DDT was introduced into the environment on a large during the latter parts of WWII to eradicate malaria which is transmitted by mosquitoes. It was so effective that it was offered to farmers to spray on crops and to local governments to control mosquitoes and other biting insects which cause human discomfort.
In the late 1950's and early 1960's a number of wildlife ecologists had noticed a decline in a number of fish eating birds in a number of ecosystems. This was caused by thinning of eggshells when the bird laid eggs. When they sat on them to incubate they broke and so the chicks failed to develop.
Another problem was that DDT was being detected in significant amounts in dairy products in Europe. This is because the substance is soluble in fat and so would appear in high amounts. Then it was detected in human breast milk.
DDT has a number of characteristics that lead to these observations:
1. Long stability and activity. It can act in a sprayed area for a long time.
2. Fat soluble. This means that it is stored in the fats and oils that are produced by plants and animals.
3. It is not metabolised (broken down) by living things very easily.
Because of this, the amount of DDT can build up in tissues, if the rate of intake is more rapid than breakdown. This is called BIOACCUMULATION. For example if a human can break down 5 units of DDT/year but takes in 11, he/she will accumulate 6 units DDT every year. After 10 year, 60 units will be stored.
On top of this there is also BIOMAGNIFICATION. This is where the accumulation of DDT is magnified every level on the food chain.
Looking at the example here the DDT is in very low levels in the water. However, it is picked up and stored in the tissues of zooplankton. Then if the minnows eat 12 zooplankton, they get 0.48 ppm DDT, then if the needlefish eat 4 minnows they get their 2 ppm of DDT. The bird only has to eat 13 needlefish to get the dosage as shown in the figure. Biomagnification shows large jumps in the amount of DDT at each stage of a food chain, that is, the dose is magnified. Even though low levels of environmental DDT wouldn't affect the bird if it was the only means of exposure, the fact that the concentrations get so high through the food chain means that it will have an effect on birds, and in this case its eggshells.
With the detection of DDT in human breast milk and foods that humans consume at levels considered high, countries began banning its use in that late 1960's onwards. Other pesticides such as organophosphates suffered the same fate a bit later for the same reasons.
DDT is still used in limited amounts in countries with high malaria risks.
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