Many of the substances naturally found in the environment or released by modern, industrialized society are harmful to humans and other living creatures. Common toxins include heavy metals (lead, cadmium, aluminum, mercury, and manganese), chlorine, organic chemicals (such as pesticides and herbicides), and radiation.

These substances may be found in the home, workplace, or backyard, in the food and water people eat and drink, and even in medications. Most of the chemicals attacked in Rachel Carson's 1962 book Silent Spring (Boston: Houghton Mifflin) are still used in the United States, and some that have been banned have been replaced by substances even more toxic.

Why Are Toxins Toxic?

A toxin is a substance—bacterial, viral, chemical, metal, fibrous, or radioactive—that poisons or harms a living organism. A toxin may cause immediate, acute symptoms such as gastroenteritis, or cause harm after long-term exposure...