DDT

For other uses, see DDT (disambiguation).

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

DDT was the first modern pesticide and is arguably the most well known organic pesticide. It is a highly hydrophobic colorless solid with a weak, chemical odor that is nearly insoluble in water but has a good solubility in most organic solvents, fat, and oils. DDT is also known under the chemical names 1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane and dichloro-diphenyl-trichloroethane (from which the abbreviation was derived).

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, and as an agricultural insecticide. The Swiss chemist Paul Hermann Müller of Geigy Pharmaceutical in Switzerland 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, American biologist 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 insecticide being banned for agricultural use in the USA, and was one of the signature events in the birth of the environmental movement. DDT was subsequently banned for agricultural use in many countries in the 1970s due to what many believe is a 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.

Properties

DDT is a colourless crystalline substance which is practically insoluble in water but highly soluble in fats and most organic solvents.

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.

History

DDT was first synthesized in 1874 by Othmar Ziedler, but its insecticidal properties were not discovered until 1939, by the Swiss scientist Paul Hermann Müller, 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 used extensively during World War II by Allied troops and certain civilian populations to control insect typhus and malaria vectors (nearly eliminating typhus as a result). Civilian suppression used a spray on interior walls, which kills mosquitoes that rest on the wall after feeding to digest their meal; resistant strains are repelled from the area. Entire cities in Italy were dusted to control the typhus carried by lice. DDT also sharply reduced the incidence of biting midges in Great Britain, and was used extensively as an agricultural insecticide after 1945.

DDT was responsible for eradicating malaria from Europe and North America. Though today malaria is considered a tropical disease, it was more widespread prior to an extensive malaria eradication program carried out in the 1950s. Though this program was initially highly successful worldwide (reducing mortality rates from 192 per 100,000 to a low of 7 per 100,000), resistance emerged in many insect populations over time. DDT was less effective in tropical regions due to the continuous life cycle of mosquitoes and poor infrastructure. It was not pursued aggressively in sub-Saharan Africa due to these perceived difficulties, with the result that mortality rates in the area 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.

In the 1970s and 1980s, agricultural use of DDT was banned in most developed countries, and DDT was replaced in most antimalarial uses by less persistent, but more expensive, alternative insecticides. DDT was first banned from use in Norway and Sweden in 1970, but was not banned in the United Kingdom until 1984.

As of 2006, DDT continues to be used in other (primarily tropical) countries where mosquito-borne malaria and typhus are serious health problems. Use of DDT in public health to control mosquitoes is primarily done inside buildings and through inclusion in household products and selective spraying; this greatly reduces environmental impact compared to the earlier widespread use of DDT in agriculture. It also reduces the risk of resistance to DDT. This use only requires a small fraction of that previously used in agriculture; for the whole country of Guyana, covering an area of 215,000 km², the required amount is roughly equal to the amount of DDT that might previously have be used to spray 4 km² of cotton during a single growing season.

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 for controlling malaria, so public health use of DDT is exempt from the ban until such alternatives are developed. Malaria Foundation International states:

The outcome of the treaty is arguably better than the status quo going into the negotiations over two years ago. For the first time, there is now an insecticide which is restricted to vector control only, meaning that the selection of resistant mosquitoes will be slower than before.

Environmental impact

Overall, DDT concentrates in biological systems (particularly in body fat), it is a toxin across a certain range of phyla, and it bioaccumulates up the food chain, reaching its greatest concentrations in higher animals such as humans.

DDT is a persistent organic pollutant and is highly persistent in the environment. It has a reported half life of between 2-15 years and is immobile in most soils. Its half life is 56 days in lake water and approximately 28 days in river water. 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.

In the United 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 later 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 hormonal-mimicking activity has been observed when DDT is used in laboratory studies involving mice and rats as 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 [1]. DDT and its breakdown products are toxic to embryos and can disrupt calcium absorption thereby impairing egg-shell quality [2]. 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 little impact on some non-predatory birds, such as the chicken.

DDT is 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.

By the 1950s, in some cases, 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 had a tendency to become more concentrated at higher levels in the food chain.

Impact on human health

There are no substantial scientific studies so far which prove that DDT is particularly toxic to humans or other primates, compared to other widely-used pesticides. DDT can be applied directly to clothes and used in soap, with no demonstrated ill effects. Indeed, DDT has on rare occasions been administered orally as a treatment for barbiturate poisoning.

Most of the precise toxicological data on DDT and its metabolites comes from animal experiments; epidemiological and toxicological studies on humans are less precise, because they come from populations who are either exposed to the compounds in manufacturing or spraying, or are third world populations; in either case, they are exposed to multiple pesticides and many other risk factors.

