An important note from Rachel Carson’s seminal book, Silent Spring, released in 1962: 
A NOTE FROM RACHEL CARSON
Malathion, another of the organic phosphates, is almost as familiar to the public as DDT, being widely used by gardeners in household insecticides, in mosquito spraying, and in such blanket attacks on insects as the spraying of nearly a million acres of Florida communities for the Mediterranean fruit fly. It is considered the least toxic of this group of chemicals and many people assume they may use it freely and without fear and harm. Commercial advertising encourages this comfortable attitude.
The alleged “safety” of malathion rests on rather precarious ground, although – as often happens – this was not discovered until the chemical had been in use for several years. Malathion is “safe” only because the mammalian liver, an organ with extraordinary protective powers, renders it relatively harmless. The detoxification is accomplished by one of the enzymes of the liver. If, however, something destroys this enzyme or interferes with its action, the person exposed to malathion receives the full force of the poison.
Unfortunately for all of us, opportunities for this sort of thing to happen are legion. A few years ago a team of Food and Drug Administration scientists discovered that when malathion and certain other organic phosphates are administered simultaneously a massive poisoning results – up to 10 times as severe as would be predicted on the basis of adding together the toxicities of the two. In other words, 1/100 of the lethal dose of each compound may be fatal when the two are combined.
This discovery led to the testing of other combinations. It is now known that many pairs of organic phosphate insecticides are highly dangerous, the toxicity being stepped up or “potentiated” through the combined action. Potentiation seems to take place when one compound destroys the liver enzyme responsible for detoxifying the other. The two need not be given simultaneously. The hazard exists not only for the man who may spray this week with one insecticide and next week with another; it exists also for the consumer of sprayed products. The common salad bowl may easily present a combination of organic phosphate insecticides. Residues well within the legally accessible limits may interact.
The full scope of the dangerous interaction of chemicals is as yet little known, but disturbing findings now come regularly from scientific laboratories. Among these is the discovery that the toxicity of an organic phosphate can be increased by a second agent that is not necessarily an insecticide. For example, one of the plasticizing agents may act even more strongly than another insecticide to make malathion more dangerous. Again, this is because it inhibits the liver enzyme that normally would “draw the teeth” of the poisonous insecticide.
What of other chemicals in the normal human environment. What, in particular, of drugs? A bare beginning has been made on this subject, but already it is known that some organic phosphates (parathion and malathion) increase the toxicity of some drugs used as muscle relaxants, and that several others (again including malathion) markedly increase the sleeping time of barbiturates.
– Rachel Carson, “Silent Spring,” 1962.
Top image: Silent Spring cover upon release
Photo of Carson and cat Moppet, 1962, the year Silent Spring was published: Alfred Eisenstaedt/Getty Image
Other pieces of interest:
Why Evolution is True from their piece on 50 Years on-Silent Spring. (2012)
The Guardian: What is the legacy of Rachel Carson’s Silent Spring? (2012)