The DDT facts – examining the evidence after 50 years

The DDT facts – examining the evidence after 50 years

DDT facts and myths have been part of our shared environmental consciousness for two generations. Most of our beliefs about DDT, a powerful insecticide long-banned by most countries, came from Rachel Carson’s best selling book, Silent Spring, published over 50 years ago.

Carson was an aquatic biologist, working for the US Department of Fisheries, who became a champion of the environmental movement across the world. Her influence on environmental policy is still felt today. It would not be an exaggeration to claim that her movement lead to the creation of the US Environmental Protection Agency (EPA) in 1970.

Silent Spring was an influential book that drove pro-environmental policies and thinking of many of us who grew up in that era. Essentially, the book outlined the environmental disaster caused by the indiscriminate use of synthetic pesticides, especially on agricultural lands. She seemed to advocate for a complete ban on DDT and other pesticides based on some anecdotal and statistical correlation between DDT use and certain environmental issues.

But it was too late. The DDT myth (or facts, depending on the evidence) had started, and it was imprinted into the American consciousness. In 1972, DDT was banned for use in agriculture in the USA, which has lasted until today. It’s ironic that the Reagan administration, a notoriously anti-environmental group, refused to reconsider the ban on DDT.

Predictably, the chemical industry lashed out against the Ms. Carson and her book. But given the nature of the times, they really had no shot, and the environmental movement was born.

However, what do we make of the strength or weaknesses of DDT facts? Is it a myth? Or were some of Ms. Carson’s points valid? I think after 50 years we can answer some of that, but DDT has evolved into a word that induces fear and loathing in most people across the world. Let’s take a look at it.

What is DDT?

DDT, formally known as dichlorodiphenyltrichloroethane, is a colorless, crystalline, tasteless and almost odorless organic molecule that is known for its insecticidal properties. The product used to be marketed in many forms, powder, sprays, and liquids, that made it easy to apply.

DDT was invented by Swiss chemist Paul Hermann Müller, who 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 other words, it was considered an important product in controlling diseases across the world.

For example, the chemical was used extensively during World War II by the Allies, in both the Pacific and Europe, to control the insect vectors of typhusdengue fever and malaria, nearly eliminating the disease in Europe. DDT was spectacularly successful in eradicating these diseases in Australia, the South Pacific, South America and other areas of the world after WWII. At the time, the goal of defeating those diseases far exceeded any risk known at that time.

Maybe you’ve noticed that malaria is nearly non-existent in the USA, despite the fact that about a quarter of the country, from Tennessee south to Florida, has mosquito loving areas that used to be prime locations for malaria. Not any more, because of DDT.

By 1950, the US Department of Agriculture, which had jurisdiction over insecticide use at the time, was beginning to establish regulations to limit DDT’s use. But it took Ms. Carson’s book to get it banned.

DDT is still used in some areas. It is still an effective means of controlling certain insect vectors, especially in areas where governments cannot afford more expensive insecticides or the latest medical treatments, especially for malaria. But you’ll not find it in the USA or other countries.

There is a silly myth that “scientists thought that DDT was good for people,” part of the trope that scientists are complete idiots and make mistakes all the time. Actually, real science knew that it had certain dangers, but the benefits, at the time, appeared to outweigh any issues. And science changes with new data, unlike beliefs. But that’s a discussion for another time.

 

DDT facts – it’s a poison

Well, everything is a poison, even pure water from a melting glacier from Greenland. If you understand anything about the field of toxicology, it’s the dose that makes the poison. Carson didn’t discriminate between what constitutes a poisonous dose (to either humans or the environment at large) and what is not. In other words, she seemed to ignore the whole cost:benefit calculation that we must do to make reasonable scientific decisions.

In his article, The Lies of Rachel Carson, J. Gordon Edwards, who is way too pro-chemical industry for my taste, wrote that:

“This implication that DDT is horribly deadly is completely false. Human volunteers have ingested as much as 35 milligrams of it a day for nearly two years and suffered no adverse effects.’

But there is strong evidence that chronic close contact with DDT may have some known health risks, especially in endocrine disruption. But the whole point of “dose makes the poison” is that at what dose do the health risks outweigh the benefits? Carson (and frankly so do a lot of pro-DDT people) appeared to take a false dichotomy approach to this discussion. Carson essentially says “DDT is bad, ban it.” The other side seems to say “DDT is good, un-ban it.”

 

DDT facts – the environment

There is some evidence that DDT was, at least partially, responsible for the thinning of shells of many bird species, especially raptors like bald eagles and falcons. Apparently, DDT accumulated in prey species, and the endocrine disruption of high doses of DDT may be responsible for the egg-thinning.

However, the evidence seems to vary all over the place – environmentalists can cherry pick papers that DDT (more properly, DDE, a breakdown product) causes thinner shells, whereas, pro-industry types claim that it’s overstated.

Although there seems to be a huge correlation between the use of DDT and bird shell thickness, we all know that correlation doesn’t not mean causation. There is some evidence that egg shells were thinning prior to the introduction of DDT, and that raptor egg shells appear to still be somewhat thinner than they used to be, despite the near elimination of DDT and DDE from the environment. I suspect that DDT is not innocent, but that are a number of other confounding factors, like humanity’s total disregard for the environment until just a few decades ago. Maybe it was DDT/DDE plus a dozen other chemicals.