Taking these limitations into account, the EPA estimates with "medium" confidence (due to "shorter duration than desired" of the studies) based mainly on liver toxicity in rats, that no non-carcenogenic effect will be seen at an oral exposure of less than 5 x10^-4 mg/kg-day as a conservative limit including a 10-fold safety factor for generalizing from rats to humans, and another 10-fold factor to account for human subpopulations which may be exceptionally sensitive.

Similarly, the EPA classifies DDT as class B2, a probable human carcinogen, based on observed carcinogenicity in animals, i.e. tumors (generally of the liver) in seven studies in various mouse strains and three studies in rats, and on structural similarity to other carcinogens such as DDE, DDD, dicofol, and chlorobenzilate. The risk factor for oral ingestion is estimated at 3.4x10^-1 per mg/kg-day or 9.7x10^-6 per ug/L for drinking water, which translates into a cancer risk of 1 in 10,000 for 10 ug/L, 1 in 100,000 for 1 ug/L, or 1 in 1,000,000 for 0.1 ug/L; the risk factor for inhalation is estimated at 9.7x10^-5 per ug/m³, which translates into a cancer risk of 1 in 10,000 for 1 ug/m³, 1 in 100,000 for 0.1 ug/m³, or 1 in 1,000,000 for 0.01 ug/m³.

A review article in The Lancet concludes

Although DDT is generally not toxic to human beings and was banned mainly for ecological reasons, subsequent research has shown that exposure to DDT at amounts that would be needed in malaria control might cause preterm birth and early weaning, abrogating the benefit of reducing infant mortality from malaria. ... DDT might be useful in controlling malaria, but the evidence of its adverse effects on human health needs appropriate research on whether it achieves a favourable balance of risk versus benefit.

Future perspectives

Although acute toxic effects are scarce, toxicological evidence shows endocrine-disrupting properties; human data also indicate possible disruption in semen quality, menstruation, gestational length, and duration of lactation. The research focus on human reproduction and development seems to be appropriate. DDT could be an effective public-health intervention that is cheap, longlasting, and effective. However, various toxic-effects that would be difficult to detect without specific study might exist and could result in substantial morbidity or mortality. Responsible use of DDT should include research programmes that would detect the most plausible forms of toxic effects as well as the documentation of benefits attributable specifically to DDT. Although this viewpoint amounts to a platitude if applied to malaria research in Africa, the research question here could be sufficiently focused and compelling, so that governments and funding agencies recognise the need to include research on all infant mortality when DDT is to be used.

Conflicting Studies

• Direct studies have not found a link between DDT and breast cancer in humans.
• Some evidence suggests a link between DDT and breast cancer in humans. For example, diminishing rates of breast cancer in Israel have paralleled a precipitous decline in environmental contamination with DDT and benzene hexachloride. [3] [4] (See also , , )
• Dr. Mary Wolf published a 1993 article in the Journal of the National Cancer Institute indicating a statistically significant correlation between DDT metabolites in the blood and the risks of developing breast cancer in the general population. Others have disputed this research.
• In one study, primates were given DDT (20mg/kg) with their diet for 130 months. No conclusive link with cancer was established.
• A study of 692 women, half of them control subjects, over a period of twenty years established no correlation between serum DDE and breast cancer. DDE is a matabolite of DDT, and correlates with DDT exposure.
• A study examined 35 workers exposed to 600 times the average DDT exposure levels over a period of 9 to 19 years. No elevated cancer risk was observed.
• In another study, humans voluntarily ingested 35 mg of DDT daily for about two years, and were then tracked for several years afterward. No elevated risk was observed.

The review discussed above summarizes the available evidence, in slightly more detail:

In people, DDT use is generally safe; large populations have been exposed to the compound for 60 years with little acute toxicity apart from a few reports of poisoning. Doses as high as 285 mg/kg taken accidentally did not cause death, but such large doses did lead to prompt vomiting. One dose of 10 mg/kg can result in illness in some people. Subclinical and subtle functional changes have not been meticulously sought until the past few decades.

Occupational exposure to DDT was associated with reduced verbal attention, visuomotor speed, sequencing, and with increased neuropsychological and psychiatric symptoms in a dose-response pattern (ie, per year of DDT application) in retired workers aged 55–70 years in Costa Rica. Although DDT or DDE concentrations were not determined in this study, they probably were very high.

Although extensively studied, there is no convincing evidence that DDT or its metabolite DDE increase human cancer risk. Mainly on the basis of animal data, DDT is classified as a possible carcinogen (class 2B) by the International Agency for Research on Cancer (IARC) and as a reasonably anticipated human carcinogen by the US National Toxicology Program.

