DDT

• 1. Introduction

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  Dichlorodiphenyltrichloroethane, known as DDT, is a colorless, tasteless and almost odorless crystal that does not occur anywhere in nature. In Sri Lanka, spraying it helped cut malaria from about one million cases a year to just 18 in 1963. The next year there were 29. Then officials halted the program to save money. Cases rebounded to 600,000 across 1968 and the first quarter of 1969. That swing captures the whole puzzle of DDT. It appeared on wartime posters, it was blamed for cracked eggshells, and it is still sprayed inside homes today. How did one compound become a hero, then a villain, then something harder to label? Why did insects learn to survive it? And who decides when a poison is worth its price?

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• 2. Wartime Miracle

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  In 1874, the Austrian chemist Othmar Zeidler first synthesized DDT while working under Adolf von Baeyer, and almost nobody noticed. A dissertation by W. Bausch described it again in 1929, followed by two publications in 1930. In 1934, Wolfgang von Leuthold patented a group of chlorinated compounds with insect-killing properties. The true discovery came in 1939, when the Swiss chemist Paul Hermann Müller found what DDT does to insects. In 1948 he won the Nobel Prize in Physiology or Medicine "for his discovery of the high efficiency of DDT as a contact poison against several arthropods".

  The Second World War gave the chemical its mission. British and American scientists hoped it would stop malaria, typhus, dysentery and typhoid among soldiers in tropical places. The older alternative, pyrethrum, was hard to obtain because it came mainly from Japan. In 1942 and 1943, the US War Production Board placed DDT on military supply lists and encouraged production for overseas use. In the South Pacific, crews sprayed it from the air against malaria and dengue fever, with spectacular effects. Better application equipment, sound organization and enough manpower mattered as much as the chemistry.

  A 1944 issue of Life magazine featured the Italian program, with photographs of uniformed American health officials spraying DDT on Italian families. Government posters showed Americans fighting both the Axis powers and insects. In 1945, DDT reached farmers, and the FDA judged food safe at up to 7 parts per million, despite thin research. Large economic incentives pushed it toward farmers, governments and households alike. Yet the FDA pharmacologist Herbert O. Calvery had already voiced concern about possible hazards back in 1944.

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• 3. Silent Spring

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  In 1947, a physician and nutritionist named Bradbury Robinson, practicing in St. Louis, Michigan, warned that farm use of DDT was dangerous. His town was awkward ground for such a warning. The Michigan Chemical Corporation, later bought by Velsicol Chemical Corporation, researched and made DDT there, and it had become important to the local economy. Citing Michigan State University research from 1946, Robinson feared the spray would upset natural balances. He expected it to kill beneficial insects, and also fish, birds and other wildlife that ate poisoned insects or swallowed the poison directly.

  By October 1945 the public could already buy DDT, both for farms and as a household insecticide. Elsewhere, women such as Dorothy Colson and Mamie Ella Plyler of Claxton, Georgia, gathered evidence about its effects. They wrote to the Georgia Department of Public Health, the National Health Council in New York City, and other organizations. Their efforts drew little attention. The science was slippery, because DDT affected species differently, and repeated exposures could accumulate into damage comparable to one large dose.

  In 1957, The New York Times reported an unsuccessful struggle to restrict DDT in Nassau County, New York. Around then, a friend named Olga Huckins wrote to the naturalist-author Rachel Carson. She enclosed a Boston Globe article she had written about the devastation of her local bird population after DDT spraying. William Shawn, the editor of The New Yorker, urged Carson to write a piece on the subject. It grew into her 1962 book Silent Spring, which argued that pesticides were poisoning wildlife and endangering human health. The book was a best seller, and public reaction to it launched the modern environmental movement in the United States.

  The year after it appeared, President John F. Kennedy ordered his Science Advisory Committee to investigate Carson's claims. The journal Science said the committee's report "added up to a fairly thorough-going vindication of Rachel Carson's Silent Spring thesis". The committee recommended a phaseout of "persistent toxic pesticides". In 1965, the US military dropped DDT from its supply system, partly because body lice had developed resistance to it. Lindane took its place.

