DDT: Secret Life of a Pesticide

Is DDT a killer of birds, a savior against malaria, or a little of each?

by Brian Dunning

Filed under Environment, General Science, Health

Skeptoid #230
November 2, 2010
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Also available in Russian

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 that DDT harms bird populations through eggshell thinning [Correction: Silent Spring did not specify the eggshell thinning mechanism; that was proposed later by other researchers - BD]. 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.

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 millions of brown and white pelicans [Correction: the birds that were destroyed were 12 million poultry. Only four wild birds were found to be infected, none of them pelicans - BD]. 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.

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. Up to three million people die of malaria each year, most 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. [Additional info: 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. In Africa, the exemption allows indoor use only, like wearing armor on half your body - 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.

Tip Skeptoid $2/mo $5/mo $10/mo One time

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 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.

Note - I've gotten a lot of criticism for including a link to the JunkScience.com web page on DDT in the References & Further Reading section below. Bizarrely, most of the criticism attributed Milloy's assertions to me, even though I clearly did not say any of it or agree with it. I try to always offer Further Reading suggestions from both sides of Skeptoid topics (you don't think I really listed Rhonda Byrnes' book The Secret because I think you can wish yourself wealthy, do you?). Steven Milloy, the author of JunkScience.com, gets lots of well-deserved criticism for cherrypicking and adhering to a conservative political agenda. I've said it before, and I'll say it again: Skeptoid is not here to tell you what to think. I want you to read and research for yourselves. You'll find that virtually every Skeptoid episode lists sources you are going to disagree with; if I've done my work well, the sources should cover the full spectrum. I will not whitewash the world and pretend certain points of view do not exist.

With that said, corrections will be made to erroneous assertions in this episode in a future Things I'm Wrong About episode, and are noted in the transcript above. The errors that have been pointed out to me so far are not, in my opinion, significant enough to alter my conclusions. I welcome any and all further corrections.

Brian Dunning

© 2010 Skeptoid Media Copyright information

References & Further Reading

Attaran, A., Roberts, D., Curtis, C., Kilama, W. "Balancing risks on the backs of the poor." Nature Medicine. 1 Jan. 2000, Number 6: 729-731.

Campbell, L. Endangered and threatened animals of Texas: Their life history and management. Austin: Texas Parks & Wildlife, Resource Protection Division, Endangered Resources Branch, 1995. 58.

Carson, R. Silent Spring. New York: Fawcett Crest, 1964.

Edwards, J., Milloy, S. "100 things you should know about DDT." JunkScience.com. Steven J. Milloy, 7 Jan. 2007. Web. 2 Nov. 2010. <http://www.junkscience.com/ddtfaq.html>

EPA. "DDT - A Brief History and Status." Pesticides: Topical & Chemical Fact Sheets. US Environmental Protection Agency, 16 Jan. 2008. Web. 9 Dec. 2010. <http://www.epa.gov/pesticides/factsheets/chemicals/ddt-brief-history-status.htm>

Gates Foundation. "Our Work in Neglected Diseases: Visceral Leishmaniasis, Guinea Worm, Rabies - Overview & Approach." Bill & Melinda Gates Foundation. Bill & Melinda Gates Foundation, 14 Jun. 2010. Web. 30 Oct. 2010. <http://www.gatesfoundation.org/topics/Pages/neglected-diseases.aspx>

Gladwell, M. "The Mosquito Killer." The New Yorker. 2 Jul. 2001, Annals of Public Health: 42.

Lambert, T. "Skeptoid Fact Check (parts 1 and 2)." Deltoid. ScienceBlogs LLC, 22 Nov. 2010. Web. 22 Nov. 2010. <http://scienceblogs.com/deltoid/2010/11/skeptoid_fact_check_part_1.php?>

Miller, H. "Utterly Repellent." Forbes.com. Forbes.com LLC, 24 Feb. 2010. Web. 29 Oct. 2010. <http://www.forbes.com/2010/02/24/bill-gates-malaria-vaccines-opinions-contributors-henry-i-miller.html>

Stokstad, E. "Can the Bald Eagle Still Soar After It Is Delisted?" Science. 22 Jun. 2007, Volume 316, Number 5832: 1689-1690.

Reference this article:
Dunning, B. "DDT: Secret Life of a Pesticide." Skeptoid Podcast. Skeptoid Media, 2 Nov 2010. Web. 4 Oct 2015. <http://skeptoid.com/episodes/4230>


10 most recent comments | Show all 141 comments

@Tom H: I was referring to the discussion at skepticblog, where Brian was certainly participating, and, as I saw it, playing the "political" card to avoid answering factual objections.

He states here that, "With that said, corrections will be made to erroneous assertions in this episode in a future Things I'm Wrong About episode, and are noted in the transcript above. The errors that have been pointed out to me so far are not, in my opinion, significant enough to alter my conclusions." If you check out the link, you'll see that there are many more objections than the two Brian acknowledges (or are corrected in the body of the original podcast). If he wishes not to debate them there, and not to debate them here, then where? And if he's unwilling to defend them...

