DDT. Dichlorodiphenyltrichloroethane.
Paul Herman Muller is the Swiss chemist credited with the discovery of this compound. Indeed, he won a Nobel Prize in Medicine for his groundbreaking work along with other accolades. Yet today, those three letters - DDT - conjure up images of death and destruction.
How did one of the most successful synthetic pesticides transition from a miraculous ally to a destructive enemy?
Some of the story of DDT is tied up with Muller himself. As a young man, he was an average student in chemistry and physics until he discovered the joys of scientific experimentation in high school. Then his grades fell even lower as he spent all of his free time in his home laboratory neglecting his school work. Fed up with school, he dropped out at 17 and spent the first two years of World War I working for chemical companies in Basel.
Two significant events helped to shape his future. The first was a serious food shortage. Neutral Switzerland is a landlocked country and with everyone else in Europe embroiled in war, it had only its own resources to grow food. Being mountainous, with most of its arable land in pasturage, it could only grow half the grain it needed. Every kernel needed to be protected from insects. Food shortages were common.
The second crisis occurred shortly after World War I and the ensuing Bolshevik Revolution when the greatest typhus epidemic in history erupted. An estimated 25 to 30 million Russians contracted the deadly lice-borne disease. Three million died. Hundreds of thousands more perished throughout Eastern Europe.
The need to protect people and food from insects shaped the direction of chemical research in the period between the two World Wars.
In 1925, Muller received his Ph.D. in dye chemistry and began working with a chemical company, Geigy, in Basel. Unfortunately, the fumes in the dye department gave him asthma so he transferred to research. His main interests were still in the discovery of new dyes but he had a good deal of free time and he used this to explore other chemistry.
He invented a light-resistant tanning material that turned hides pure white. He developed a seed disinfectant free of poisonous mercuric compounds which helped to control smut fungus.
In the meantime, Geigy had moved into a new field of research - insecticides. Specifically, they developed a chemical that could mothproof wool. The next logical step for Geigy was to invent other kinds of insecticides, especially if insecticides that could be mass produced more cheaply than natural chemicals such as pyrethrum and nicotine.
Muller began to study the science of plant protection and pest control. After years of work, he realized that not all insects eat in the same way. That many of the serious diseases, such as typhus and malaria, are spread by insects that only suck human blood for food. He needed a contact poison.
Muller generated a list of desirable characteristics: toxic to insects but harmless to mammals, fish, birds, and plants; rapid action; no irritating odor; and inexpensive. To this list he added two more properties - it should affect many kinds of insects and it should be chemically stable allowing it to persist in the environment.
The latter two proved to be DDT's downfall.
In his search for a better insecticide, Muller had several clues to work with. The mothproofing compound was a chlorinated hydrocarbon and early experiments confirmed that a chlorinated organics had insecticidal properties. He read a 1934 article that described the preparation of diphenyltrichloroethane which was mildly poisonous to flies.
Putting all of this data together convinced Muller that a chlorinated compound should make a good insecticide. By the autumn of 1939, Muller had tested 349 compounds. Number 350 was DDT.
Muller initially tried the compound on flies and was amazed as they fell onto their backs in ten minutes. In every test he tried, on every insect species, the insects died although sometimes it took several hours. Further, Muller was astounded to discover that his experimental cage remained poisonous for weeks killing any fly that touched the walls.
In the parlance of insecticide research, DDT had a "slow knockdown" but a "sure kill" effect. Here was a compound that met with almost all of the criteria on Muller's list.
In a critical test, Muller dusted DDT on to potato plants with Colorado beetles. Almost at once, beetle larva dropped to the ground. DDT was definitely a contact poison.
DDT seemed to be a miracle. It was cheap to manufacture. It was a contact poison without any objectionable odor. It was stable in air and light. It was so powerful, only miniscule doses were required. And it dissolved so poorly in water that warm-blooded organisms absorbed only traces.
The fact that DDT dissolves readily in fats and oils didn't seem important at the time. But we will continue with the story next week.
