The bitter-sweet taste of toxic substances
Box 3 | DDT and biological concentration
Dichloro-diphenyl-trichloroethane, better known as DDT, is a inexpensive, highly effective insecticide with relatively low toxicity to mammals. DDT molecules insinuate themselves into the cell walls of insect nerves, where they allow sodium and potassium ions to leak out. The loss of ions affects the transmission of nerve impulses and the insect dies of paralysis.
DDT was widely used after it was first manufactured before World War II. Perhaps its greatest triumph has been in the control of the Anopheles mosquito, which transmits malaria.
But, after years of widespread use, it was realised that DDT is also a hazard and it is no longer used in Australia. The problem with DDT is that it is chemically stable and doesn’t break down readily in the environment. Because of this persistence in the environment, DDT has been suspected of causing deleterious effects to organisms higher up the food chain. This is because of a phenomenon known as biological concentration or amplification.
Biological concentration
When a potentially toxic and persistent substance is released into the environment, its concentration may be so low that is causes no obvious damage. It may move into and remain in the plants at the same low concentration in which it exists in water or soil. But a herbivore (a plant-eating animal) must eat about 10 grams of living matter to make 1 gram of itself. So herbivores will, on average, take in as much of the potentially toxic substance as was found in 10 individual plants. A carnivore (a meat-eating animal) will accumulate the toxin to a concentration about 10 times that found in a herbivore. Thus, the animals at the top of the food chain may contain the compound at a high enough concentration to be damaging, even though the concentration in the environment or in other species may be too low to cause harm. Biological concentration only works with non-biodegradable substances (ie, substances that are not easily degraded by the chemical processes of living things.)
An example of biological concentration
Imagine a field sprayed from the air with a pesticide. A cow grazing in the field will eat the pesticide that is on the plants. The pesticide will remain in the cow's tissues. Assuming that the pesticide is not degraded, the more the cow eats, the more pesticide it will accumulate. If the cow is slaughtered and its meat eaten, the human consumer ingests the pesticide at a much higher concentration than the cow did. If the person regularly eats meat from a herd of cows that grazed in that paddock, as time passes that person will also steadily accumulate the pesticide.
It is known that humans have accumulated compounds such as DDT in their tissues, but we cannot be certain that this has caused harmful effects. However, studies on carnivorous birds (eg, the peregrine falcon) have shown that accumulated DDT in their tissues caused a reduction in the amount of calcium deposited in their egg shells. This made the eggs very fragile, breaking even when the mother sat on them to keep them warm, and reduced the number of offspring.
Boxes
Box 1. Chances and risks
Box 2. Cyanide and arsenic
Related site
Bioaccumulation (Extension Toxicology Network, USA)
Posted February 1999.