Breast cancer has been examined most closely for an association with p,p'-DDE. In a study in 1993, 37 breast cancer patients had higher serum DDE concentrations (11·8 μg/L) than controls (7·7 μg/L), and results from several subsequent studies supported such an association. However, large epidemiological studies and subsequent pooled and meta-analyses failed to confirm the association.

With detailed work history of chemical manufacturing workers to estimate DDT exposure, a nested case-control study reported occupational DDT exposure associated with increased pancreatic cancer risk. A weak association of self-reported DDT use with pancreatic cancer was reported in another case-control study. A report indicated a higher standardised mortality ratio for pancreatic cancer in outdoor workers with a history of DDT exposure of less than 3 years, but the standardised mortality ratio of DDT workers with exposure of 3 years or more was not significantly raised.

The U.S. ban on the use of DDT

In 1962, Rachel Carson's book Silent Spring was published. The book argued that pesticides, especially DDT, were poisoning both wildlife and the environment and also endangering human health. Public reaction to Silent Spring launched the modern environmental movement in the United States, and DDT became a prime target of the growing anti-chemical and anti-pesticide movements during the 1960s. In fact, Carson devoted a page of the book to thoughtful consideration of the relationship between DDT and malarial mosquitoes, but with cognizance of the development of resistance in the mosquito, concluding:

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."

However, Carson also made the controversial claim that DDT caused cancer in humans, a claim which is still widely held by the public. 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." However, as pesticide research was still immature when it was written, many of the claims made in Silent Spring were ultimately scientifically inaccurate.

During the late 1960s, pressure grew within the United States to effect a ban on DDT. In January 1971, the U.S. District Court of Appeals ordered William Ruckelshaus, the EPA's first Administrator, to begin the de-registration procedure for DDT. Initially, after a six-month review process, Ruckelshaus rejected an outright ban, citing studies from the EPA's internal staff stating 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. In the summer of 1972, Ruckelshaus announced a ban on virtually all uses of DDT in the U.S., where it was classified as an EPA Toxicity Class II.

This decision has been vigorously criticized by pro-DDT advocates, including Steven Milloy, Roger Bate and Richard Tren, whose critiques draw on the work of entomologist J. Gordon Edwards, a witness at the hearings who stated that there was no evidence to substantiate the claims that DDT posed a threat to human health. They report that, at the end of the hearings, 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 Mark Commissions) that had both come to the opposite conclusion. He did not actually attend any of the EPA commission's hearings, and according to his aides did not read any transcripts of the hearings. Ruckelshaus overturned Sweeney's ruling, arguing 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."

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 and use of the hormone diethylstilbestrol (DES). In addition, the placement of the bald eagle on the endangered species list—in large part because of the overuse of DDT—was also a strong factor leading to its being banned in the United States.

The malaria controversy

Malaria afflicts between 300 million and 500 million people every year. The World Health Organization estimates that around 1 million people die of malaria and malaria-related illness every year. About 90% of these deaths occur in Africa, mostly to children under the age of 5. The economic impact includes costs of health care, working days lost due to sickness, days lost in education, decreased productivity due to brain damage from cerebral malaria, and loss of investment and tourism.

Most prior use of DDT was in agriculture. 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. Current use for disease control requires only a small fraction of the amounts previously used, and at these levels the pesticide is much less likely to cause environmental problems. Residual house spraying involves the treatment of all interior walls and ceilings with insecticide, and is particularly effective against mosquitoes, which favour indoor resting before or after feeding. Advocated as the mainstay of malaria eradication programmes in the late 1950s and 1960s, DDT remains a major component of control programmes in southern African states, though many countries have abandoned or curtailed their spraying activities. Swaziland, Mozambique and Ecuador are examples of countries that have very successfully reduced malaria infestations with DDT.

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 countries worst hit by malaria.

The growth of resistance to DDT and the fear that DDT may be harmful both to humans and insects led to the U.N., donor countries and various national governments restricting or curtailing the use of DDT in vector control. At the same time, use of DDT as an agricultural insecticide was often unrestricted, and restrictions were often evaded, especially in developing coutries where malaria is rife, so that resistance continued to grow.

This has generated two related controversies. The first, involving debate among professionals working on malaria control concerns the appropriate role of DDT. The range of disagreement here is relatively small. Few believe either that large scale spraying should be resumed or that the use of DDT should be abandoned altogether. The debate focuses on the relative merits of DDT and alternative pesticides as well as complementary use of interior wall spraying and insecticide-treated bednets.

