Scientific Tipping Points: The Banning of DDT, Part I

Tim Londergan, Sept. 19, 2018

This is the first installment in a series called Scientific Tipping Points.  In this series, we will review the history of a number of different controversial issues in science.  In each of these, we will attempt to identify a “tipping point:” that is, a single incident that turned the tide in a struggle between competing points of view.

As we point out in the introduction to this series, it is frequently difficult to identify a single “turning point” in what is often a long and protracted struggle to determine the truth and convince the public regarding a controversial issue.  As a result, these posts will be less certain than the other entries in our “Debunking Denial” blog posts.  Even if we could show that the tide of public opinion or scientific consensus followed chronologically from the event that we choose as a “tipping point,” this does not necessarily prove that the shift in consensus was caused by the event in question.

However, in our first case, “The Banning of DDT,” there is no question that the eventual ban on DDT production in the United States was a direct result of a single individual and a single event.  That would be the publication in 1962 of the book Silent Spring by Rachel Carson.   This post consists of two parts.  In the first, we will give a history of DDT and its uses, from its initial synthesis until its ban in the U.S.  The second part will consist of a biography of Rachel Carson and the publication and legacy of Silent Spring.

1) Synthesis of DDT and Early Uses:

Dichlorodiphenyltrichloroethane, commonly known as DDT, is a synthetic organochlorine crystalline chemical compound that was first synthesized in 1874.  Fig. 1.1 shows the molecular structure of the dominant so-called (p,p’) isomer of DDT.   The blue dots represent carbon atoms or, in 9 of the 14 occurrences, carbon atoms bonded to hydrogen atoms.

Fig. 1.1: molecular structure of the dominant (p, p’) isomer of DDT

DDT was first synthesized by Othmar Zeidler, who was working for Adolf von Baeyer.  For the initial few decades DDT did not have any significant commercial applications.  However, in 1939 the Swiss chemist Paul Hermann Müller discovered that DDT was extraordinarily effective as an insecticide.  For his developments in this area, Müller was awarded the 1948 Nobel Prize in Physiology and Medicine.

As an insecticide, DDT works in the following way.  It opens sodium ion channels in neurons, which causes the neurons to fire spontaneously.  This causes spasms and eventual death in the insect.  Initially, DDT was found to be exceptionally effective at killing insects.  Its first major applications occurred during World War II.  In that conflict, Allied troops were susceptible to typhus, a name given to a group of infectious diseases spread by different insect vectors (one type is carried by body lice, a second by chiggers and a third by fleas).

The use of DDT was a major factor in tamping down epidemics of typhus.  It was applied to troops in the European theater, and in addition was used for European populations where typhus spread rapidly in wartime conditions.  Fig. 1.2 shows a woman being sprayed with DDT to combat typhus, after she was liberated from the Bergen-Belsen concentration camp in 1945.

Fig. 1.2: a woman being sprayed with DDT to combat typhus in 1945.

The use of DDT was a major factor in the elimination of typhus from much of Europe.  Following the success of the anti-typhus program, DDT was used with great initial success in campaigns against malaria.  For example, malaria was largely eliminated in Europe and North America.  In 1955, the World Health Organization commenced a global campaign to combat malaria, an infectious disease caused by a Plasmodium parasite.  That parasite is transmitted to humans by the bite of infected mosquitoes.

Fig. 1.3:  Film about use of DDT in World War II

The anti-malaria campaign relied heavily on DDT spraying to control the mosquitoes.  It had great success in many areas, including the former Soviet Union, Taiwan, much of the Caribbean, the Balkans, parts of northern Africa, the northern region of Australia, and a large swath of the South Pacific. Significant improvements in fighting malaria also occurred in India and Sri Lanka.  In addition, DDT was also effective in fighting dengue fever, another mosquito-borne illness found in tropical climates.   Fig. 1.3 above shows a scene from a film describing the effectiveness of DDT in World War II in controlling diseases; it is sub-titled “Miracle Insecticide Mosquito Control.”

