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Home » DDT effects – Paul Offit tries to set the record straight

DDT effects – Paul Offit tries to set the record straight

I don’t judge people by their looks, intelligence, bank account or fame. I only judge people by the good things they have done to save and improve lives. It’s a simple equation. Using a similar life calculator, Dr. Paul Offit, in an article in the Daily Beast, examined the legacy of Rachel Carson, and her groundbreaking book, Silent Spring. Published in the early 1960s, Carson was the first to warn that DDT effects include accumulation in the environment, and by doing so, it could bring harm to wildlife. She also warned that its overuse could make it ineffective. And finally, she said that we should use natural means for pest control, like bacteria that killed the mosquito larvae.

If you’re unfamiliar with Paul Offit, he is an inventor of a lifesaving vaccine and provider of scientific information about vaccines – he absolutely cares about human lives, despite the nastiness thrown his way. Dr. Offit’s rotavirus vaccine, which he invented, has saved millions of lives across the world. Who amongst us can make that claim, of saving so many lives?

But Dr. Offit looked at something that is generally ignored with regards to the most important of DDT effects – it killed malaria carrying mosquitoes that kills millions of lives. Today, because of DDT, there is no malaria in the USA. But it’s more than just America, Dr. Offit looks carefully at other successes of the pesticide:

As malaria rates went down, life expectancies went up; as did crop production, land values, and relative wealth. Probably no country benefited from DDT more than Nepal, where spraying began in 1960. At the time, more than two million Nepalese, mostly children, suffered from malaria. By 1968, the number was reduced to 2,500; and life expectancy increased from 28 to 42 years.

It’s hard to imagine, but Nepal had a 99% decrease in malaria infections just because of DDT. From our cozy homes in the wealthy developed world, malaria seems like some distant disease that matters not. But it wasn’t too long ago that malaria was rampant in many areas of the developed world, like Italy, the American south, Greece, and other areas. It’s not some boring disease, it kills.

And since DDT was banned, malaria has come screaming back. According to Dr. Offit, “since the mid 1970s, when DDT was eliminated from global eradication efforts, tens of millions of people have died from malaria unnecessarily: most have been children less than five years old. While it was reasonable to have banned DDT for agricultural use, it was unreasonable to have eliminated it from public health use.”

There is a claim out there that whether we chose DDT, and killed ourselves and the environment, or choose malaria with no DDT, it was all the same. But in fact, real scientific studies have since shown us that the danger from DDT was overstated, while the danger from malaria stayed the same.

It’s the 0,1 binary scale of decision making that we see by a lot of anti-science types. DDT may save lives of by preventing malaria, but any harm to the environment is bad. Either an insecticide must be 100% safe, or it’s 100% unacceptable.

Let’s go into more detail about DDT and Rachel Carson – the story is complicated.

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, mosquitoes, of typhusdengue fever and malaria, nearly eliminating malaria in Europe. DDT was spectacularly successful in eradicating most of 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.

As I mentioned above, 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.

Just to go a bit off-topic, 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 effects – 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, mentioned by Dr. Offit,  that we must do to make reasonable scientific, medical and environmental 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.”

One of Carson’s fears was that DDT was leading to higher cancer rates. But, as we know, cancer rates have barely moved in decades, even during the times that DDT was prevalent in the environment. Then again, high quality meta reviews have shown that DDT has little, mostly no, effect on cancer risks.

There are older, less well designed articles that have shown very weak links, many of those have been overturned with better epidemiological research. The powerful systematic reviews are just not showing any link.


DDT effects – 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 be highly variable. Whatever side of the DDT effects on the environment one sits, there are all kinds of scientific articles that support one side over the other. I guess one could cherry pick their favorite (and that will happen), but from my perspective, the science appears to be so conflicting, it would be difficult to either ban or un-ban DDT without some conclusive evidence.

There appears to be a huge correlation between the use of DDT and bird shell thickness, we all know that correlation doesn’t 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. Maybe it’s DDT plus climate change (don’t discount climate change as having many more effects than just rising oceans).

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.

So, if we agree that dose makes the poison to the environment, we need extraordinary and unbiased evidence to establish a “safe” level of DDT application for the environment. Or maybe keep DDT for just mosquito control. There are many choices that are better for human life (destroying malaria carrying mosquitoes) and keeping the amount of DDT safe for human life and the environment. It’s not an “either or” fallacy, it’s a complex calculation that requires real science and real analysis.



DDT effects – malaria

This is the other side of the examination of the DDT effects, the other half of the cost:benefit equation. Are there still benefits to using DDT carefully? If I could divine Dr. Offit’s response, it would be a carefully considered “yes.”

I think there are several points that must be considered with an analysis of the benefits of DDT:

  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, which has lead to the testing and release of genetically modified mosquitos to prevent reproduction.
  3. Malaria, a mosquito borne disease, still infects over 214 million people, and leads to the death of over 438,000 people worldwide. 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 the chemical 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. Moreover, 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 we save beautiful birds or we save human life. It actually can be both. There are too many cases of malaria, which, sadly, lead to too many deaths. And with the continuing destruction of the environment through uncontrolled climate change, malaria will probably not be just a “third world problem.” The mosquito vector will star moving further north and north, and the USA and Europe will have to deal with a disease that we just don’t associate with the “developed” world.

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, there is no evidence otherwise, 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. Whatever we discover will probably be a harsh chemical, and we will have to decide how to apply it to kill mosquitoes while protecting the environment. I maybe naïve, but I think that every cost:benefit equation has a point where the benefits overwhelm the costs, and you can move forward.


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.

Dr. Paul Offit got it right on how we should look at DDT effects. Yes, this story is very nuanced. In high doses, DDT can be damaging to human and environmental health. But we need to protect human lives from malaria. Unless we want to say “forget about malarial deaths, the environment should be our only concern,” then there is a point which we can eliminate (or at worse, minimize) harm to the environment while saving millions of lives.

Am I saying bring back DDT for common use? Not really, but I think that the unintended consequence of Rachel Carson’s book is millions upon millions of tragic deaths. Dr. Offit has taken a risk to step forward and tell us there is a price for eliminating DDT.


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