DDT Chemical name 4,4'-(2,2,2-trichloroethane-
1,1-diyl)bis(chlorobenzene)Chemical formula C14H9Cl5 Molecular mass 354.49 g/mol Melting point 108.5 °C Boiling point 260 °C CAS number 50-29-3 SMILES ClC(Cl)(Cl)C(C1=CC=C(Cl)
C=C1)C2=CC=C(Cl)C=C2[image]
DDT was developed as the first of the modern insecticides early in World War II. It was initially used with great effect to combat mosquitoes spreading Malaria, Typhus, and other Insect-borne human diseases among both military and civilian populations. The Swiss chemist Paul Hermann Müller was awarded the Nobel Prize in Physiology or Medicine in 1948 "for his discovery of the high efficiency of DDT as a contact poison against several arthropods."
In 1962, an American activist named Rachel Carson published the book Silent Spring, which alleged that DDT caused Cancer and harmed bird reproduction by thinning egg shells. The book resulted in a large public outcry which eventually led to the pesticide being banned for agricultural use in the USA, and was one of the signature events in the birth of the environmental movement. It was subsequently banned for agricultural use in many countries in the 1970s due to its negative environmental impact. There is still a great controversy regarding the extent of this impact and the reduced use of DDT to fight human diseases.
DDT is created by the reaction of Trichloroethanol 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 has potent insecticidal properties; it kills by opening sodium channels in insect neurons, causing the neuron to fire spontaneously. This leads to uncontrolled spasming and eventual death. DDT was responsible for eradicating malaria from Europe and North America, and was also extensively used as an agricultural insecticide after 1945. Insects with certain mutations in their sodium channel gene may be resistant to DDT and other similar insecticides.
In the States, human blood and fat tissue samples collected in the early 1970s showed detectable levels in all samples. A later study of blood samples collected in the latter half of the 1970s showed that blood levels were declining further, but DDT or metabolites were still seen in a very high proportion of the samples.
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.
DDT and its metabolic products accumulate through the food chain, with apex predators such as raptors having a higher concentration of the chemicals than other animals sharing the same environment. In particular, DDT has been cited as a major reason for the decline of the Bald Eagle in the 1950s and 1960s. In general, however, DDT in small quantities has very little effect on birds; its primary metabolite, DDE, has a much greater impact. DDT and DDE have had little impact on some birds which are not apex predators, like the chicken.
DDT is very highly toxic to aquatic life, including Crayfish, daphnids, sea shrimp and many species of fish. DDT may be moderately toxic to some Amphibian species, especially in the larval stages. In addition to acute toxic effects, DDT may bioaccumulate significantly in fish and other aquatic species, leading to long-term exposure.
DDT is not particularly toxic to humans, compared to other widely used pesticides. In particular, no link to cancer has yet been established. Numerous studies have been conducted, including one in which humans voluntarily ingested 35 mg of DDT daily for almost two years. DDT is often applied directly to clothes and used in soap, with no demonstrated ill effects.
Some chemical and pesticide proponents claim there is not a single known human death from DDT poisoning. However, a study ( Haun & Cueto, 1967) states, "In a 9-month-old child, poisoned with a 2:1 mixture of camphechlor and DDT, death occurred after convulsions and respiratory failure." Studies into organochlorines indicate "a dose of approximately 10 mg/kg leads to convulsions. An oral median lethal dose (LD50) is higher than 50 mg/kg in animal studies."
Overall it is known that DDT can kill, DDT does not decompose quickly, DDT adheres to the fat of animals and plants, and that DDT becomes more concentrated in animals higher on the food chain; such as Humans.
DDT was responsible for eradicating malaria from Europe and North America. Though today malaria is thought of as a tropical disease, it was more widespread prior to an extensive malaria eradication program carried out in the 1950s. Though this program was highly successful worldwide (reducing mortality rates from 192 per 100,000 to a low of 7 per 100,000), it was less effective in tropical regions due to the continuous life-cycle of the parasite and poor infrastructure. It was not pursued aggressively in sub-Saharan Africa due to perceived difficulties, with the result that mortality rates there were never reduced to the same dramatic extent, and now constitute the bulk of malarial deaths worldwide, especially following the resurgence of the disease as a result of microbe resistance to drug treatments and the spread of the deadly malarial variant caused by Plasmodium falciparum.