In 1945, DDT was made available to farmers as an agricultural pesticide.  Its effectiveness against insects, coupled with what appeared to be little or no adverse impact on human health, made it the most widely-used pesticide in the U.S.  It was hailed by many as a “miracle substance.”   Fig. 1.4 shows an advertisement that emphasized the benefits of DDT, and more importantly its lack of any negative effects on people, animals or plants.

Fig. 1.4: Advertisement, “DDT is good for me-e-e!” stressing lack of toxicity for humans, animals or plants.

In the period following World War II, a number of chemicals that had been developed in the past few decades were applied to combat diseases, fight agricultural pests and develop new synthetic materials.  This era could be called the “Golden Age of Chemicals.”  Fig. 1.5 shows an advertisement by Union Carbide, part of a series of ads touting the benefits of synthetic materials in improving one’s quality of life.  This ad is titled “Synthetics – why they spell a better life for you.”

Fig 1.5: Union Carbide advertisement emphasizing the benefits of synthetics.

And Fig. 1.6 shows the slogan for DuPont, the largest American chemical company: “DuPont – Better things for better living … through chemistry.”   My father was a research chemist for DuPont, and he was justifiably proud of the many products developed by his company.   My father’s colleagues, friends of my family, were idealistic scientists who were convinced that their products were creating a better, healthier and more prosperous world.

Fig. 1.6: slogan of the E.I. DuPont de Nemours chemical company

2) What’s The Problem With DDT?

As we have noted, DDT was exceptionally effective at killing insects.  It had proved effective in campaigns against the vectors of infectious diseases, including typhus and malaria.  When applied to farms, it proved to be lethal to many pests that attacked crops.  DDT was also the centerpiece of campaigns against particular insects such as the spruce budworm that attacked spruce trees, the gypsy moth that was a threat to Eastern hardwoods, and the insect that transported the fungus that caused Dutch elm disease.

In addition, DDT appeared to have no significant short-term effects on human health.  Workers who handled or sprayed DDT showed no obvious immediate adverse effects (provided that they took appropriate precautions).  So, given these remarkable successes, what were the problems, if any, from the use of DDT?

The “tipping point” regarding the use and safety of DDT was the publication in 1962 of Rachel Carson’s book Silent Spring.  That book presented a scathing attack on the widespread use of pesticides, and Carson focused specifically on the potential dangers of DDT.  Part II of this series includes a detailed biography of Carson and the impact of her book; in this Part we simply summarize the major accusations in Carson’s book.

First, Carson noted that DDT was indiscriminate.  In addition to the targeted pests, it tended to kill all insects.  Carson pointed out that large-scale spraying of rivers and lakes might kill off insects that were vital in the diet of fish.  Thus, large-scale aerial spraying campaigns often resulted in dramatic fish kills when the sprayed areas included bodies of water.  Some of the fish appeared to be directly killed by the chemicals, and in other cases the fish perished because their source of food had been eliminated.  In addition, such campaigns often resulted in the deaths of large numbers of songbirds and some game birds (particularly quail).   DDT would also kill off pollinating insects, and there were concerns that bees might be particularly sensitive to pesticides such as DDT.

To many entomologists, the deaths of insects, birds and other wildlife amounted to justifiable collateral damage.  Because DDT and other powerful chemicals were lethal to a wide spectrum of wildlife, Carson suggested that they be called ‘biocides’ rather than ‘pesticides.’

Carson also accused the chemical companies of using powerful chemicals without understanding potential long-term problems for their use.  For example, DDT and its breakdown products were stored in the fatty tissues of birds and other animals.  The levels of pesticide residues could end up being concentrated in animals at the top of the food chain.  At the time Silent Spring was published, it was uncertain whether the resulting concentrations of pesticide residue would lead to health issues.  However, Carson pointed out that some raptors and pelicans were having issues raising young birds, and she speculated that one cause could be thinning of eggshells that would cause the shells to break before the young birds hatched.  Scientists had found a correlation between the thinning of eggshells and amounts of DDE, a breakdown product of DDT, in the tissues of birds.

