as well as between
species. Large, fat, long-lived individuals or species with low rates of
metabolism or excretion of a chemical will bioaccumulate more than small,
thin, short-lived organisms. Thus, an old lake trout may bioaccumulate much
more than a young bluegill in the same lake.
SUMMARY
Bioaccumulation results from a dynamic equilibrium between exposure from
the outside environment and uptake, excretion, storage, and degradation
within
an organism. The extent of bioaccumulation depends on the concentration of a
chemical in the environment, the amount of chemical coming into an organism
from the diet, water, or air, and the time it takes for the organism to
acquire the chemical and then excrete, store, and/or degrade it. The nature
of
the chemical itself, such as its solubility in water and fat, affects its
uptake and storage. Equally important is the ability of the organism to
degrade and excrete a particular chemical. When exposure ceases, the body
gradually metabolizes and excretes the chemical.
Bioaccumulation is a normal process that can result in injury to an
organism only when the equilibrium between exposure and bioaccumulation is
overwhelmed, relative to the harmfulness of the chemical. Sometimes
bioaccumulation can be a protective mechanism in which the body accumulates
needed chemicals.
This TIB is part of the EXTOXNET Pesticide Information Notebook. For
more information, contact the Pesticide Management Education Program, Cornell
University, 5123 Comstock Hall, Ithaca, N.Y. 14853.
DISCLAIMER: The information in this brief does not in any way replace or
supersede the information on the pesticide product label/ing or other
regulatory requirements. Please refer to the pesticide product label/ing.