DDT was also extensively used as an agricultural insecticide after 1945. DDT spraying in agricultural contexts was often orders of magnitudes greater in quantity than that employed for public health purposes, which required as little as 2g/m2 of DDT; by comparison, a single cotton field may have used a ton of DDT.
By the 1950s, in some uses, doses of DDT and other insecticides had to be doubled or tripled as resistant insect strains developed. In addition, the evidence began to grow that the chemical became more concentrated at higher levels in the food chain.
In the 1970s and 1980s DDT was banned in most developed countries. DDT was first banned from use in Norway and Sweden in 1970 and was not banned in the United Kingdom until 1984.
It is more sensible in some cases to take a small amount of damage in preference to having none for a time but paying for it in the long run by losing the very means of fighting [is the advice given in Holland by Dr Briejer in his capacity as director of the Plant Protection Service]. Practical advice should be 'Spray as little as you possibly can' rather than 'Spray to the limit of your capacity'.
Charles Wurster, the chief scientist for the Environmental Defense Fund, was quoted in the Seattle Times of 5 October 1969, as saying: "If the environmentalists win on DDT, they will achieve a level of authority they have never had before. In a sense, much more is at stake than DDT." (Tren & Bate, 2004). However, as pesticide research was still immature when it was written, many of the claims made in Silent Spring were ultimately scientifically inaccurate. A 2004 study in the Journal of American Physicians and Surgeons concludes "Public pressure was generated by one popular book and sustained by faulty or fraudulent research. Widely believed claims of carcinogenicity, toxicity to birds, anti-androgenic properties, and prolonged environmental persistence are false or grossly exaggerated." .
During the late 1960's, pressure grew within the United States for a ban on DDT. In January 1971, the US District Court of Appeals ordered Ruckelshaus to begin the deregistration procedure for DDT. Initially, after a six-month review process, the EPA's first Administrator William Ruckelshaus rejected an outright ban, citing studies from the EPA's internal staff that stated that DDT was not an imminent danger to human health and wildlife. However, the findings of these staff members were criticized, as they were performed mostly by economic entomologists inherited from the United States Department of Agriculture, whom many environmentalists felt were biased towards agribusiness and tended to minimize concerns about human health and wildlife. The decision not to ban thus created public controversy.
The EPA held seven months of hearings in 1971-1972, with scientists giving evidence both for and against the use of DDT. At the end of the hearings, the hearing examiner, Edmund Sweeney, ruled that the scientific evidence provided no basis for banning DDT. In the summer of 1972 Ruckelshaus reviewed evidence collected during the agency's hearings as well as reports prepared by two DDT study groups (the Hilton and Mrak Commissions) that had both come to the opposite conclusion. He did not actually attend any of the EPA commission's hearings however, and according to his aides did not read any transcripts of it. Ruckelshaus overturned Sweeny's ruling and announced a ban on virtually all uses of DDT in the U.S., where it was classified in EPA Toxicity Class II. Ruckelshaus argued that the pesticide was "a warning that man may be exposing himself to a substance that may ultimately have a serious effect on his health." (Tren & Bate, 2004)(Milloy, 1999).
The 1970s ban in the U.S. took place amid a climate of public mistrust of the scientific and industrial community, following such fiascoes as Agent Orange, Love Canal, and use of the hormone Diethylstilbestrol (DES). In addition, the fact that the Bald Eagle was placed on the endangered species list in large part because of the overuse of DDT was also a strong factor leading to its banning in the United States.