A third issue was that indiscriminate use of pesticides on a wide scale would hasten the development of pesticide resistance in insects.  When insects develop a resistance to a pesticide, it requires progressively larger doses of the pesticide to kill them.  Although pesticide resistance can occur in various ways, the most common form of pesticide resistance occurs through natural selection.  This is defined as ‘a heritable change in the sensitivity of a pest population that is reflected in the repeated failure of a product to achieve the expected level of control when used according to the label recommendation for that pest species’.

When a pesticide is first applied to a population of insects, it kills off most of the insects.  The surviving individuals can tolerate a larger dose of poison before succumbing.  Repeated applications of the pesticide will continue to select out the most tolerant individuals, and they can pass that tolerance on to their offspring.  Thus after time, the remaining insects will be progressively more resistant to that chemical.  The problem is magnified by several factors.  First, insect pests tend to breed more rapidly and prolifically than their predators.  So broad-spectrum chemicals tend to be more effective in reducing populations of predators than the targeted pests.  Second, chemicals that persist for long periods in the environment continue to develop resistance in insect populations for some time after the chemicals have been applied.

Initially, it was believed that the new organic insecticides such as DDT would not produce insecticide resistance.  However, those hopes were soon dashed as cases of housefly resistance to DDT were discovered as early as 1947.  Through her contacts in the scientific community, Rachel Carson understood that pesticide resistance represented a serious challenge to the large-scale use of pesticides.  Paradoxically, the continued use of DDT on a massive scale in agriculture, together with campaigns that used DDT against infectious diseases and environmental pests, would insure that insects would develop resistance to DDT.

In fact, the larger the scale on which DDT was used, the faster resistance would develop.  This would lead to a vicious cycle: as pesticide resistance began to appear, either the community would have to steadily increase the dosage of DDT; or they would have to switch to a more potent chemical, until resistance to that chemical occurred.  As an example, at present the Colorado potato beetle has developed resistance to 52 different compounds, belonging to all known insecticide classes!

A final accusation leveled by Carson was that DDT might be damaging to human health.  She pointed to various studies that appeared to be cause for concern.  It had recently become possible to measure trace levels of DDT, and some recent studies found levels of DDT in human tissues or in breast milk that could conceivably suggest a link between DDT levels and cancer.  Carson suggested that over the long haul, it might turn out that DDT was carcinogenic.

 3) The Debate Over DDT:

Rachel Carson’s book Silent Spring raised national concerns over the use of chemical pesticides and in particular the use of DDT.  In 1962, President Kennedy directed the President’s Science Advisory Council to study issues of chemical pesticides.  The PSAC report issued in May 1963 largely vindicated claims made by Rachel Carson.  It recommended phasing out the use of “persistent toxic pesticides.”  At this point, the federal government changed the way pesticides were assessed.  Prior to 1962, the only requirements on chemical pesticides were that they were authentic and effective.  Now, Congress passed legislation that included safety considerations in the labeling of pesticides.

The use of DDT began to decline in the late 1950s.  Some states had attempted to regulate pesticides such as DDT, and in addition resistance to DDT had begun to appear in various insects.  In 1967, motivated in part by Silent Spring, a group of scientists and lawyers formed the Environmental Defense Fund (EDF).  One of the goals of this organization was the banning of DDT from use in the U.S.  The EDF petitioned the government to ban DDT, and filed lawsuits in an attempt to achieve such a ban.

On Jan. 1, 1970, President Richard Nixon signed the National Environmental Policy Act.  This Congressional statute led to the formation of the Environmental Protection Agency (EPA).  Indiana attorney William Ruckelshaus, who had previously served in the Justice Department, was named the first administrator of the EPA.

Fig. 1.7: Nixon signing statute that led to creation of the EPA. First EPA administrator William Ruckelshaus is at left.

The EPA was given the responsibility for enforcing environmental regulations.  It thus consolidated functions that had previously been scattered over as many as fifteen different federal agencies.  For example, the Department of Interior had been responsible for oversight of water quality; Health, Education and Welfare had regulated air pollution and solid waste disposal; and the Department of Agriculture (USDA) had regulated pesticides (while simultaneously being charged with promoting agricultural production).

