An object is falling from an apartment balcony. Will it hurt you?

Time for some quick thinking. What is it, how big is it, and is it going to hit you?

Well, it's a huge concrete flower pot, and if it hits you, it will kill you. But not to worry - you're on the other side of the street!

Welcome to the art of risk assessment!

Converted to the language of scientific risk assessment, you took three steps to make your final judgment:

1. You identified a hazard.
2. You determined the size of the "dose" you would get if
   you were exposed to it.
3. You determined that you weren't going to get any dose at all;
   therefore, there was no risk to you.

There are hazards all around us. But a hazard doesn't become a risk unless you are exposed to it. And, if you are exposed, the exposure has to be at a level that might do harm; it makes a difference if it's the flower pot or just the flower that falls off the apartment balcony and hits you. Making the distinction is important to our well-being, because, as The Reporter's Environmental Handbook states, "…the risks that make people angry or frightened may or may not be the risks that endanger their health and environment." *

Take the subject of chemicals in the environment. In the 15th Century, a doctor with the name Theophrastus Phillippus Aureolus Bombastus von Hohenheim, more comfortably known as Paracelsus, theorized that it wasn't the balance of "humours" inside the body that made people sick or well, but agents outside the body that got in and caused good or harm. He learned that the same chemical could do both - a little could make a person better, but a lot could kill. His discoveries have been distilled into the toxicologist's axiom: "the dose makes the poison."

Pretty much any chemical that gets into our bodies, even the chemical called salt (sodium chloride) can become a risk if the dose we ingest is large enough. That's where science and government team up to help.

From studies of human data, or environmental data, or from laboratory studies on animals, scientists can usually calculate the point at which a hazard might become a risk. For chemicals, regulatory agencies set exposure limits, using scientific data to determine a level where no effects have been observed and then dividing by ten, a hundred, a thousand, or more, to build in a large margin of safety. Or, if safe levels can't be defined or observed, the government restricts the use of the chemical.

Last Updated: July 16, 2003