DDT orDichloro-diphenyl-trichloroethane (ClC6H4)2CH(CCl3) is a colourless crystalline organochloride insecticide. It is very soluble in fats and most organic solvents and practically insoluble in water.
PropertiesDDT is created by the reaction of trichloromethanal with chlorobenzene (C6H5Cl). Trade or other names for DDT include Anofex, Cesarex, Chlorophenothane, Dedelo, p,p'-DDT, Dichlorodiphenyltrichloroethane, Dinocide, Didimac, Digmar, ENT 1506, Genitox, Guesapon, Guesarol, Gexarex, Gyron, Hildit, Ixodex, Kopsol, Neocid, OMS 16, Micro DDT 75, Pentachlorin, Rukseam, R50 and Zerdane.
DDT is persistent in the environment, with a reported half life of between 26 days in river water (U.S. EPA, 1989) to 15 years and is immobile in most soils. Routes of loss and degradation include runoff, volatilization, photolysis and biodegradation (aerobic and anaerobic). These processes generally occur slowly. Breakdown products in the soil environment are DDE (1,1-dichloro-2,2-bis(p-dichlorodiphenyl)ethylene) and DDD (1,1-dichloro-2,2-bis(p-chlorophenyl)ethane), which are also highly persistent and have similar chemical and physical properties.
It was first synthesized in 1873, and its insecticidal properties were discovered by the Swiss scientist Paul Hermann Müller in 1942 who was awarded the 1948 Nobel Prize in Physiology and Medicine for his efforts. DDT is the best known of a number of chlorine-containing pesticides used in the 1940s and 1950s. It was extensively used during World War II among Allied troops and certain civilian populations to control insect typhus and malaria vectors. Entire cities in Italy were dusted to control the typhus carried by lice. DDT was also extensively used as an agricultural insecticide after 1945. In the 1950s, in some uses doses of DDT and other insecticides had to be doubled or tripled as some resistant insect strains developed, and evidence began to grow that the chemical was concentrated in the food chain. Civilian suppression of typhus and malaria mosquito vectors uses a spray on interior walls, which kills mosquitoes which rest on the wall, while resistant strains are repelled from the area, and thus humans are protected. The compound is stable and concentrates in fatty tissue, reaching dangerous levels in carnivores high in the food chain. It is also excreted in milk.
DDT was responsible for eradicating malaria from Europe and North America. Many people today think of malaria as a tropical disease, but that is the result of an extensive malaria eradication program carried out in the 1950's. When it started India was losing 800,000 people per year to malaria deaths. By the late 1960's the death rate was near zero.
When present at comparatively low levels in birds, DDT has been alleged to cause the birds to lay eggs with thin shells. Prior to the U.S. ban of DDT, some believed that raptors accumulated enough DDT in their bodies to lay eggs with thin, membraneous shells that would break before hatching. Newer technology indicates DDE may be a more significant factor although effects of both vary in different birds. Before the U.S. ban, some bird populations were being reduced while others were increasing.
DDT is not particularly toxic to humans, compared to other widely used pesticides. Some studies show that if DDT were used responsibly with the regulatory oversight present today, its safety would be comparable to other widely used insecticides.
Because of DDT's singular effectiveness in the control of the mosquito, which vectors malaria, it is still used in many countries throughout the world where malaria has proven difficult to control. Use of DDT to control mosquitoes is primarily done inside buildings and through inclusion in household products and selective spraying; this greatly reduces the risk of strong environmental impact.
DDT is an organochlorine. Some organochlorines have been shown to have weak estrogenic activity, that is, they are chemically similar enough to estrogen to trigger hormonal responses in contaminated animals. This sort of activity has been observed in DDT in laboratory studies involving mouse and rat test subjects, but available epidemiological evidence does not indicate that these effects have occurred in humans as a result of DDT exposure.
Studies of alligators in Florida swamps have found extensive oestrogenation due to high levels of DDT exposure. Many male crocodiles in the area have deformed genitalia and feminised bodily features, while their eggs are showing high rates of infertility and abnormal fetal development. Some researchers believe that this is echoed in the human population. Fertility studies in Scandinavia (not entirely sure which country), where DDT was widely used to control pests, have found that the average male sperm count has dropped by almost 50% since DDT started to be used, while there is an increased rate of certain cancers of the reproductive organs compared to former years.