With pressure being applied by EDF and other environmental groups, one of the first major decisions by the EPA would concern the status of DDT.  We have listed various charges leveled by Rachel Carson about the use of DDT in agriculture, forestry and disease control.  The use of DDT resulted in significant collateral damage to animals other than the pests or disease vectors – in particular, DDT killed other insects including pollinators, songbirds and game birds, and fish.  DDT and its breakdown products persisted in the environment, and built up in the fatty tissues of animals, particularly those at the top of the food chain.  And the widespread use of DDT produced DDT resistance in the targeted insects.

All of these issues led to the conclusion that the use of DDT should be regulated, with the goal of ramping down its use over time.  However, a major factor in the eventual decision to ban DDT in the U.S. was its effect on human health.  This was such a central issue of debate that we will devote the next subsection to this question.

4) DDT and Human Health:

Silent Spring focused much of its attention on the effects of DDT, the most widely-used pesticide in the 50s.  After its publication, a major debate ensued regarding the benefits and hazards of DDT.  Elena Conis has written an article in Public Health Reports that summarizes this debate from the viewpoint of two of its most articulate protagonists; in this section, we will follow her argument.

Charles Wurster (see Fig. 1.8) was a chemist and professor of Environmental Sciences at Stony Brook University.  Wurster had been inspired by Rachel Carson’s Silent Spring.  Initially, Wurster’s focus on DDT arose from several issues.  The first was his postdoctoral research, where he studied the effects of DDT on robins.  Another catalyzing event was a 1965 campaign against the gypsy moth on Long Island, that involved aerial spraying of a DDT mixture.  Wurster advised a group of landowners who petitioned to stop the campaign, on the grounds that their homes and farms were being sprayed with a dangerous chemical, animals and fish on their property were being killed, and vegetables and milk from local farms were contaminated with DDT residue.  The Long Island campaign against DDT led directly to the formation of the Environmental Defense Fund.  Wurster was one of the founding members of EDF, and the Long Island anti-DDT effort was repeated in various other communities.

Fig. 1.8: Stony Brook professor Charles Wurster.

Thomas Jukes (see Fig. 1.9) was a British-born biologist who worked for a while at the University of California at Davis, and then worked at the American Cyanamid Corporation, before joining Cal-Berkeley as a biology professor.  Jukes was a generation older than Wurster, so he began his career during World War II, when DDT first appeared and was considered a ‘miracle drug.’  Following the publication of Silent Spring, Jukes and his Cyanamid colleague Robert White-Stevens were among the most vehement critics of Carson’s book.  While Wurster and his colleagues focused on possible harmful effects of DDT on the ecosystem, Jukes emphasized the benefits derived from the use of DDT: crops were spared from the ravages of insect pests; and DDT had been spectacularly successful against infectious diseases such as typhus and malaria.  In his signature hyperbolic polemical style, Jukes claimed that “We need … insecticides … and every other possible aid from science in the struggle for survival of our species.”

Fig. 1.9: Berkeley professor Thomas Jukes.

Although the initial arguments between Jukes and Wurster focused on the effects of DDT on the ecosystem, the debate between them rapidly settled on the possible adverse effects of DDT on human health.  Jukes and his allies maintained that one of the major successes of DDT was its positive effects on human health.  They emphasized the success of the campaigns against typhus and malaria.  Furthermore, they stressed that there were no iron-clad studies that linked DDT exposure with cancer.  Previous studies had not found significant adverse health effects in soldiers exposed to DDT sprays, and tests where prisoners had eaten DDT had not shown any short-term health damage.

Opponents of DDT such as Wurster emphasized the toxicity of DDT to insects and wildlife.  They pointed out that DDT worked as an endocrine disruptor, which suggested potential health problems in humans and animals.  They stressed issues such as the persistence of DDT in the environment, the buildup of DDT and its breakdown products in fatty tissues, and possible adverse effects on development in newborns and children.

