An important
process by which chemicals can affect living organisms is through
bioaccumulation. Bioaccumulation means an increase in the concentration of a
chemical over time in a biological organism 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 (metabolised) 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, or 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 man-made chemicals, the
term bioconcentration 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 own food. It occurs when a chemical becomes more and
more concentrated as it moves up through a food chain -- the dietary
linkages from single-celled plants to increasingly larger animal species.
A
typical food chain includes algae eaten by a 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 though contamination levels in the air, water, or soil are low.
Fortunately, bioaccumulation does not always result in biomagnification.
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.
So it is 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,
the 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) and potentially cause harmful
effects.
Likewise, if
the concentration in the environment decreases, the amount inside the organism
will also decline. If the organism moves to a clean environment, in which there
is no exposure, then the chemical eventually will be eliminated from the
body.
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 tend to 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.