What is toxicology?

Toxicology is the study of chemicals that can cause problems for living things. It’s a wide-ranging field: The chemicals may occur naturally or have been created in a lab or factory; the living things may be humans, pets, livestock, or microbes living in a pond. And the problems that concern toxicologists stretch from inconvenience to disaster—from mild skin irritation, for instance, to death.

iStock_000000169599_MediumBut the challenge at the heart of toxicology is not as simple as identifying dangerous chemicals and safe ones. There’s a saying among toxicologists that the dose makes the poison—a chemical that is perfectly safe at one dose may be lethal at another. You can’t live without water, for instance, but if you drink enough of it fast enough, it can kill you by disturbing the balance of sodium and water in your blood. (In 2007, a contestant in a radio station’s water-drinking contest, “Hold Your Wee for a Wii,” died of this imbalance, called hyponatremia.) But some chemicals are more dangerous than others, which is why a gallon of bleach has warnings on its label that you don’t see on a jug of drinking water. Toxicologists spend their time figuring out, among other things, where on the spectrum of risk a chemical lies—its toxicity—and what can be done to limit its damage.

The hazard posed by a chemical depends on a number of factors. A substance that dissolves in water may cause more problems in case of a spill than if it just sank to the bottom of a lake or river and remained there to be scooped up and carted away. That’s an obvious distinction, but other factors take more time and effort to suss out. Toxicologists want to know what exactly goes on when a human, animal, or plant is exposed to the chemical in question. They’re interested not just in whether it causes a rash or makes it hard to breathe, for example, but why: Is it fitting into a receptor on cell membranes? That can trigger a chemical reaction, or block one. Is it getting inside the cell and attaching itself to DNA? That can start a process that can lead to cancer or birth defects. Is it bonding with a protein in the blood? That can create big clumps of stuff that gum up the machinery of the body, perhaps getting stuck in the small vessels of the urinary or circulatory system, causing a blockage or shutdown.

All this means that a toxicologist must do a series of analyses in order to figure out if a chemical is hazardous. He or she might investigate:

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Phototoxicity assay (photo courtesy of the Institute for In Vitro Sciences)

  • Its physical properties. Does it dissolve in water? Is it acidic? Does it tend to react with other chemicals or just sit there?
  • How it enters an organism. If it gets on your skin, for instance, will it be absorbed? What about if you inhale it or swallow it? Does it have to be injected into the bloodstream to have an effect?
  • Its mechanism of action. In addition to looking at whether the substance binds to cell receptors or DNA, a toxicologist will also want to know what happens as it is broken down in the body. Sometimes a chemical is itself innocuous, but creates troublesome compounds when it’s metabolized.
  • The dose at which effects can be seen. Just a few molecules of some chemicals are enough to wreak havoc on living things, while a more benign substance may not cause problems until levels get high.

These and other factors help determine the hazard posed by a chemical—the kinds of damage it can cause. But that’s just half the story, since a hazardous chemical may not be worth worrying about if there’s little chance that anyone will actually experience that damage. “Risk” is the term toxicologists use to describe the real-life danger a chemical poses, which combines how inherently hazardous it is, how likely it is that someone will come into contact with it, and how vulnerable that individual or group of individuals is.

Since no chemical is completely without risk (even water!), much of what toxicologists do involves risk management. They figure out the rules and safety measures that can keep people and animals reasonably safe when a chemical is used. Often, that comes down to establishing safe doses and exposures.