A central point of contention turned out to be the possibility that DDT might be a carcinogen.  This tapped into a national concern with chemical causes of cancer; for example, 1964 had seen the publication of the Surgeon General’s Report on Smoking and Health, which demonstrated the link between smoking and cancer.  A widely-cited report was a Swedish study that showed the presence of DDT in human breast milk at a level twice that recommended as a maximum by the World Health Organization.  Wurster and the EDF proponents made this a major centerpiece of their argument to ban DDT, while Jukes and his colleagues stressed that no studies had unambiguously shown a link between DDT exposure and cancer.

Wurster also invoked the Delaney Clause to argue for a ban on DDT.  That clause was a 1958 amendment to the Food, Drug and Cosmetic Act that established a zero-tolerance limit for any food additive found to cause cancer in humans or animals.  Although Wurster could not demonstrate conclusively the link between DDT and cancer, he made the following argument.  “The overwhelming majority of chemicals are not carcinogens, but when laboratory tests give clear warning, we must prevent human exposure to such agents. There is hope for preventing many future cancers if we are prudent and place human lives ahead of some chemicals.”

Jukes turned the argument around.  He argued that a DDT ban in the U.S. would necessarily lead to a world-wide ban on DDT.  This would mean death and decreased life expectancy for millions of Third-World citizens. Referring to U.S. hearings on the dangers of DDT, Jukes wrote “It is… noteworthy that the World Health Organization was not represented at the hearings. Also unrepresented were 30 million Africans with onchocerciasis, which produces blindness. The issue of banning DDT is unquestionably a genocidal one.”   Note that although Wurster argued for a U.S. ban against DDT, he did support allowing the continued use of DDT to fight infectious diseases in Third-World countries.

5) Restrictions on the Use of DDT:

Following the creation of the EPA, one of the first major issues was the disposition of DDT.  There had been four prior reports that analyzed DDT and related pesticides.  The first was the 1963 report of the President’s Science Advisory Council (PSAC) to Kennedy in 1963.  This was followed by a November 1965 report of the PSAC Environmental Protection Panel; a May 1969, National Research Council report on pesticides to the USDA; and the December 1969 Mrak Commission Report on pesticides. All four reports recommended an orderly phasing out of the pesticide over a limited period of time, with an exemption for “those uses essential to the preservation of human health and welfare.”

William Ruckelshaus initially declined to suspend registration of DDT products, because there was no evidence that DDT caused an imminent hazard to public welfare.  Environmental groups attacked this ruling on the basis that Ruckelhaus’ action was based on prior reports that had been generated by the USDA; the claim was that USDA had a conflict of interest because it was responsible both for promoting the use of pesticides in agriculture, and for certifying the safety of those pesticides.

In August 1971, the EPA began hearings on the uses of DDT.  The hearings continued for 8 months, and eventually contained testimony from 125 expert witnesses and produced 9,312 pages of transcripts.  The final conclusions of the report were that “The present reported annual level of DDT usage does not present an imminent hazard to human health in terms of bodily functions and safety” (based on a definition of “imminent hazard” arrived at by the EPA). A second conclusion was that “DDT and its derivatives are serious environmental pollutants and present a substantial threat to the quality of the human environment through widespread damage to some nontarget organisms.  There is, therefore, an imminent danger to human welfare in terms of maintaining healthy desirable flora and fauna in man’s environment.”   The report recommended that the U.S. continue to reduce the use of DDT in the U.S. at an accelerated rate.

However, on June 14, 1972 EPA administrator Ruckelshaus decided to cancel all remaining crop uses of DDT in the U.S. effective Dec. 31; the purpose of the delay was to allow time for an orderly transition to alternative pesticides.  The order did not affect uses of DDT for public health and quarantine uses, or exports of DDT to other countries.  Ruckelshaus based his decision on findings of persistence, transport, biomagnification, toxicological effects and on the absence of benefits of DDT in relation to the availability of effective and less environmentally harmful substitutes. 

Immediately following the ruling by Ruckelshaus, both the pesticide manufacturers and the EDF filed lawsuits.  While the manufacturers wanted Ruckelshaus’ ruling overturned, the EDF wanted his ruling extended to include all domestic uses of DDT.  On December 13, 1973, the U.S. Court of Appeals for the District of Columbia ruled that there was “substantial evidence” in the record to support the EPA Administrator’s ban on DDT.

