Biomedical Engineering Reference
In-Depth Information
chronic illness and mortality (e.g., leukemia and radiation
disease) that could be directly linked to radiation expo-
sure. Following are a few cases where the effects did not
become apparent until after a protracted lag period.
associated with dioxin exposure, then there is also likely
to be residual dioxin contamination in the treated areas
of Vietnam. Dioxin is highly persistent.
Scientists from Vietnam, the United States, and 11
other countries have discussed the state of the science of
research in the health effects of dioxin. The Vietnamese
and US have agreed to a plan that addresses the need for
direct research on human health outcomes from expo-
sure to dioxin and research on the environmental and
ecological effects of dioxin and Agent Orange.
Chlorinated dioxins have 75 different forms and there
are 135 different chlorinated furans, depending on the
number and arrangement of chlorine atoms in the mol-
ecules. The compounds can be separated into groups
that have the same number of chlorine atoms attached
to the furan or to the dioxin ring. Each form varies in
its chemical, physical, and toxicological characteristics
( Figure 8.2-5 ) The primary concerns in Vietnam from
prolonged exposure to dioxin are reproductive and de-
velopmental disorders that may be occurring in the
general population.
Dioxins are created only unintentionally during
chemical reactions, especially combustion processes; that
is, they have never been synthesized for any other reason
than for scientific investigation, for example, to make
analytical standards for testing. The most toxic form
is the TCDD isomer, which is a byproduct when certain
Agent orange
The use of Agent Orange during the Vietnam War (used
between 1961 and 1970) dramatically demonstrates the
concept of ''latency period,'' where possible effects may
not be manifested until years or decades after exposure.
Agent Orange is a defoliant and weed-killing chemical
that was used by the US military during the Vietnam War.
It was sprayed to remove leaves from the trees behind
which the enemy troops would hide. Agent Orange was
dispersed by airplanes, helicopters, trucks, and backpack
sprayers. In the 1970s, years after the tours of duty in
Vietnam, some veterans became concerned that exposure
to Agent Orange might be the cause of delayed health
effects. One of the chemicals in Agent Orange contained
small amounts of the highly toxic compound 2,3,7,8-
tetrachlorodibenzo- para -dioxin (TCDD).
The US Department of Veteran Affairs (VA) has
listed a number of diseases, which could have resulted
from exposure to herbicides like Agent Orange. The law
requires that some of these diseases be at least 10%
disabling under VA's rating regulations within a deadline
that began to run the day a person left Vietnam. If there
is a deadline, it is listed in parentheses after the name of
the disease as follows:
Chloracne or other acneform disease consistent with
chloracne (must occur within one year of exposure
to Agent Orange)
Chronic lymphocytic leukemia
Diabetes mellitus, type II
Hodgkin's disease
Multiple myeloma
Non-Hodgkin's lymphoma
Acute and subacute peripheral neuropathy (for the
purpose of this section, the term acute and subacute
peripheral neuropathy means temporary peripheral
neuropathy that appears within weeks or months of
exposure to an herbicide agent and resolves within
two years of the date of onset)
Porphyria cutanea tarda (must occur within one year
of exposure to Agent Orange)
Prostate cancer
Respiratory cancers (cancer of the lung, bronchus,
larynx, or trachea)
Soft-tissue sarcoma (other than osteosarcoma,
chondrosarcoma, Kaposi's sarcoma, or mesothelioma)
The issue is international. After all, if it is true that US
soldiers exposed to Agent Orange show symptoms often
1
9
O
8
2
3
7
O
4
6
Dioxin structure
9
1
8
2
3
7
O
4
6
Furan structure
O
Cl
Cl
Cl
Cl
O
2,3,7,8-Tetrachlorodibenzo- para -dioxin
Figure 8.2-5 Molecular structures of dioxins and furans. Bottom
structure is of the most toxic dioxin congener, tetrachlorodibenzo-
para-dioxin (TCDD), formed by the substitution of chlorine for
hydrogen atoms at positions 2, 3, 7, and 8 in the molecule.
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