The banning of DDT
In 1962 Rachel Carson's book Silent Spring was published. The book argued that pesticides, and especially DDT, were poisoning both wildlife and the environment and also endangering human health. The book received little support from the mainstream scientific community. Nonetheless, the public reaction to Silent Spring launched the modern environmental movement, and DDT became a prime target of the growing anti-chemical and anti-pesticide movements during the 1960s.
DDT was first banned from use in Norway and Sweden in 1970 (it was not banned in the United Kingdom until 1984). In the United States, the EPA's first Administrator, Environmental Defense Fund supporter William Ruckelshaus, defying his science advisors, announced a ban in 1972 on virtually all uses of DDT in the U.S., where it is classified in EPA Toxicity Class II. Despite the U.S. ban on usage, chemical factories in the U.S. continued to manufacture and export DDT to Third World countries for years.
As of 2004, DDT continues to be used in other (primarily tropical) countries where mosquito-borne malaria and typhus are greater health problems than DDT's potential toxicity.
The 1970s ban in the U.S. took place amid a climate of public mistrust of the scientific and industrial community, following such fiascos as Agent Orange, Three Mile Island, Love Canal, and use of the hormone DES (diethylstilbestrol). In understanding the public policy landscape that led to the ban, it is important to realize that there were essentially no restrictions in the U.S. on pesticide manufacture and use during the 1940s and 1950s. This led to impure products, little knowledge of any risks on the part of the pesticide users, overapplication, and near total disregard for any long-term environmental damage that might occur.
In this light, it is not surprising the DDT was overapplied, and its accumulation in the soil and in sensitive watersheds has been rigorously documented. There was little question that DDT accumulation in the environment led to the precipitous decline in raptor populations starting around 1960, with many birds including the bald eagle having been placed on the endangered species list during this period. DDT accumulates through the food chain, with apex predators such as raptors having a higher concentration of the chemical than other animals sharing the same environment.
Support for using DDT
Controversy remains in some scientific circles over DDT's actual toxicity, however.
- Some scientists have protested that the laboratory animal studies done in 1969 (and which led to the banning of DDT in much of the developed world) which showed that DDT caused an increase in liver cancer was inconsistent with observations in the wild, given that DDT had been used widely during the preceding three decades with no increase in liver cancer in any of the human populations among whom it had been sprayed.
- When the World Health Organization investigated the 1969 mouse study, they found that both experimental and control groups had developed a surprising number of tumors. Further investigation revealed that the food fed to both groups were moldy and contained aflatoxin, a carcinogen. When the tests were repeated using uncontaminated foods, neither group developed abnormal numbers of tumors.
- A DDT study of quail which produced thinner eggshells was flawed by an abnormally low amount of calcium in the diet.
- Thin eggshells can be caused by many factors, including calcium deficiencies and high temperatures. Investigation is complicated by the global cooling of 1945-1975 being during the same period, what is more relevant is the actual weather over habitat and migration areas.
In many African nations, the health problems resulting from millions of malaria per year are viewed as greater than the potential dangers of DDT. In 2001, after a five-year ban led to more than a ten-fold increase in malaria cases, South Africa permitted its use again. Uganda also began permiting its use in anti-malarial efforts despite a threat that its agricultural products to Europe would be banned as a result. One of the arguments against continuing its ban was that DDT was being used anyway in uncontrolled amounts.
Dr. Elizabeth Whelan, the president of the American Council on Science and Health claims that 60 million or more lives "have been needlessly lost since the ban on DDT took effect. ... It's a real tragedy that DDT has been so demonized over the years by activist organizations such as Environmental Defense and the regulatory bodies that they have duped." Whelan's estimates come from estimates that 2.5 million people die of malaria each year: mostly African children. According to the U.N. World Health Organization (WHO), malaria kills one child under the age of 5 every 30 seconds.