The Stockholm Convention, ratified in 2001 and effective as of 17 May 2004, calls for the elimination of DDT and other persistent organic pollutants, barring health crises. The Convention was signed by 98 countries and is endorsed by most environmental groups. However a total elimination of DDT use in many malaria-prone countries is currently unfeasible because there are few affordable or effective alternatives, so public health use of DDT is exempt from the ban until alternatives are developed. Malaria Foundation International states:
Most of the prior use of DDT was in agriculture. Current use for disease control requires only a small fraction of the amounts used previously and is much less likely to cause environmental problems. Residual house spraying involves the treating of all interior walls and ceilings with an insecticide, and is effective against mosquitoes that favour indoor resting before or after feeding. Advocated as the mainstay of malaria eradication programmes in the late 1950s and 1960s, it remains a major component of control programmes in southern African states, though many countries have abandoned or curtailed their spraying activities due to disillusionment over the failure to achieve eradication, concerns over the safety and environmental impact, and administrative, managerial and financial constraints on implementation. Such limited use of DDT has not become ineffective due to resistance in areas where it is used inside homes. Swaziland, Mozambique and Ecuador are examples of countries that have very successfully reduced malaria incidence with DDT. However, its use has been greatly limited in many other countries due to reluctance by aid agencies in the developed world to fund DDT spraying and opposition from Western environmental groups. (Africa fighting malaria FAQ, 2005).
There are insecticide alternatives to DDT, and Vietnam is often mentioned as a country that has seen a continued decline in malaria cases after involuntarily switching from DDT in 1991. However, Vietnam's neighbour Thailand has continued to use DDT and has a much smaller malaria rate despite similar conditions. The insecticide alternatives are generally more expensive, which limits their use in poor nations and in situations where anti-malarial efforts are already underfunded. It is doubtful that they are more environmentally friendly or as efficient, easy to use and safe for humans as DDT.
However, in some areas DDT has lost much of its effectiveness, especially where outdoor transmission is predominant form, like in India. "The declining effectiveness of DDT is a result of several factors which frequently operate in tandem. The first and the most important factor is vector resistance to DDT. All populations of the main vector, An. culicifacies have become resistant to DDT. The excito-repellent effect of DDT, often reported useful in other countries, actually promotes outdoor transmission" (Current Science 85 1532-1537) (pdf file)
In the period from 1934-1955 there were 1.5 million cases of malaria in Sri Lanka resulting in 80,000 deaths. After an extensive anti-mosquito program with DDT there were only 17 cases reported in 1963 and the program was halted. Malaria later rebounded to 600,000 cases in 1968 and the first quarter of 1969. Some proponents of DDT consider this an example of environmentalism trumping public well-being even though the use of DDT was ended more due to the lack of continuing need than due to environmental concerns. Spraying with DDT was resumed but many of the local mosquitoes had acquired resistance to DDT in the interim, presumably because of the continued use of DDT for crop protection, and so it was not nearly as effective as it had been previously. Switching to the more-expensive Malathion in 1977 reduced the malaria infection rate to 3,000 by 2004. A recent study notes, "DDT and Malathion are no longer recommended since An. culicifacies and An. subpictus has been found resistant." (Malaria Journal 2005 4:8)
In many African nations, the problems resulting from malaria are viewed as greater than the potential dangers of DDT. After South Africa stopped using DDT in 1996, the number of malaria cases in KwaZulu Natal province rose from 8,000 to 42,000 cases. By 2000, there had been an approximate 400 percent increase in malaria deaths. Today, thanks to DDT, the number of deaths from malaria in the region is less than 50. South Africa could afford and did try newer alternatives to DDT but they proved less effective (Tren & Bate, 2004). Uganda also began permitting its use in anti-malarial efforts despite a threat that its agricultural products to Europe could be banned if contaminated with DDT. (EU warns Uganda over plans to use DDT to fight malaria, 2005). The country states it cannot achieve its development goals without first eliminating malaria. The GDP shows a striking co-relation between malaria and poverty, where malaria is estimated to reduce per capita growth by 1.3 percent per annum. (Lirri & Ntabadde, 2005).
Malaria cases increased in South America after stopping DDT use. Only Ecuador, which has continued to use DDT, has seen a reduction in the number of malaria cases in recent years . Other mosquito-borne diseases are also on the rise. Until the 1970s, DDT was used to eradicate the Aedes aegypti mosquito from most tropical regions of the Americas. The reinvasion of Aedes aegypti since then has brought devastating outbreaks of Dengue fever, dengue hemorrhagic fever, and a renewed threat of urban Yellow fever (Bate, 2001).
