Polychlorinated Biphenyls
Polychlorinated biphenyls are a family of man-made chemicals that contain
two
hundred and nine individual compounds with varying toxicity. Many
different
combinations are possible. In chemical terminology, "phenyl"
denotes a ring
structure of six carbon atoms attached to something else;
"biphenyl" results
when two such rings are attached to each other.
Polychlorinated biphenyl (PCB)
is any molecule having multiple chlorine atoms
attached to the carbon atoms of a
biphenyl nucleus. Chlorine atoms can be
placed at any or all of ten available
sites. PCB’s have been used widely as
coolants and lubricants in transformers,
capacitors, and other electrical
equipment. The manufacture of PCBs stopped in
the United States in October
1977 because of evidence that PCBs accumulate in
the environment and may
cause health hazards for humans. Although PCBs are no
longer manufactured,
human exposure still occurs. Many older transformers and
capacitors still
contain fluids that contain PCBs. The useful lifetime of these
transformers
can be thirty years or more. Prior to 1977, certain occupational
settings
had, and may still have, higher levels of human exposure. Animal
experiments
have shown that some PCB mixtures produce adverse health effects
that include
liver damage, skin irritations, reproductive and developmental
effects, and
cancer. With these observations, it is most considerably dangerous
for
humans. The U.S Department of Health and Human Services has determined
that
PCBs may reasonably be expected to be carcinogens. Human studies to
date show
that irritations, such as acne-like lesions and rashes, can occur
in PCB-exposed
workers. Other studies of people with occupational exposure
suggest that PCBs
might cause liver cancer. Reproductive and developmental
effects may also be
related to occupational exposure and eating of
contaminated fish. While the role
in producing cancer, reproductive and
developmental effects can not be clearly
determined, the suggestive evidence
provides an additional basis for public
health concerns about humans who
might be exposed to PCBs. It has also been
suggested that chemicals such as
PCBs in the environment can mimic the body’s
natural hormones. This could
lead to infertility, certain types of cancer, and
other hormone-related
disorders. In the past, discharges of PCB-filled wastes
into rivers, streams
and open landfills were considered acceptable, legal and
hazard-free
practices. PCBs were also sometimes intentionally released into
the
environment to reduce dust emissions from dirt roads, or as extenders in
some
agricultural pesticides formulations. These practices were inappropriate
and
potentially harmful. There have also been accidental releases of PCB
and
associated contaminants into the environment via leaking of sealed PCB
fluid
compartments during commercial use of transformers and capacitors, as a
result
of improper disposal of PCB-containing equipment or chemical products.
The
contamination of food for animal and human consumption has also occurred
from
PCBs leaking from malfunctioning heating coils into foods during
manufacturing.
PCBs can also enter the food chain through the ingestion
of contaminated fish
from PCB-contaminated water. Finally, transformer and
capacitor fires have led
to the environmental release of PCBs and thermal
degradation products. Concern
about the presence of PCBs in our environment
began around 1966 when research in
Sweden revealed the presence of PCBs
in environmental samples being screened for
DDT. Further study confirmed
suspicions that the rate of natural breakdown was
very slow for some of the
PCB congeners. Concern continued to rise due to
widespread human poisoning
episodes in 1968 and 1978. PCBs are now regarded as
environmental
contaminants that build up in the food chain. They build up in the
food chain
due to their relative insolubility in water and high solubility in
fats. To
date, there are tests to determine PCBs in blood, body fat and breast
milk.
These tests are not routine clinical tests, but they can detect PCBs
in
members of the general population as well as in workers with
occupational
exposure to PCBs. Although these tests indicate if one has been
exposed to PCBs,
they do not predict potential health effects. Blood tests
are the easiest,
safest and perhaps, the best method for detecting recent
large exposures. As
well as tests, remediation techniques and disposal
technologies continue to
evolve. Many approaches have been tested out but
none has emerged as a
technically and economically viable solution for the
endless environmental
situations encountered. A useful method for cleansing
water will probably not be
effective in treating soil. Highly chlorinated
PCBs will not be as readily
decomposed as less-chlorinated PCBs. These types
of factors contribute to the
challenge of effective and cost-efficient
remediation of PCB contaminated sites.
Bacterial build-up or breakdown of
PCBs in soil and sediment is potentially
useful technology that is receiving
much attention. Introductory results on the
breakdown of soil samples,
including those taken from the Hudson River, show
promise. A "thermal
blanket" approach, in which soil is heated from the
surface downward and PCBs
are removed under vacuum, is also investigated.
Another approach to
reduce PCB problems is finding alternatives to PCBs. There
are various
chemical alternatives to PCBs (including silicon fluids,
fluorocarbons, high
molecular weight hydrocarbons, low molecular weight
chlorinated hydrocarbons
and high boiling oils and esters) which will work in
electrical equipment;
however, a transformer constructed for use with PCB fluid
may not operate at
the same power load with a substitute chemical. Such a
transformer becomes
less efficient, resulting in a higher operating cost as well
as added risk of
fire. Precautionary measures must be taken to ensure fire
safety and proper
disposal of waste. Clearly shifting from PCBs to alternatives
has involved
tradeoffs, but not elimination, of health and safety risks. In
conclusion,
because of the ban on PCB production, decreasing use of PCBs and
efforts to
remediate contaminated sites, general population exposure to PCBs in
fish and
other foods has been significantly reduced, and PCB levels in human
blood are
also decreasing. Industry and government should focus on the
development of
novel and cost-effective remediation and disposal techniques, as
well as on
scientifically improved health risk analysis to reduce the
uncertainty
associated with PCB exposure and health effects in
humans.
Bibliography
Page Internet, Polychlorinated Biphenyls,
‘Yahoo’ 1992 http://mail.odsnet.com/TRIFacts/92,html
Internet,
Organochlorine and Metal Pollution, ‘About’ 04/20/98
http://chemistry.about.com
Wall, E. Byron, Science in Society, Wall &
Emerson, Inc. Toronto, Ontario.
1989 Pages 326-329