The popular controversy involves claims that scaremongering by the green movement that DDT cause cancer in human led to restriction of the use of DDT in vector control by various national governments, donor countries and international aid agency including U.N, which resulted to millions of unnecessary deaths. The most prominent proponent of this view in internet is Africa Fighting Malaria which advocate expansion of the use of DDT in vector control. This claim has been widely repeated in newspaper articles, and promoted by a range of conservative/right lobby groups which criticise green movenents.

Restrictions on use of DDT

In many developing countries, spraying programmes (especially using DDT) were stopped due to concerns over safety and environmental impacts, as well as constraint and incompetence (or corruption) in administrative, managerial and financial implementation. Such limited use of DDT has not become ineffective due to resistance in areas where it is used inside homes.

The pro-DDT advocacy group Africa Fighting Malaria assert that USAID and some other international donor organizations have refused to fund public health DDT programs. Similarly, Roger Bate of AFM asserts that many countries have been coming under pressure from international health and environment agencies to give up DDT or face losing aid grants, and that Belize and Bolivia have gone on record to say that they gave in to pressure on this issue from the US. Agency for International Development.

USAID's Kent R. Hill states that the agency has been misrepresented:

USAID strongly supports spraying as a preventative measure for malaria and will support the use of DDT when it is scientifically sound and warranted.

However, USAId, indeed, "favour" DDT alternatives in its funding which is exactly what the Africa Fighting Malaria asserted in the first place.

Contrary to popular belief, USAID does not "ban" the use of DDT in its malaria control programs. From a purely technical point of view in terms of effective methods of addressing malaria, USAID and others have not seen DDT as a high priority component of malaria programs for practical reasons. In many cases, indoor residual spraying of DDT, or any other insecticide, is not cost-effective and is very difficult to maintain. In most countries in Africa where USAID provides support to malaria control programs, it has been judged more cost-effective and appropriate to put US government funds into preventing malaria through insecticide-treated nets, which are every bit as effective in preventing malaria and more feasible in countries that do not have existing, strong indoor spraying programs.

Overall Effectiveness of DDT

A recent editorial in the British Medical Journal argues that the campaign against malaria is failing, that funding of malaria control should therefore be increased, and that use of DDT should be considered since DDT has "a remarkable safety record when used in small quantities for indoor spraying in endemic regions."

One insecticide supply company states on its website:

DDT is still one of the first and most commonly used insecticides for residual spraying, because of its low cost, high effectiveness, persistence and relative safety to humans. [...] In the past several years, we supplied DDT 75% WDP to Madagascar, Ethiopia, Eritrea, Sudan, South Africa, Namibia, Solomon Island, Papua New Guinea, Algeria, Thailand, and Myanmar for Malaria Control project, and won a good reputation from WHO and relevant countries' government.

In the period from 1934-1955 there were 1.5 million cases of malaria in Sri Lanka, resulting in 80,000 deaths. After the country invested in an extensive anti-mosquito program with DDT, there were only 17 cases reported in 1963. Thereafter the program was halted, and malaria in Sri Lanka rebounded to 600,000 cases in 1968 and the first quarter of 1969. Although the country resumed spraying with DDT, many of the local mosquitoes had acquired resistance to DDT in the interim, presumably because of the continued use of DDT for crop protection, so the program was not nearly as effective as it had been before. 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."

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% increase in malaria deaths. Today, thanks to DDT, the number of deaths from malaria in the region is less than 50 per year. South Africa could afford and did try newer alternatives to DDT, but they proved less effective. Uganda also began permitting the use of DDT in anti-malarial efforts, despite a threat that its agricultural exports to Europe could be banned if they were contaminated with DDT. The Ugandan government has stated that it cannot achieve its development goals without first eliminating malaria. The GDP shows a striking correlation between malaria and poverty, where malaria is estimated to reduce per capita growth by 1.3 percent per annum.

Malaria cases increased in South America after countries in that continent stopped using DDT. 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 has brought devastating outbreaks of dengue fever, dengue hemorrhagic fever, and a renewed threat of urban yellow fever.

Mosquito resistance against DDT

In some areas DDT has lost much of its effectiveness, especially in areas such as India where outdoor transmission is the predominant form. According to V.P. Sharma, "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."

One study that attempts to quantify 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."

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 has largely lost its effectiveness. Use of DDT for agricultural purposes was banned in India in 1989, and its use for anti-malarial purposes has been declining. Use of DDT in urban areas of India has halted completely. Food supplies and eggshells of large predator birds still show high DDT levels. Parasitology journal articles confirm that malarial vector mosquitoes have become resistant to DDT and HCH in most parts of India. Nevertheless, DDT is still manufactured and used in India.

