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Effects observed at various subsequent samplings, but not at the
end of the study period.
IMPACTS WHICH INFLUENCE ENVIRONMENTAL EFFECTS AND While the ecosystem is a major recipient of many toxic substances, humans are also a part of the
ecosystem and poisonings do oc cur in humans as well as in other non -target organisms. The
incidence of cases of poisoning of humans is of obvious importance but is well documented.
The following section will focus more on the environmental impact of toxic substances.
All substances are potentially toxic, it is the dose or the exposure that makes the poison. The
likelihood that adverse environmental effects will result from the release of a substance to the
environment is governed by a number of factors indicat ed in the equation below.
RISK α ( PERSISTENCE) x ( MOBILITY) x (TOXICITY) x ( VOL OF USE) These very important properties of the substance are seldom found to act alone — they are
usually found in combinations of two or more of:
● Toxicity towards non -target organisms (this is fixed by the chemical properties of the
substance and the biology and biochemistry of the receptor organism).
Amount of substance used or released (this is under the control of humans).
Persistence of the substance (this depends on the physicochemistry of the substance and the
environment into which it is released).
Potential for movement in the environment (like the previous point, this is affected by the
properties of the substance and the characteristics of the environment). The effects of toxic substances in ecosystem situations are best illustrated in case histories which
demonstrate some of the principles of environ mental toxicity discussed above. Although this
section will focus on a narro w range of toxic substances, the principles discussed are very
relevant to many other substances and situations. The first of these, the case of DDT and
Atlantic salmon, illustrates the need for toxicity data for all life stages of an organism as well as
their source of food.
3.3.1 DDT and the Atlantic Salmon Atlantic salmon spend from 2 -4 years in fresh water rivers and about 2 years in the ocean. Adult
salmon will return to spawn in the rivers from whence they came. Atlantic salmon used to be
found in most of the rivers of Europe and were also very common in the rivers of North America
when the early settlers arrived. They were used as a source of food by native Indians as well as
the colonists and became an important export item to Europe. With the development of dams
(for mills) and the pollution caused by wool, pulp and paper mills, the salmon were exposed to a
double jeopardy. They no longer had access to the upper reaches of rivers and streams for
spawning and the quality of the water was such that they died from toxic effects of the pollutants
in the water and/or a lack of oxygen. The catch suffered and, by 1840, only a total of 500 kg of
salmon was exported to Europe from Eastern Canada.
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