Toxic substances (toxics) are chemicals that can harm living organisms. They fall into two broad categories: synthetic and natural. Natural toxics are not to be taken lightly: Arsenic, a basic element that sometimes leaches into groundwater, can cause cancer and nervous system damage in humans.

But synthetic toxic substances are a particular problem because there are quite a lot of them, and many are persistent chemicals (meaning they don’t readily degrade over time). According to the U.S. Environmental Protection Agency (EPA), more than 80,000 chemicals are used in the United States alone. And some 1,000—2,000 new chemicals enter the consumer market each year.

The debate over how to regulate these chemicals—how to determine what quantity of any particular compound is safe for humans or the environment and then how to ensure that exposure levels stay well below those quantities—began in 1962, with a book called Silent Spring.

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In this book, legendary environmental activist Rachel Carson asked her readers to imagine a world without the sounds of spring, a world in which the birds, frogs, and crickets had all been poisoned to death by toxic chemicals. Just 20 years had passed since the widespread introduction of herbicides and pesticides (like DDT), she explained, but in that relatively short time, they had thoroughly permeated our society.

These chemicals were obviously great for killing off weeds and pests; they had done an amazing job conquering mosquito-borne diseases like malaria during World War II and combating world hunger by boosting global food production. But no one seemed terribly concerned about the effects they might have on nontarget species or on their (or our) ecosystems. After all, they were designed to kill living things. Wasn’t it at least possible that what killed one organism might also kill others?

Carson went on to identify three specific concerns that were being overlooked at the time: Some chemicals can have large effects at small doses, certain stages of human development are especially vulnerable to these effects, and mixtures of different chemicals can have unexpected impacts. The book created an uproar, which led to much stricter regulations for chemical pesticides in general, and, in the United States, a complete ban on DDT in particular. But half a century later, we are still struggling to effectively regulate the chemicals in our world.