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• 4. Bans and Treaties

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  In 1966, a fish kill in Suffolk County, New York, was linked to a 5,000-gallon DDT dump by the county's mosquito commission. A group of scientists and lawyers sued to stop further use. A year later, the group, led by Victor Yannacone and Charles Wurster, founded the Environmental Defense Fund with the scientists Art Cooley and Dennis Puleston. They brought a string of lawsuits against DDT and other persistent pesticides in Michigan and Wisconsin.

  In 1971, responding to an EDF suit, the US District Court of Appeals ordered the EPA to begin de-registering DDT. After a six-month review, the agency's first Administrator, William Ruckelshaus, declined an immediate suspension. He cited internal staff studies finding no imminent danger. Critics objected that economic entomologists inherited from the United States Department of Agriculture had done most of that work. Many environmentalists felt those scientists leaned toward agribusiness and understated risks to people and wildlife.

  Seven months of hearings followed in 1971-1972, with scientists testifying for and against DDT. In the summer of 1972, Ruckelshaus announced the cancellation of most uses, exempting public health uses under some conditions. The EDF and the manufacturers both sued the EPA immediately. Industry wanted the ban overturned, while the EDF wanted a comprehensive one. In 1973, the United States Court of Appeals for the District of Columbia Circuit ruled that the EPA had acted properly. During the late 1970s, the EPA went on to ban chemically similar organochlorines, including aldrin, dieldrin, chlordane, heptachlor, toxaphene and mirex.

  The scale of what was being restricted was large. About 15 American companies made DDT, among them Monsanto, Ciba, Montrose Chemical Company, Pennwalt and Velsicol. Production peaked in 1963 at 82,000 tonnes per year, and usage peaked in 1959 at about 36,000 tonnes. More than 600,000 tonnes were applied in the US before the ban. An estimated 1.8 million tonnes have been produced worldwide since the 1940s. Some exemptions lingered. In June 1979, the California Department of Health Services was permitted to use DDT against fleas that carry bubonic plague. American factories kept producing it for foreign markets until 1985, when over 300 tons were exported.

  Abroad, Hungary banned agricultural use in 1968, although it continued through at least 1970. Norway and Sweden followed in 1970, and West Germany and the US in 1972. The United Kingdom waited until 1984. East Germany used DDT until 1988, including large forestry applications in 1982-1984 against bark beetle and pine moth. Forest soils in eastern Germany still hold significantly more DDT than those in the former west. By 1991, at least 26 countries had total bans, among them Cuba in 1970, Singapore in 1984, Chile in 1985 and the Republic of Korea in 1986.

  The Stockholm Convention on Persistent Organic Pollutants took effect in 2004 and was ratified by more than 170 countries. It restricted DDT to vector control and exempted public health use within World Health Organization guidelines. Malaria Foundation International said, "The outcome of the treaty is arguably better than the status quo going into the negotiations. 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." Even so, agricultural use continued in India, North Korea and possibly elsewhere.

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• 5. Eggshells and Persistence

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  Peregrine falcons, bald eagles, ospreys and brown pelicans shared a grim symptom as the lawsuits mounted. Their eggshells grew so thin that eggs often cracked before hatching. The toxicologist David Peakall measured DDE levels in the eggs of peregrine falcons and California condors. Higher levels matched thinner shells. The effect was first conclusively proven at Bellow Island in Lake Michigan, in University of Michigan-funded studies of American herring gulls in the mid-1960s.

  Birds of prey, waterfowl and song birds proved more sensitive than chickens and their relatives. The biological mechanism is not fully understood. Strong evidence indicates that p,p-DDE blocks calcium ATPase in the membrane of the shell gland, reducing the calcium carbonate delivered to the shell. Other evidence suggests that o,p'-DDT disrupts the development of the female reproductive tract. Different species may suffer through different routes. In 2010, California condors at Big Sur still showed thin shells. They fed on sea lions that in turn fed near the Palos Verdes Shelf, part of the Montrose Chemical Superfund site, though DDT's role in the condor's decline is disputed.

