DDT as a pollutant

DDT is a synthetic chemical compound called dichloro-diphenyl-trichloroethane. It is a chlorinated hydrocarbon. and among the most persistent and bioaccumulative of the persistent organic pollutants. Once it is released into the environment, it lasts for years and even decades, and is capable of being transported long distances through the atmosphere and other environmental media.

DDT does not occur naturally. The presence of DDT in the environment is generally a result of contamination due to its application as a pesticide. This broad spectrum insecticide can kill a wide range of insects and other arthropods, and persists breaking down very slowly and retaining its effectiveness for long periods after application.

Humans can be exposed to DDT, and its sister compounds DDE and DDD, primarily by eating food that contains small amounts of these compounds. Even in countries where DDT is banned, its previous use and persistence means that small amounts of DDT and DDE are found in soil and, under certain conditions, may be transferred to crops grown on this soil. In addition, imported foods may have been directly exposed to DDT. Once inside the body, these compounds are stored most readily in fatty tissue. Stored amounts leave the body very slowly, primarily in urine, also in breast milk.

Short-term exposure to high doses of DDT affects primarily the nervous system. People who have swallowed very high amounts of DDT experienced excitability, tremors, and seizures. These effects on the nervous system appeared to be reversible once exposure stopped. Some people who came in contact with DDT complained of rashes or irritation of the eyes, nose, and throat. People exposed for a long-term at low doses, such as people who made DDT, had some changes in the levels of liver enzymes, but there was no indication that DDT caused irreversible harmful (non-cancer) effects.

Tests in animals suggest that exposure to DDT may have a harmful effect on reproduction, and long-term exposure may affect the liver. Studies in animals have shown that oral exposure to DDT can result in an increased occurrence of liver tumours. The US Department of Health and Human Services has determined that DDT may reasonably be anticipated to be a carcinogen.

DDT interferes with bird's ability to metabolize calcium, resulting in the production of eggs with shells so thin that they break when nesting parents sit on them. Some species of birds in certain environments accumulate enough DDT to cause death. DDT also has potential for causing cancer, mutations and birth defects.

DDT, 1,1,1-trichloro-2,2-bis-(p-chlorophenyl)ethane, was one of the most widely used chemicals for controlling insect pests on agricultural crops and controlling insects that carry such diseases as malaria and typhus. Technical DDT is primarily a mixture of three forms, all of which are white, crystalline, tasteless and almost odourless solids. In addition, DDE, 1,1-dichloro-2,2-bis(p-chlorophenyl)ethylene, and DDD, 1,1-dichloro-2,2-bis(p-chlorophenyl)ethane, are found in small amounts as contaminants in technical DDT. DDD was also used to kill pests, and one form of DDD was used medically to treat cancer of the adrenal gland.

The amount of DDT in crops has been decreasing and is expected to continue to decrease with time. In the USA, the average amount of DDT and DDE eaten daily in food in 1981 was 2.24 micrograms per day (æg/day) (0.000032 mg/kg/day), with root and leafy vegetables containing the highest amount. Meat, fish, and poultry also contain very low levels of these compounds. Levels in most air and water samples are low and exposure by these pathways is of little concern. Because inhaled DDT, DDE, or DDD particles are generally too large to pass through the lungs into the body, they are more likely coughed-up and ingested. These compounds are very poorly absorbed through the skin.

The two primary uses of DDT that have resulted in its environmental dispersal are in public health and agriculture. In the former it has largely been employed as a biocide against adult mosquitoes in malarial control. According to WHO, in 1999 approximately 20 countries use DDT, and many of these countries do not have the resources or infrastructure to abandon its use at this time. In agriculture, one of its principal applications is as a biocide in the protection of cotton crops.

Restrictions upon its agricultural uses have already been enacted in the USA, Canada, Japan, Sweden and the former Soviet Union following observations of unwanted mortalities and morbidities among non-target organisms. For example, although DDT was banned for agricultural use in the USA in 1973, it is permitted in Latin American countries. Songbirds that winter in Latin American countries continue to be exposed to DDT.

The biggest users of DDT are china and India. The World Bank has lent India £130 million to find alternatives to DDT.

DDT was directly responsible for the threatened condition of the Peregrine falcon, osprey, bald eagle and brown pelican in North America until it was banned in 1972.

Ecuador has increased the use of DDT since 1993 and has seen a 60 per cent decline in new malaria cases. By comparison Paraguay, Bolivia and Peru which have stopped using DDT, the incidence of malaria has increased by 90 per cent.

In Mexico several bat species are being threatened due to ingestion of insects that contain DDT.