If arsenic is unhealthy, then you'd think that the less arsenic the better. Same with radiation, mercury, dioxin and a whole host of environmental toxins.

But zero tolerance is expensive. There's no free lunch. Getting rid of the last molecule of dioxin in the environment means we'll have less money to get rid of something else that might give us more bang for the health buck.

It's not just a question of money. Think about air quality. We support government regulations on pollution, because we want clean air. But most of us don't want perfectly clean air. Most of us are willing to accept some pollution in order to have the freedom that comes with our cars. Attaining zero pollution is too expensive, not only because it hurts our pocketbook, but also because there are other things we value besides cleaner air.

Using the same logic, we ride airplanes and cars, and live in big cities. All of these choices are more dangerous than staying at home on a ranch in rural Missouri. (And it would have to be a horse-free ranch, because horses are dangerous, too.) Yet we understand that it is sometimes better to accept a little more risk in return for something of greater value.

Now along comes Edward Calabrese, a toxicologist at the University of Massachusetts, whose review of research done on toxins shows that a little bit of exposure to toxins can actually be good for your health. That's about the closest thing you can find to a free lunch. The phenomenon is called "hormesis," from the Greek work for excite. Thousands of studies confirm that at super-low levels, toxins like radiation, arsenic and mercury have beneficial effects on plants, people and other living things. (You can find out more about Calabrese's work at www.discover.com by searching on calabrese)

How can hormesis possibly be true? Poison stimulates. It mobilizes the body in complicated ways. At high levels, the body is overwhelmed. But at very low levels, the body can respond in good ways. Think of so-called healthy things like Vitamin C. Too much Vitamin C is harmful. A little television can be educational. A lot of television turns you into a couch potato. One man's meat is another man's poison. And, as it turns out, one man's poison is another man's meat.

Calabrese is not a crazy man. He doesn't advocate the ingestion of just the right amount of mercury, or hanging out near a lead smelter. His work doesn't mean we can ignore the dangers of toxic chemicals. It doesn't mean we should gut environmental regulations. The tipping point where a poison becomes harmful is hard to measure, and it will differ for every individual. Better safe than sorry remains a good working hypothesis. But Calabrese's work does challenge our obsession with creating a risk-free world. And measuring the correct relationship between good health and exposure to various chemicals may lead to important health breakthroughs in the future.

A clear example of the benefits from understanding hormesis is called alcohol. We know that excessive drinking can rot your liver and kill you. But the evidence appears overwhelming that a little bit of alcohol is good for you, particularly for men over the age of 40. A drink or two at dinner apparently reduces the risk of heart disease and stroke.

So is alcohol a good idea or a bad idea? It depends. We may long for the black and white of "Don't drink." But a lot of life's most interesting choices involve risk, ambiguity and the ability to embrace moderation.

Edward Calabrese's analysis shows the wisdom in the old proverb, "Danger and delight grow on the same stalk." It's not a bad lesson to remember. Some risks are worth taking, because the benefits outweigh the costs.

And some risks might not even be risks at all. For some of us, at least when we look at the wine list at dinner, it's a lesson that might help us live long, and live well.