In 1976 Congress passed the Toxic Substances Control Act. It directed the EPA to protect the public from “unreasonable risk of injury to health or the environment.” The EPA subsequently took action to ban or severely restrict all six of the chemical pesticides mentioned by Rachel Carson in Silent Spring — DDT, chlordane, heptachlor, dieldrin, aldrin, and endrin.   The EPA also took on the responsibility for testing new chemicals.

Following the U.S. ban on DDT, the chemical began to be banned or severely restricted in several other countries.  By 1991, DDT was banned in 26 countries.  The Stockholm Convention on Persistent Organic Pollutants took effect in 2004, and it instituted a global ban on several persistent organic compounds.  The use of DDT was not banned but was restricted to vector control; so an exemption was provided for DDT use in combating certain infectious diseases.

6) Continued Uses of DDT Today:

Malaria continues to be a significant health issue in many developing countries.  A method called indoor residual spraying (IRS) was developed against disease-carrying insects.  In the IRS protocol, the interior walls of dwellings are sprayed with DDT.  This limited use of DDT has been shown to be an effective treatment option.  It avoids the old blanket-spraying techniques that promoted the development of resistance in both the mosquitoes, and also in the parasite causing malaria.  As a comparison, treating 40 hectares (99 acres) of cotton during a typical U.S. growing season requires the same amount of chemical as roughly 1,700 homes [using IRS].

 The World Health Organization (WHO) had actively promoted IRS spraying in countries suffering epidemics of malaria.  In the 1980s they stopped recommending this technique because of concerns about the dangers of DDT.  However, malaria has continued to be a serious health problem in Third World countries.  So in 2006, WHO recommended that IRS techniques be used in countries with epidemics of malaria, and also in countries with constant high rates of malaria transmission.   The use of IRS techniques is supported by several environmental groups, including the Environmental Defense Fund, the Sierra Club, and the Endangered Wildlife Trust.

Several countries now use IRS in treating malaria.  It can be particularly effective against mosquitoes since they tend to rest on walls before and after feeding.  In addition, IRS is generally combined with other techniques such as mosquito nets (frequently treated with insecticide), and destruction of open water breeding grounds for mosquitoes.

In hindsight, it appears that the use of chemicals such as DDT to combat infectious diseases could have been ramped down more slowly.  There are several examples where the use of DDT in a limited, targeted approach could have saved numerous lives from diseases such as malaria.  As a result, I have some sympathy for Jukes’ defense of DDT.  Overall, however, his defense of pesticides was over-the-top, and he deliberately minimized any negative effects of chemical pesticides.

As an aside, one of Jukes’ most important contributions was a demonstration that livestock experienced rapid weight gains if continuous doses of antibiotics were added to their feed.  Adding antibiotics to animal feed became a major factor in modern agriculture.  However, it is now clear that the addition of copious antibiotics has led to the development of antibiotic resistance in a number of dangerous pathogens.  This further demonstrates how Jukes failed to consider long-term negative effects of chemicals in the environment.

It is worth noting that, even today more than 50 years after the publication of Silent Spring, DDT is listed as a “suspected carcinogen.”  We do not yet have definitive studies that link DDT exposure to specific cancers.  There were compelling reasons to ban DDT for all uses except fighting infectious disease; however, the precise effect of DDT on human health is still not a settled issue.

to be continued in Part II

Source Material:

Rachel Carson
Silent Spring
pesticide resistance
DDT Regulatory History to 1975
Environmental Defense Fund
Debating the Health Effects of DDT: Thomas Jukes, Charles Wurster, and the Fate of an Environmental Pollutant, Elena Conis, Public Health Rep. 2010 Mar-Apr; 125(2): 337.
The Golden Age of the EPA, Karen Hao, New Republic, Apr. 2017.
National Environmental Policy Act
Charles F. Wurster
Thomas H. Jukes
Food Additives Amendment of 1958
Toxic Substances Control Act of 1976
Indoor Residual Spraying
2006, WHO Recommends DDT to Combat Malaria