  The damage lasted because DDT barely dissolves in water and clings to soils and sediments. Its soil half-life ranges from 22 days to 30 years, depending on conditions. The National Pesticide Information Center lists its half-life in aquatic environments at 150 years. A process called global distillation carries DDT and its breakdown products from warmer regions to the Arctic, where they build up in the food web. Stored in body fat, the chemicals magnify up the food chain, so apex predators such as raptors concentrate the most.

  In humans, the half-life of DDT is 6 years, and that of DDE is up to 10 years. A 1974 study found significantly more DDT in the breast milk of mothers in New Brunswick than in Nova Scotia, possibly because of heavier past spraying. The US Centers for Disease Control detected DDT and its relatives in almost all blood samples tested in 2005, though levels have since fallen sharply. In 2018, researchers found residues still lingering in European soils and Spanish rivers.

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• 6. Malaria Campaigns

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  In 1955, the World Health Organization launched a program to eradicate malaria wherever transmission was low to moderate. It paired DDT spraying with rapid diagnosis and treatment. The program eliminated the disease in North America, Europe, the former Soviet Union, Taiwan, much of the Caribbean, the Balkans, parts of northern Africa, northern Australia and a large swath of the South Pacific. It also sharply cut deaths in Sri Lanka and India. Success came only in areas with high socio-economic status, well-organized healthcare systems and less intensive or seasonal transmission.

  Sub-Saharan Africa told a different story. Colonial states applied DDT there, but the global eradication program left the region out. DDT worked less well in tropical regions, where mosquitoes breed continuously and infrastructure is poor. That region now accounts for the bulk of malarial deaths worldwide, helped along by drug resistance and the spread of the deadly Plasmodium falciparum variant. Eradication was abandoned in 1969, and attention shifted to control and treatment, including insecticide-impregnated bednets. Experts blame poor leadership, management and funding, plus poverty, civil unrest and increased irrigation, for the resurgence.

  In 2015, there were 214 million cases of malaria worldwide and an estimated 438,000 deaths, 90% of them in Africa. Views of DDT's role range from a "miracle weapon that is like Kryptonite to the mosquitoes" to "toxic colonialism". In 2006, the WHO reversed a longstanding policy and recommended DDT as an indoor pesticide where malaria is a major problem. The shift followed the appointment of Arata Kochi to head its anti-malaria division. The method is indoor residual spraying, which coats interior walls and ceilings. It suits mosquitoes well, since many species rest on an indoor wall before or after feeding. DDT is one of 12 WHO-approved insecticides for it.

  Indoor spraying also limits environmental harm. Treating 40 hectares of cotton during a typical US growing season takes as much chemical as treating roughly 1,700 homes. China ceased production in 2007, leaving India as the only manufacturer and the largest consumer. In 2009, 3,314 tonnes were produced for malaria control and visceral leishmaniasis. By 2019, only five countries still used DDT for indoor spraying.

  Two cases show how stark the arguments can be. In 1996, South Africa switched to alternative insecticides, and malaria incidence increased dramatically. Returning to DDT and adding new drugs brought it back under control. Between 1993 and 1995, Ecuador increased its DDT use and achieved a 61% drop in malaria rates, while neighbors that gradually reduced their use saw large increases. Meanwhile, the WHO has aimed for a 30% cut in worldwide DDT application by 2014 and a total phase-out by the early 2020s if not sooner.

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• 7. Mosquitoes Fight Back

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  Inside an insect, DDT forces open voltage-sensitive sodium ion channels in neurons. The neurons fire spontaneously, which causes spasms and eventual death. Insects with certain mutations in their sodium channel gene resist it. Others resist by making more cytochrome P450 enzymes, which break the toxin down into inactive products. Studies of the fruit fly Drosophila melanogaster showed that high-level resistance is polygenic, with several mechanisms at work.