Alternatives to DDT

There are some insecticide alternatives to DDT. One agent that is being used as a substitute is methoxychlor. Vietnam is often mentioned as a country that has seen a continued decline in malaria cases after involuntarily switching from DDT to other insecticides in 1991. However, Thailand, another Southeast Asian nation, has continued to use DDT and has a much smaller malaria rate despite similar climate 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. These alternatives are not necessarily more environmentally friendly, nor as efficient, easy to use and relatively safe for humans as DDT.

In Mexico, however, the use of a range of effective and affordable chemical and non-chemical strategies against malaria has been so successful that the Mexican DDT manufacturing plant ceased production voluntarily, due to lack of demand. Furthermore, while the increased numbers of malaria victims since DDT usage fell out of favor would, at first glance, suggest a 1:1 correlation, many other factors are known to have contributed to the rise in cases [See below: Arguments for and against DDT].

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 mosquitoes 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.

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), at very least casting some doubt on the unexamined assumption that DDT was 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% cheaper for Mexico to spray a house with synthetic pyrethroids than with DDT.

Furthermore, a more effective way of measuring cost-effectiveness or efficacy of malarial control would not only measure the cost in dollars of the project, as well as the number of people saved, but would also take into account the negative aspects of insecticide use on human health and ecological impact. Preliminary studies regarding the impact of DDT show that it is likely the detrimental impact on human health approaches or exceeds the beneficial reductions in malarial cases, except perhaps in malarial epidemic situations.

Controversy

Supporters of DDT state that millions of malaria deaths are due to an international ban: 90,500,000 as of January 2006, according to the ever-increasing "deathclock" at junkscience.com, and 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.

One of the salient pro-DDT arguments is that the ban shows a lack of compassion for sufferers in the Third World: treatments were used long enough to eliminate insect-borne diseases in the West, but now that it is only needed in poorer nations in Africa, Asia and elsewhere, it has 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 expended 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 controversial.

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 mosquitoes. Indeed, Paul Russell, a former head of the Allied Anti-Malaria campaign, observed that eradication programs had to be wary of relying on DDT for too long as "resistance has appeared [after] six or seven years."

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.

Some believe that if DDT were used in the way its supporters propose, it might do more harm than good in the fight against malaria. While some like to paint a picture of environmental radicals endangering human lives to save a few birds, Carson pointed out in Silent Spring that:

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.

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, the primary worry of many experts is not the usage of DDT per se, but its potential overuse.

External links

Toxicity

• Agency for Toxic Substances and Disease Registry: ToxFAQs for DDT, DDE and DDD
• CDC ATSDR DDT toxicity reference list (PDF)
• EXTOXNET: Pesticide Information Profiles—DDT
• Oregon State University EXTOXNET DDT toxicity reference list
• Scorecard: The Pollution Information Site—DDT

Environmental impact

• Microbial Degradation of Pesticides
• DDT and its Derivatives - Environmental Aspects
• Aerobic pathway of DDT metabolization
• Anaerobic pathway of DDT metabolization
• Pesticide residues in food 2000 : DDT
• Pesticide residues in food—1984
• Environmental Fate Evaluation of DDT, Chlordane and Lindane
• DDT, Eggshells, and Me
• EPA : Pesticides and Public Health

Malaria and DDT

• "If Malaria's the Problem, DDT's Not the Only Answer", a Washington Post column by entomologist May Berenbaum
• "The Mosquito Killer", a New Yorker article about Fred Soper by Malcolm Gladwell
• The DDT ban myth
• Malaria and the DDT Story
• Malaria and DDT
• Putting Myths to Bed
• The Kill Malarial Mosquitoes NOW! coalition, a project of the international NGO Africa Fighting Malaria
• DDT Ban Myth Bingo
• Nets for fighting malaria

References

• Bailey, R. (12 June 2002). Silent Spring at 40: Rachel Carson’s classic is not aging well, Reason Online,
• Lundholm, C. E. (1997) DDE-induced eggshell thinning in birds: Effects of p, p'-DDE on the calcium and prostaglandin metabolism of the eggshell gland. Comparative Biochemistry and Physiology Part C: Pharmacology, Toxicology and Endocrinology 118 (2), 113-128.
• Risebrough, R. W. (1998). Endocrine Disrupters and Bald Eagles: A Response.
• Roberts, D. R. (2004) Testimony, United States Senate, Committee on Foreign Relations.

DDT also stands for Ding Dong Teacher