DDT: Secret Life of a Pesticide

by Brian Dunning
November 2, 2010

Put on your respirator and hazmat suit, because today we're pointing the skeptical eye at the claims on both sides of the DDT question. DDT is an insecticide in use since the 1930's. At first, its basic use was to kill mosquitos that transmit malaria, lice that transmit typhus, and other insect disease vectors like tsetse flies, at which DDT is extremely effective. It was so successful in World War II that its discoverer was awarded the Nobel Prize in medicine in 1948. Subsequently it was used in agriculture to protect crops from a variety of pests, and once again, it's highly effective in doing so. But a few decades later, DDT became a two-sided issue, with detractors pointing to health effects on humans and animals; most notably, eggshell thinning in various bird species, and a number of potentially severe health effects in humans. In response to these concerns, DDT has now been banned for the most part in many countries. But the controversy continues. While the ban has been credited with the rebound of bird species, it has also been criticized as overzealous, with many now saying the detrimental effects were overblown and did not outweigh the many lives saved from malaria in the third world. It is in fact making a comeback, with production increasing today in India, China, and North Korea, for both agricultural and anti-malaria uses.

And so we ask the question: Is one side completely wrong and one side completely right, or do we equivocate and conclude that DDT has its place, albeit a limited one?

DDT is dichlorodiphenyltrichloroethane. It's a completely synthetic compound that does not exist in nature. It's a white, powdery, waxy substance that's hydrophobic: It doesn't dissolve in water and so does not contaminate it, but readily dissolves in solvents and oils. It's applied as a white smokey mist. DDT kills insects by chemically enhancing the electrical connections between their neurons, short-circuiting them into spasms and death. DDT's hydrophobic nature is both a blessing and a curse. It can't contaminate water sources, which is good; but it also doesn't get dissolved away by them and diluted into virtual nothingness, so it hangs around for a long time.

DDT probably never would have been banned if it were not for the 1962 publication of Silent Spring, the title of which alluded to dead birds. Author Rachel Carson was a much beloved nature writer who died only two years after the book came out. She'd been a marine biologist for the U.S. Fish & Wildlife Service (then called the Bureau of Fisheries), but was able to retire with the publication of a trilogy of books about the sea. All became bestsellers in the 1950s, with the public enamored by her poetic presentation of all things pertaining to beaches, islands, the deep sea, and the creatures living there. Following this trilogy, her writing turned toward environmental issues and became increasingly critical of industry, government, and the effect of humans on the planet. Silent Spring was serialized in The New Yorker before its publication, and it was probably the most scathing of her works, though beautifully written. It charged DDT with being a health hazard and with widespread environmental destruction, particularly to bird populations, and was unquestionably the turning point which resulted in DDT's bans in the United States and other countries. In fact, as one Environmental Protection Agency writer put it:

Silent Spring played in the history of environmentalism roughly the same role that Uncle Tom's Cabin played in the abolitionist movement.

Rachel Carson's list of posthumous honors is a long one, showing what deep roots Silent Spring thrust not only into the environmental movement, but also into the public psyche. President Jimmy Carter awarded her the Presidential Medal of Freedom. She appeared on postage stamps in several countries. A bridge in Pittsburgh is named after her, as is a government building in Harrisburg. The number of schools, parks, nature refuges, scholarships and scholarly prizes named for Rachel Carson would fill a page.

Silent Spring's principal thesis was to link the declines in bird populations to pesticide use, and later research found that eggshell thinning was one cause and noted the correlation to DDT. The mechanism by which DDT does this is now largely, but not completely, understood. In summary, it interferes with the delivery of calcium carbonate to the eggshell gland, and the eggs that are laid have thinner shells. Shells that are too thin can lead to the death of the embryo. This eggshell thinning is the primary environmental concern over DDT.

Correction: An earlier version of this wrongly said that Rachel Carson discussed the eggshell thinning mechanism for the population declines, but this was in fact proposed by later researchers. —BD

It's been about five decades since Silent Spring was published, and we've learned a lot in those years. One thing we've learned is that DDT is only one of many causes of eggshell thinning. Other culprits include lead and mercury toxicity, oil, phosphorus and calcium deficiency, and dehydration. Perhaps most significantly, birds in captivity in order to undergo testing are under stress, and this stress alone is enough to produce eggshell thinning. Although DDT's mechanism for eggshell thinning is plausible, many studies throughout the 1970's and 1980's failed to correlate such thinning with high levels of DDT, even extremely high levels. Other studies have confirmed Rachel Carson's findings. My own conclusion based on a review is that there probably is a correlation, but it's not a strong one; and at best it's only one of many causes. Whether DDT is used or not would probably not have a large impact on bird populations.