Despite evidence being all over the place, I think the bulk of it supports, at least, a correlation between large-scale use of DDT and the thinning of raptor eggs. And because of the biological plausibility (an important factor in determining whether correlation does imply causation) that DDT/DDE is an endocrine disruptor at high doses, one can conclude, at least in this case, there is a link between massive uses of DDT and the near-destruction of part of the world’s wildlife.

Because DDT is fairly persistent in the environment, it can take anywhere from a few weeks to decades to break down, depending on conditions. This means it’s still detectable in wildlife, food, and humans, even 40 years after banning. And again, there’s enough evidence, outside of Carson’s book, that this is not a good thing.

I think it’s clear to us that massive agricultural application of DDT is environmentally dangerous. It should be banned, despite some overstatements by both sides of the DDT question, because there’s sufficient evidence that it harms the environment. Not all farmers would use DDT carelessly, but there’s an odd belief that if a little is good, a lot must be really good. DDT is so cheap that being judicious may not be a high priority.

Furthermore, because of the issues with DDT in the environment, large scale use, even careful use, may still be too much.

DDT facts

DDT facts – malaria

This is the other side of the examination of the DDT facts, and the other half of the cost:benefit equation. Are there still benefits to using DDT carefully.

And I think we should look at one application of DDT, but it would have to be carefully considered and weighed. This would be the judicious use of DDT to control disease carrying mosquito vectors.

I think several points have to be considered with this part of the benefit discussion:

  1. As climate change increases temperatures, tropical and subtropical diseases start moving north. Dengue fever, for example, has entered the Florida Keys, because the mosquito vector for the virus, Aedes aegypti, can now survive that far north.
  2. Current mosquito abatement tools, such as several insecticides, aren’t working as well as they have in the past. A. aegypti is now resistant to four of six commonly used insecticides for mosquitos. Hence the testing and release of genetically modified mosquitos to prevent reproduction.
  3. Malaria, a mosquito borne disease, still infects over 200 million people. The cost of treatment and prevention is huge, in the area of billions of dollars. Invariably, like the movement of the Dengue fever mosquito into the Florida, malaria carrying mosquitos could return to North America and southern Europe as the climate warms.

DDT has a couple of benefits for mosquito control. First, it kills them. Of course, mosquitos can evolve a resistance to DDT, but there’s another characteristic of DDT – mosquitos avoid it for some unknown reason.

For example, a light application of DDT powder to screens can prevent access of the insect into a home. In addition, careful use of DDT around a home can reduce mosquito breeding in the immediate area. In addition, after feeding, mosquitos like to rest on a nearby surface. So a light application of DDT, which would have little impact on humans or the environment, could kill mosquitos.

In fact, despite the global ban on DDT, it is still acceptable to use it for household mosquito control. Just to be clear, it still bans its use for agriculture, so it’s a matter of the dose, again, with respect to the environment.

In Silent Spring, Carson asked, “Who has decided—who has the right to decide—for the countless legions of people who were not consulted that the supreme value is a world without insects, even though it be also a sterile world ungraced by the curving wing of a bird in flight? The decision is that of the authoritarian temporarily entrusted with power.”

This is a valid question. But again, it’s a false dichotomy. Our choices aren’t between beautiful birds and human life. It can be both. There are over 200 million cases and almost 1 million deaths every year from malaria. And if you think that Europe and the USA, are safe, both areas have had several thousand cases (some, obviously, brought from other areas) and a few deaths.

And remember, it’s not just malaria. Zika virus, which has become a major concern for epidemiologists, is spread by mosquitos.

If DDT can prevent much of that, and it can, then the benefits have to outweigh the risks. DDT is one of the more powerful tools in the arsenal against mosquitos. No, there isn’t some “natural” method to kills a large number of mosquitos at a low cost.

Again, massive application to farmland is not ever going to be considered – but careful use to control mosquitos? Sure.

 

TL;DR version

  1. DDT was banned 40 years ago as a result of Rachel Carson’s, Silent Spring, based on some evidence available at the time.
  2. DDT has a known correlation with some environmental issues, such as thinning of raptor eggs.
  3. DDT has some known effects on humans, although at fairly high doses.
  4. DDT has been and should continue to be banned for large-scale agricultural use. This was the single most important cause of environmental contamination throughout the food chain.
  5. DDT kills mosquitos and prevents infestation that lead to several diseases, especially malaria.

Like most stories in science, it is fairly nuanced. Yes, DDT can be an issue, but we know that now. We can control its use, because of the benefits it has to humanity. Are there alternatives? Yes, but they are going to take a long time. One of the strategies is GMO mosquitos, and I can hear the complaining from privileged white liberals in Oregon – NO GMOs. Then they’ll recommend pouring some eucalyptus oil on your head, and you won’t get malaria. Or some such nonsense.

Am I saying bring back DDT for common use? Not really, but it’s being used. Maybe there should be a consumer version to be used in areas that are plagued by malaria, Dengue fever and other mosquito based diseases.

 

Key citations:

 
 
The Original Skeptical Raptor
Chief Executive Officer at SkepticalRaptor
Lifetime lover of science, especially biomedical research. Spent years in academics, business development, research, and traveling the world shilling for Big Pharma. I love sports, mostly college basketball and football, hockey, and baseball. I enjoy great food and intelligent conversation. And a delicious morning coffee!