  Paul Russell, the former head of the Allied Anti-Malaria campaign, observed in 1956 that "resistance has appeared after six or seven years". Resistance is largely blamed on agricultural use in far greater quantities than disease prevention requires. It has been detected in Sri Lanka, Pakistan, Turkey and Central America, where malathion or bendiocarb largely replaced DDT. In Sri Lanka, mosquitoes had evolved resistance by the time spraying resumed, and malaria kept resurging into the 1980s. In KwaZulu-Natal Province, South Africa, 63% of sampled mosquitoes were susceptible to the standard 4% DDT test, against an average of 87% for the same species caught in the open.

  Resistant mosquitoes often avoid DDT-sprayed walls, and a 2007 study reported that they avoided treated huts. Some researchers consider that a benefit. Others argue it slows eradication, and in India, where outdoor sleeping is common, it may even promote outdoor transmission. One analysis concluded, "For these reasons, when comparisons have been made, better malaria control has generally been achieved with pyrethroids than with DDT."

  Cost complicates the choice. Pyrethroids such as deltamethrin cost more per kilogram but are applied at 0.02-0.3 grams per square meter, against 1-2 grams for DDT, so the net cost per house is about the same. DDT stays active for 6 to 12 months, pyrethroids for 4 to 6, and organophosphates and carbamates for 2 to 6. Where transmission runs year-round, shorter-lasting insecticides mean repeating costly spray campaigns. A Thailand study put DDT spraying at US$1.87 per case prevented, 21% above the US$1.54 for lambda-cyhalothrin nets.

  Other strategies sometimes did better. A WHO study released in January 2008 found that insecticide-treated nets and artemisinin-based drugs cut malaria deaths in half in Rwanda and Ethiopia. Vietnam achieved a 97% mortality reduction after switching in 1991 to prompt treatment, bednets and pyrethroids. In Mexico, alternatives worked so well that the DDT plant closed for lack of demand. Draining standing water is impractical across much of Africa, since Anopheles gambiae breeds in numerous small rain pools that are hard to predict. Spraying also needs at least 80% of homes and barns covered, and many residents object to the lingering smell and stains on their walls.

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• 8. Health and Blame

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  A review in The Lancet reported that exposure to DDT at the levels needed for malaria control might cause preterm birth and early weaning. DDT is an endocrine disruptor and is considered likely to be a human carcinogen, although most studies suggest it does not directly damage genes. In 2015, the International Agency for Research on Cancer classified it as Group 2A, "probably carcinogenic to humans". The WHO rates it only "moderately hazardous" in acute poisoning, based on a rat oral dose of 113 mg/kg. In one controlled test, humans given 6 mg/kg developed perspiration, headaches and nausea. Convulsions appeared at doses of 16 mg/kg or higher.

  Breast cancer remains an open question. Several meta-analyses found no overall relationship, and in 2012 the United States Institute of Medicine concluded that a causal link could be neither proven nor disproven. A 2007 study using archived blood samples found a 5-fold increase in risk among women born before 1931 who had high serum DDT levels in 1963. Its authors reasoned that the ages of 14-20 were a critical period. A 2015 study found an odds ratio of 3.4 for breast cancer in daughters exposed in the womb.

  The politics are as heated as the toxicology. Roger Bate of the pro-DDT group Africa Fighting Malaria and the Competitive Enterprise Institute blamed restrictions for large numbers of deaths, sometimes in the millions. The former WHO scientist Socrates Litsios called these arguments "outrageous". The entomologist May Berenbaum said, "to blame environmentalists who oppose DDT for more deaths than Hitler is worse than irresponsible". Critics often cite the 1972 US ban as if it were worldwide, though it never prohibited use against disease. Carson, for her part, never pushed for a ban. John Quiggin and Tim Lambert wrote that "the most striking feature of the claim against Carson is the ease with which it can be refuted". The chemist Michael Palmer points to rising manufacturing costs as the main reason for declining use.

  The hardest problem may be control. DDT meant for malaria sometimes ends up elsewhere. It is reportedly used in Ethiopian coffee production and for fishing in Ghana. In India it is widespread in mango growing, and it is reportedly used by librarians to protect books.

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