But despite the likelihood that it would have some impact, it's now known that the species Rachel Carson focused on (most notably bald eagles) were already in massive decline from unrelated pressures even before DDT's introduction. Habitat loss and hunting had been, by far, the greater causes of bald eagle deaths. Hunting had reduced the populations to just a few hundred nesting pairs in the mountains, and lowland eagles were already gone from habitat loss. Rachel Carson did not ignore these issues in her book, but the popular perception that banning DDT was all that was needed to magically restore bald eagle populations was naïve. In the end, it was the Bald Eagle Protection Act and the bird's 1967 placement on the endangered species list, combined with increased penalties for poaching, that ultimately led to the bald eagle's successful return to remaining habitats.

Brown pelicans are another species often cited as having been decimated by DDT use in the United States, along the Gulf coast and in California. Massive declines were indeed correlated with DDT use, but it may have been a coincidence in each case. Along the Gulf coast, hunting by angry fishermen had reduced the pelican population in Texas from 5,000 annual births to just 200 in 1941. The California populations suffered a double whammy in the years following Silent Spring's publication; first with an oil spill off Santa Barbara in 1969, and then with an outbreak of Newcastle Disease in 1971 that unfortunately required the culling of some 12 million poultry. DDT certainly didn't help; but it was another case where the bird populations would have dropped sharply whether DDT was in the picture or not.

Correction: An earlier version of this wrongly said the millions of poultry culled were wild pelicans. Nope, only four of those birds were wild, and none were pelicans. —BD

Of course, it would be completely wrong to overlook DDT's potential for causing harm simply because there are other things that cause harm too. All we can do is our best to quantify exactly what the risk really is, and then the decision to ban or not to ban becomes a cost/benefit analysis, which is no longer a science question. Everyone has the right to their own opinion on what's most important, and in the United States, we chose the birds.

Silent Spring's legacy may have been good for the birds, but not so much for human populations in the third world. DDT is one of the most effective pesticides ever discovered for fighting malaria. Although DDT remains legal for insecticide use in most areas where malaria is a major killer, the money for fighting the mosquitos often comes from donors in wealthy countries like the United States. Such wealthy donors have often had little personal exposure to the issues, and can sometimes have a skewed perspective when it comes to bald eagle eggshells in the United States versus the deaths of children in Mozambique. Writing in the Nature Medicine journal, malaria advocate Prof. Amir Attaran criticized American environmental groups for opposing the public health exceptions of DDT bans:

Environmentalists in rich, developed countries gain nothing from DDT, and thus small risks felt at home loom larger than health benefits for the poor tropics. More than 200 environmental groups, including Greenpeace, Physicians for Social Responsibility and the World Wildlife Fund, actively condemn DDT.

As a result of these pressures, many donations now coming from wealthy nations are now contingent upon DDT not being used, which leaves the poor nations with fewer options, often too expensive and less effective, and children die. Between six and seven hundred thousand people die of malaria each year, 95% of them in Africa. DDT, while it does have environmental and health concerns like all pesticides, is not known to have ever killed anyone. If we shelve our most effective tools hoping that something perfect will come along that has no potential downside, we'll wait forever, and thousands will continue dying every day. These are the cases where wealthy environmental groups appear to do their best to justify their elitist stereotype, at the expense of brown people.

Addendum: The World Health Organization's ban on DDT does include limited exemptions for malaria control in many regions, but money for its use still often depends on qualified foreign aid. —BD

Correction: An earlier version of this incorrectly overstated annual deaths from malaria. —BD

Rachel Carson absolutely acknowledged DDT's importance to fighting malaria, but was quick to point out another downside: acquired resistance. After six or seven years, mosquito populations develop resistance to DDT. However, this is the case with all pesticides, it is not a reason to avoid DDT per se. Moreover, we've since learned that it is still effective against resistant mosquitos, only a little less so. Susceptibility in resistant strains goes down to 63%, as opposed to 87% in non-resistant strains.

Even among resistant mosquitos, DDT is an exceptionally effective repellent. Houses treated with DDT are avoided by all mosquitos, resistant or not.

But like all synthetic chemicals, DDT has been blamed for virtually any human illness imaginable. Some say it causes cancer, diabetes, hyperthyroidism, loss of fertility, that it functions as an endocrine disruptor, and more. The World Health Organization classifies it only as "moderately hazardous", and in response to all the wildly conflicting studies of its cancer-causing effects in animal tests, the U.S. Environmental Protection Agency classifies it as a "probable carcinogen". The claims that DDT definitely causes cancer or anything else are not supported by the data, but obviously it's a risky compound that we don't want to expose anyone to if we don't have to. And so, again, we're outside of science questions, and down to risk assessment.

DDT does have its place, and its current usage is probably not too far off of what it should be. The existing bans are more or less appropriate. The exception is Africa where DDT's upside far outweighs the down, and my opinion is that donors should relax their restrictions against it, and leave those decisions to the experts on the front lines in Africa. For much of the rest of the world, DDT has largely been supplanted by newer and better agricultural pesticides, and there's insufficient reason to put collateral species under pressure. A scientific review nearly always produces better focused policy, and our DDT policy is definitely due for a tuneup.

By Brian Dunning
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