Science
Read the
following content and answer the questions that follow based on the discussions
in class in your science note book. Submit on Monday, 31/10/16.
The chemicals like DDT are
either washed down into the soil or into the water bodies.
From the soil, these are absorbed by
the plants along with water and minerals, and from the water bodies these are
taken up by aquatic plants and animals. This is one of the ways by which they
enter food chain. As these chemicals are not degradable, these get accumulated
progressively at each trophic level. As human beings occupy the top level in
any food chain, the maximum concentration of these chemicals gets accumulated
in
our bodies. This phenomenon is known
as biological magnification. This is the reason why our food grains such as
wheat and rice, fruits and vegetables and even fish contain varying amounts of
pesticide residues. These chemicals cause stomach related problems or can even
affect our nervous system.
In the 1970s and
1980s, agricultural use was banned in most developed countries, beginning with Hungary in 1968 followed by Norway and Sweden in 1970, West
Germany and
the US in 1972, but not in the United
Kingdom until
1984. By 1991 total bans, including for disease control, were in place in at
least 26 countries; for example Cuba in 1970, Singapore in 1984, Chile in 1985
and the Republic of Korea in 1986.
Despite the
worldwide ban, agricultural use continued in India, North Korea, and
possibly elsewhere as of 2008. Today, about 3,000 to 4,000 tons of DDT are produced each year for disease vector
control. DDT is
applied to the inside walls of homes to kill or repel mosquitoes. DDT and other
pesticides had been shown to cause cancer and that their agricultural use was a
threat to wildlife, particularly birds.
Q1. How do chemicals reach water
bodies?
Q2. What is meant by
Bio-magnification?
Q3. Some countries like USA have
banned DDT. But some developing nations are still using DDT, why?
Q4. What are the effects of DDT on
human beings?
Hindi
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