In Mexico, however, use of a range of effective and affordable chemical and non-chemical strategies against malaria has been so successful that its DDT manufacturing plant ceased production voluntarily, due to lack of demand. pdf Furthermore, while the increased numbers of malaria victims since DDT usage fell out of favor would, at first glace, suggest 1:1 correlation, many other factors are known to have cotributed to the rise in cases [See Below: Arguments for and against DDT]. Without detailed studies of how and why the spread of malaria has increased in some parts of the world, the claim that a DDT "ban" is to blame, amounts to Post hoc ergo propter hoc .
One study that quantifies the lives saved due to banning agricultural use of DDT and thereby the spread of DDT resistance has been published in the scientific literature: "Correlating the use of DDT in El Salvador with renewed malaria transmission, it can be estimated that at current rates each kilo of insecticide added to the environment will generate 105 new cases of malaria." (Agricultural production and malaria resurgence in Central America and India, Chapin, Georgeanne & Robert Wasserstrom, Nature, Vol. 293, 1981, page 183).
Some donor organizations have refused to fund public health DDT programs. Many countries have been coming under pressure from international health and environment agencies to give up DDT or face losing aid grants: Belize and Bolivia are on record admitting they gave in to pressure on this issue from the US. Agency for International Development. . In 1977 environmental groups sued to ban exports of DDT, after which many countries could no longer obtain any. In 1986 Secretary of State George Schultz telegraphed orders to all embassies stating that "The U. S. cannot, repeat cannot, participate in programs using any of the following: (1) Lindane, (2) BHC, (3) DDT, or (4) Dieldrin."
Nevertheless, the USAID website says:
One insecticide supply company states on its website:
According to a pesticide industry newsletter, DDT is obsolete for malarial prevention in India not only owing to concerns over its toxicity, but because it is losing its effectiveness. Use for agriculture was banned in India in 1989, and use for malaria has been declining. Use of DDT in urban areas in India has halted completely. Food supplies and eggshells of large predator birds still show high DDT levels. (No Future in DDT: A case study of India. Agarwal, Ravi. Pesticide Safety News, May 2001). Parasitology journal articles confirm that malarial vector mosquitoes have become resistant to DDT and HCH in most of India. (Current scenario of malaria in India, Sharma, V.P., Parassitologia 41: 349-53, 1999)
A recent editorial in the British Medical Journal, argues that the campaign against malaria is failing, that use of DDT should be increased, and that donor organizations should abandon their reluctance to fund such programs since DDT has "a remarkable safety record when used in small quantities for indoor spraying in endemic regions".
Actual data on the cost-effectiveness of DDT versus other insecticides and/or means of fighting malaria is, in fact, lacking. One complicating factor is that the relative costs of various measures varies, depending on geographical location and ease of access, the habits of the particular mosquito vector prevalent in each area, the degrees of resistance to various pesticides exhibited by the mosquitoes, the habits and compliance of the population, among other factors.
