You've heard all about the hole in the ozone layer, the one that was eaten away by the chemicals in spray cans, the one makes getting a sun tan about as safe as walking your pet alligator.

So why is it that on hot summer days, broadcasters on Maine's TV and radio stations sometimes tell you to limit your outdoor activity because ozone levels are high? Isn't lots of ozone a good thing?

The answer to this mystery lies in the simple phrase "ozone: good up high, bad nearby."

Here's the scoop. Ozone is a molecule made up of three oxygen atoms. High above the earth's surface, there is a thin layer of naturally-occurring ozone, the "ozone layer," which makes life possible on earth. It acts as a filter, letting in light, while shielding the planet from much of the sun's harmful radiation.

But ozone can also be found right here, at the earth's surface. Although it's the same substance that is found in the ozone layer, ozone's effect at ground level is quite different. This ozone, the main ingredient in smog, makes it hard for people to breathe.

Ozone forms at the earth's surface when pollutants from cars, factories, power plants, and some household and industrial chemicals are "cooked" by the sun. This is why air quality tends to be at its worst on hot summer days.

But here in Maine, is air pollution really a problem?

Each summer, there are a number of days during which ozone levels in some areas of the state are unhealthy, especially for people with respiratory problems like asthma. Last summer, Maine saw eight of these "bad air days."

Maine isn't really responsible for this pollution, is it?

Maine is in fact on the receiving end of pollution from states to our south and west. However, pollution created in Maine does contribute significantly to our ozone problem. An analysis of last year's ozone episodes shows that the pollution that caused some of the unhealthy air came largely from Maine sources.

Maine has asked the federal government to force other states to reduce their air pollution, and the EPA is taking action to do just that. Mainers can help by tuning up their cars, carpooling, and combining car trips.

The battle against air pollution rages on. So, next time you hear your local weather forecaster say ozone levels will be good, you'll know you can breathe a little easier. But don't forget the sun screen.

Judy Landers