Brian also says, "Skeptoid is not here to tell you what to think." If you take a look at many of his other articles, he is rather willing to tell you what to think...or at least to call something BS when it is (instead of presenting it neutrally and letting the reader decide). And we applaud him for that. But when his readers point out that he has used crap information about DDT to "balance" what is presented by scientists, people are going to start scratching their heads.

I'm not sufficiently tempted to summarize the discussion, but again, if you're interested:


Peter Rott, New York, NY
February 19, 2011 11:08pm

There is a difference between telling you what he considers Bull and why, and telling you to think it is Bull. The DDT episode is no different, he put his opinion out there and some people disagreed. Calling the thred an "epic fail" is pretty much making a point with no sense of balance or objective scrutiny. It is starting form the view "He is wrong". So it does seem a little unfair to accuse Mr Dunning of the same issue. He is under no obligation to explain his opinion on any forum he does not want to use.

It is of course an entirely different issue to the subject of the podcast, but it is a rather pragmatic point that should be considered: If the podcasts are self edited, free, podcast that offers one mans opinion, and he has shown all the evidence on which he has built that opinion so others can make up their own mind, why exactly should be obliged to answer questions in a discussion whose motivation is to describe him as a "fail" (that would be a political stance) rather than to discuss if a different opinioin would have been reached based on other evidence he may or may not have encountered in his research?

Tom H, Kent, UK
February 20, 2011 6:12am

If only pesticides (like nuclear reactors) were used properly:

I worked as off-sider to a former panzer driver who was a pest controller on a military base. Called in to eradicate a massive wasps nest on the side of a hill, he filled a bucket with kerosine and added a full can of DDT. This he poured into the hole in the hill containing the wasps nest

He was just about to throw the match in when the military police drove up to ask what we were doing. I said "Ask him" and pointed at my mate.

They told us to get in the vehicle and took us round the front of the hill and then drove a little way inside a large door

Yes - it was the main ammunition dump.

"Learnt anything?" the MPs said

"Yes" I said, "I'll get him to read the label on the can next time."

Phi, Sydney
March 23, 2011 5:36pm

So Phi, any proof nuclear reactors arent used properly? Or that most farmers don't work with in pretty tight regulations? Or is this another case of the assumption your experience is indicative of the entire world?

Lets look at my experience and compare and contrast shall we; anybody who makes that kind of mistake finds themselves under an investigation for not completing a proper COSHH assesment, and probably investigated by either the HSE or the Environment agency.

Being in a good union helps, because their employer is likely to consider them a liability. That is when subcontractors find themselves with out a contract.

I have no doubt, (and good evidence…) people misuse pesticides. But that is why we have control measures and we enforce them. For actual evidence look up the coshh control sheets for pesticides.

Illuminatus, fantasy island
April 26, 2011 11:25am

Phi may be saying that nucear reactors cant be updated because of the antitechnological fringe whipping up an anti nuclear vote on reactor replacement. There are perfectly safe reactor designs and frankly, the nuclear industry is as safe as you can get.

When the public weighs an apparent scary word like nuclear against a non scary phrase like general industry they think of weapons.

Phi, in what moorcockian universe was this tale elaborated? Can you lose this periodical reference to former ss/wehrmacht?

We have to face facts, we have in the past had a little too much zeal in a lot of things. Biologically, I see DDT as about as bad a threat in our day to day chemical exposure. If some snippet says DDT is bad, I am sure another sector will say, forget DDT, floor polish kills..and maybe rightly so..But on average we live longer and more fruitfully than ever before.

The thing is overwhelming claims about a compound need overwhelming evidence. Its easy to aggrandise small and most likely non synergistic properties to demonise one thing we think we dont like in favor of something else.

DDT is essentially outré in our society and we have better compounds that are cheap for us to disseminate. I agree with Brian, the evidence seems to be mainly outcry and exampling of properties, exaggeration of claims. Its cheap and effective when used correctly (sans matchbox).

Should old nuclear reactors be replaced? Has anyone a better cheaper and safer technology? Not lately!

Henk v, sin city NSW, Oz
August 9, 2011 11:25am

DDT is more dansers for us through our crop's production.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.it would be completely wrong to overlook DDT's potential for causing harm simply because there are other things that cause harm toosobe awaoid this insfctcide

ibrahim, nangloi
December 3, 2011 11:15pm

I was at Club Med in Moorea, Tahiti in '93 and they used DDT every night just before dusk and I never got a mosquito bite the whole time. The applicator guy drove around in a golf cart with a fogger on the back, kind of reminded me of Tattoo on Fantasy Island...

Glen, Vancouver
March 21, 2012 7:20pm

Brian - for another perspective on why DDT use declined in other countries you should read "The Merchants of Doubt".

Jamie, Gilston, Australia
October 2, 2012 3:20pm

well this is good but yeah ...

Prank Sayers, Warren New South Wales
November 12, 2012 7:26pm

What about the return of bed bugs? Maybe you can cover that topic.

macsnafu, Tulsa, Oklahoma
January 25, 2013 2:01pm

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