A review of fourteen studies on the subject in sub-Saharan Africa, covering insecticide-treated nets, residual spraying, chemoprophylaxis for children, chemoprophylaxis or intermittent treatment for pregnant women, a hypothetical vaccine, and changing the first line drug for treatment, found decision making limited by the gross lack of information on the costs and effects of many interventions, the very small number of cost-effectiveness analyses available, the lack of evidence on the costs and effects of packages of measures, and the problems in generalizing or comparing studies that relate to specific settings and use different methodologies and outcome measures. The two cost-effectiveness estimates of DDT residual spraying examined were not found to provide an accurate estimate of the cost-effectiveness of DDT spraying; furthermore, the resulting estimates may not be good predictors of cost-effectiveness in current programmes. (The evidence base on the cost-effectiveness of malaria control measures in Africa C. A. Goodman and A. J. Mills, Health Policy and Planning; 14(4): 301–312 1999 pdf)
However, a study in Thailand found the cost per malaria case prevented of DDT spraying ($1.87 US), to be 21% greater than the cost per case prevented of lambdacyhalothrin-treated nets, ($1.54 US). (Cost-effectiveness and sustainability of lambdacyhalothrin-treated mosquito nets in comparison to DDT spraying for malaria control in western Thailand Kamolratanakul, P., P. Butraporn, M. Prasitisuk, C. Prasittisuk, and K. Indaratna, American Journal of Tropical Medicine and Hygiene 2001, 65(4), 279-84), at very least casting some doubt on the unexamined assumption that DDT would be the most cost-effective measure to use in all cases. The director of Mexico’s malaria control program finds similar results, declaring that it is 25 percent cheaper for Mexico to spray a house with other chemicals (synthetic pyrethroids) than with DDT. ( pdf)Arguments for and against DDT
Many supporters of DDT state that millions of malaria deaths are due to an international ban: 89,000,000 as of May, 2005, according to the ever-increasing "deathclock" at junkscience.com; hundreds of thousands according to Nicholas Kristof. Popular author Michael Crichton states in his novel State of Fear: Since the ban, two million people a year have died unnecessarily from malaria, mostly children. The ban has caused more than fifty million needless deaths. Banning DDT killed more people than Hitler. (page 487)
While this point, as Crichton raises it, is little more than hyperbole, one of the seemingly salient pro-DDT arguments is that the treatment was used long enough to eliminate insect-borne diseases in the West but now that it's only needed in poorer countries in Africa, Asia and elsewhere it's been banned. Paul Driessen, author of Eco-Imperialism: Green Power, Black Death, argues that the epidemic of malaria in Africa not only takes the lives of 2 million people a year, but leaves those who survive malaria unable to contribute to the economy while sick and more vulnerable to subsequent diseases that might kill them. Many African resources are tied up with the sick or in caring for them, leaving the world's poorest countries even poorer. While raising important questions about how the West deals with health crises in the Third World, the core of the argument made by Driessen and others is based on a fallacy.
Although the publication of Silent Spring undoubtedly influenced the U.S. ban on DDT in 1972, the reduced usage of DDT in malaria eradication began the decade before because of the emergence of DDT-resistant mosquitos. Indeed Paul Russell, a former head of the Allied Anti-Malaria campaign observed that eradication programs had to be wary relying on DDT for too long as "resistance has appeared [after] six or seven years." [Garret, 1994]
Furthermore, the application of DDT that proved most troubling to environmentalists (and indeed, health officials) was in agriculture. Even as anti-malaria programs were reducing their usage of DDT, producers of cotton and other cash crops were spraying ever increasing amounts of the pesticide, further limiting DDT's overall effectiveness. As noted above, El Salvador actually saw its cases of malaria increase during years of high DDT usage, directly contradicting the claims of Crichton and others. [Chapin & Wasserstrom, 1981]
Were DDT used in the way its supporters propose it may do more harm than good in the fight against malaria. While anti-environmentalists like to paint a picture of radicals endangering human life to save a few birds, even Carson herself laid out the simple truth of the matter in "Silent Spring"
"No responsible person contends that insect-borne disease should be ignored. . .The question that has now urgently presented itself is whether it is wise or responsible to attack the problem by methods that are rapidly making it worse."
Indeed, the problems facing health officials in their fight against malaria neither begin nor end with DDT. Experts tie the spread of malaria to numerous factors including the resistance of the malaria microbe itself to the drugs traditionally used to treat the illness and a chronic lack of funds in the worst hit countries. The latter was perhaps made worse when the U.S. withdrew funding from an early and effective eradication program in 1963 [Garret, 1994].
The primary worry of many experts is not the usage of DDT per se, but a potential overuse. The general thesis of DDT supporters is that the alternatives to DDT are generally more expensive, more toxic to humans and not always as effective at controlling malaria and insect-borne diseases. However, this argument still places the responsibility for reduced DDT usage on environmentalists, which is clearly contradicted by the facts.
The controlled usage of DDT continues to this day for the purposes of public health and, to a lesser extent, agriculture. The U.S. has continued to use DDT under the conditions of the 1972 ban.