Rachel Carson

In 2001 an international environmental treaty was signed in Stockholm to eliminate or restrict production of twelve toxic persistent organic pollutants. The delegates referred to these as the “dirty dozen.” The U.S. did not sign the treaty— even though it was Rachel Carson, an American, who first brought the world’s attention to the environmental damage of the 12 chemicals.

Rachel Carson was born in 1907 and raised in a small town outside of Pittsburgh. She was a sensitive shy girl who loved to read and write.

Carson started college as an English major. But after taking biology, she changed her major. The biology instructor, Mary Scott Skinner, became a mentor to Carson and let her know it was OK to be a sensitive student of a hard science. Carson went on to get her Masters in Biology from Johns Hopkins and joined the U.S. Bureau of Fisheries (now called the Fish and Wildlife Service) as their first female biologist in 1936.

Carson spent the ‘40’s and ‘50’s writing “feel good” articles and books about the wonders of nature, with particular focus on marine life. Her unique ability to communicate hard science in a lyrical, almost romantic prose, brought her praise and recognition in literary circles.

Then came the 1960s and Silent Spring. This was not a feel good book about nature, but an indictment of the pesticides that were harming it.

The hit list starts with DDT (dichloro diphenyl trichloroethane; structure no. 10 in the illustration). DDT is the chemical that launched the synthetic pesticide industry after WWII. The pesticide, apparently harmless to people, helped control insect-borne diseases worldwide. Countless lives were saved as DDT was used to combat malaria and typhus. The Swiss chemist, Paul Herman Muller, was awarded the Nobel prize for inventing DDT— a chemical that was clearly making the world better.

With inexpensive raw materials and a simple manufacturing process, DDT provided a low barrier of entry for companies wanting to sell into the growing synthetic pesticides market. By the 1950’s, 15 different chemical companies were making DDT in the U.S. alone. From country farms to city streets, DDT was sprayed everywhere.

What the public did not realize was that DDT was building up to dangerous levels in the environment. This buildup could occur fairly fast, as the case histories in Silent Springillustrate. In one case, the spraying of DDT to fight Dutch Elm Disease in a Midwestern city was followed a few months later by a die-off of birds all around. As Carson explains, when the leaves of DDT-treated trees fell, they were eaten by earthworms. These DDT-concentrated worms would then be eaten by birds, further concentrating DDT up the food chain.

What causes this “bio-magnification” of toxicity is the pesticide’s two key properties: it is highly fat soluble and does not break down. Once DDT is ingested—directly (through spraying) or indirectly (by eating a DDT-contaminated food)— it became part of the fat tissue and is there to stay. And depending on the cumulative concentration, it can cause hormonal disruptions or cancer.

In the case of birds, DDT also interferes with egg-shell formation. Carson’s work at the Fish and Wildlife Service gave her access to statistics that showed populations of certain birds were dropping year after year. This was particularly dramatic for birds further up the food chain, like eagles, falcons, and hawks, some of which had become endangered species.

Carson then explained how insects build immunity to pesticides, and how effective control has meant developing and using different chlorinated hydrocarbon compounds. A class of stronger pesticides were those made by the reaction of chlorine with cyclic dienes. These products include aldrin, chlordane, endrin, dieldrin, and heptachlor (structures 1-5 in the illustration).

The case of the fire ant eradication program, as narrated by Carson, is particularly illuminating. Fire ants are not native to North America. They were brought in accidentally on a ship from Argentina to Mobile, Alabama in 1918. (As such, the U.S. fire ants are also referred to as “imported fire ants.”) By the 1950’s, fire ants had spread across several southern states and were considered a serious nuisance because of the mounds they build and their nasty sting.

Making the case that the fire ant mounds were interfering with farming activities, the southern states and USDA started their eradication project in 1958. Dieldrin and heptachlor were sprayed by aircraft all over Louisiana, Alabama, Georgia, and Florida. These hard pesticides were several times more toxic than DDT. Within months of aerial spraying, populations of several fish and bird species dropped by 90%. Raccoons, dogs, cats, and livestock were also affected, many dying or becoming sick.

As for the “public enemy” itself, the number of active mounds were reduced in some areas. The fire ants, however, developed resistance and reestablished their widespread presence. Carson explained how the law of natural selection was at play here. The weaker fire ant colonies died off while the tougher ones survived, bred, and rapidly multiplied. The pesticides did not eradicate the fire ants— it toughened them.

One might think that the negative publicity brought on by Silent Spring would have stopped the USDA fire ant eradication program. That was not the case.

With dieldrin and heptachlor no longer killing the ants, a new organochlorine compound was developed: Mirex (structure 7). To reduce the collateral damage in the war on fire ants, Mirex was pelletized with ground corn cobs and coated with soybean oil. Research had shown that birds would not eat these, while ants would take them into their mounds; there, the powerful Mirex would wipe out the colony.

As it turns out, this new phase of the eradication program mainly took out a native species of ants that was fighting a low intensity war against the imported fire ants. The fire ants survived Mirex, as they had survived heptachlor and dieldrin. And with their competing ant species eliminated, fire ants flourished and spread to more states. The USDA ended the failed program in the early ‘70’s, declaring that the ant mounds were not a serious obstacle to farming activities after all.

Silent Spring became an instant bestseller and prompted a citizens movement to demand tougher pesticide regulations. At the same time, the book and its author were attacked by virtually all companies serving the $500 million (1967 dollars) pesticide market. Velsicol Chemical, the manufacturer of seven of the “dirty dozen,” was the most vocal in its attacks on Carson and threatened to sue her publisher. That never happened, and Carson died of cancer 16 months later.

Within the framework of the Stockholm convention, DDT and a few other “dirty dozen” pesticides are still used today. In fact, DDT’s effectiveness in combating malaria has increased since its indiscriminate use was stopped.

Which is just as Rachel Carson had predicted.

Ramin Abhari, P.E.

Published on February 10, 2018

Follow Ramin on twitter at @RaminAbhari