Rachel Carson, DDT, and Malaria

In an earlier post, I listed what I consider the eight most important writings in environmental ethics. I included Rachel Carson’s Silent Spring as one of the eight, but I noted that it was controversial. That controversy is the subject of this post.

Silent Spring, published in 1962, questioned the indiscriminate spraying of DDT, an insecticide, in the U.S. It questioned the logic of releasing large amounts of chemicals into the environment without understanding where these chemicals go and what their effect on human health and the environment is. The result was the rise of the environmental movement, and DDT was ultimately banned in 1972.

Since it was published in 1962, Silent Spring has received mixed reviews by scientists. In an editorial in the New York Times titled, “Fateful Voice of a Generation Still Drowns Out Real Science,” John Tierney expresses as clearly as anyone the view of scientists who do not agree with Rachel Carson. As he sees it, the human costs of banning DDT were horrific in poor countries when malaria increased after the ban. He believes the DDT ban brought about by Carson’s book substantially increased human deaths. In his view, banning DDT was reprehensible, and the science expressed in Silent Spring is bad science.

While I agree it was wrong to ban DDT (Rachel Carson also did not agree with the ban), I do not agree with the position articulated by John Tierney and others scientists. I will explain why.

DDT was used extensively during World War II to de-louse soldiers, and it was used to control mosquito populations, especially in malarial zones. It was cheap, effective, and considered safe. DDT is effective against the species of mosquito that carry malaria. How then could anyone object to using DDT to try to eradicate malaria or to the science that developed and tested it?

I will begin by listing the detrimental features of DDT when it is used to try to eradicate mosquitoes that carry malaria:

• DDT is a wide-spectrum insecticide. It kills the good insects as well as the bad. For instance, it kills virtually all species of aquatic insects that are the primary food source for fish. On land, it kills virtually all insects, such as bees, that pollinate plants. In short, it can disrupt food supplies.
• DDT and its break-down product, DDE, last for decades in soils and streams. Once applied, it can remain viable for decades.
• DDT “bio-accumulates” as it passes up the food chain. DDT sprayed at a low concentration in water is taken up by algae and passed up through zoo-plankton to fish and then to mammals and birds. DDT that is eaten is stored in fat tissues and in the milk of mammals. Birds and mammals, including man, can have concentrations of DDT in fat tissues and milk that are ten to a hundred times higher than the initial concentration sprayed. For instance, in 2005, the Center for Disease Control reported that DDT was still found in the blood of virtually all U.S. citizens, although at a lower concentration than the previous decade, even though DDT was banned in 1972. Also, the levels of DDT in salmon in the Columbia River are high enough that pregnant women are warned not to eat more than one serving per month of Columbia River salmon.
• DDT is classified as moderately toxic in acute toxicity (one-dose) tests for humans and the environment. To test for “safe” levels of DDT in water, varying concentrations of DDT are added to a series of aquaria with an equal number of fish in each. The experiment is aimed at determining the DDT concentration when fifty percent of the fish die. This is DDT’s “D50” concentration. A safety margin is added to the D50 concentration, and the resulting concentration is established as the safe concentration of DDT in water. This is the standard method for determining safe levels for any chemical in water. For humans, similar tests were conducted with rats in cages.
• DDT has been linked to diabetes in chronic (continuous low doses) toxicity tests (e.g., http://www.ehponline.org/docs/2009/0800281/abstract.html).
• DDT is less successful in eliminating the mosquitoes that carry malaria in the tropics, where they can breed year-round. Using DDT in the tropics thus leads to strains of mosquitoes that are resistant to DDT.

John Tierney and a number of scientists believe the benefits of DDT spraying outweigh these detrimental effects. I am not so sure. While I acknowledge that spraying DDT can lead to short-term declines in the mosquitoes carrying malaria, I am not sure that even the short-term benefits are worth it. Spraying DDT will cause increased reproductive problems, and high levels of DDT in mother’s milk will increase infant health problems. DDT can also potentially disrupt food supplies.

Furthermore, the long-term effects of continued DDT spraying will result in mosquitoes developing a resistance to DDT; it will no longer be effective. And that is a very serious problem. DDT should be saved in reserve for periodic serious outbreaks of malaria. It should not be used to try to eradicate malaria; not only will it  not be successful, but it will become ineffective against malarial mosquitoes when we do need to use it.

In the end, I believe that which side of this issue you are on depends largely on your philosophy of science. To the proponents of spraying, like John Tierney, science has made great strides in understanding nature. We have the necessary knowledge to control nature for the benefit of man. The scientific tests which isolate the levels of DDT from all other factors to determine the acute toxicity levels are viewed with objectivity and certainty. With this information, then, we can confidently move forward.

I am not so optimistic. I believe that nature is more complex than we can ever know. I am not so confident that we really understand the effects of DDT on humans or the environment. While I acknowledge that the acute (one-dose) toxicity tests are useful to get a general idea of the toxicity of a chemical, I find the chronic (long-term low-dose) toxicity tests less compelling. Over what range of concentrations should the tests be run and for how long? However, my biggest concern with chronic toxicity tests is that they don’t test the interaction between DDT and other pesticides. What is the interaction between DDT and other pesticides at low, long-term doses? Thousands of chemicals have been developed and used daily since World War II. How do they interact with each other? Also, what compounds do they break down into, and how do these chemicals interact with each other? We have little or no information about most of these interactions. Without this understanding, it seems to me that our knowledge is pretty limited with regard to pesticides and their effects. In short, as an ecologist, I do not believe that these simple reductionistic laboratory tests can deliver the knowledge necessary for us to confidently control nature for our benefit.