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There are two kinds of ozone - one good, the other bad. Surprisingly, both chemically are exactly the same thing (a colorless gas made of three molecules of oxygen). What makes the difference between bad ozone and good ozone is where it's located. Remember this handy phrase: "Good up high, bad nearby."

Good ozone - found in the upper reaches of the sky, about six to 30 miles above ground - is stratospheric ozone. It's better known as the "ozone layer." Akin to a protective blanket, it helps shield us from developing cataracts and skin cancer. It absorbs most of the sun's biologically damaging ultraviolet rays (UV-B) that otherwise would bring genetic damage and harm marine life.

Ground-level ozone (the nearby stuff) is bad, the main ingredient in cough-inducing, eye-stinging smog. It can cause rubber to crack and people's lungs to scar. In cities, it mainly traces to noxious chemicals - called hydrocarbons and nitrogen oxides -- that spew from the tailpipes of cars, buses, motorboats and trucks, causing a chemical reaction that forms ozone. Some people use the term "smog" and "ozone" interchangeably. The hotter the day and the more intense the sun, the more ozone formed. That's why hot summer afternoons are worst, prompting officials to issue alerts advising people with asthma or other respiratory problems to stay indoors.

Other terms: The "hole in the ozone layer" or "ozone hole" was first publicized back in the 1980s, striking worldwide concern. Technically it's a misnomer; there's no actual hole. Instead, the ozone layer is thinning. In any case, it's bad news. Up to 60 percent of the ozone becomes depleted over the Antarctic every year during its springtime (September to November). The fluctuating size hit a record in September 2000 - covering an area three times larger than the land mass of the United States.

In a model of international cooperation, concerned countries in 1987 got together and passed the Montreal Protocol calling for countries around the world to eventually discontinue the production of ozone-layer-depleting chlorofluorocarbons, carbon tectrachloride and methyl chloroform (except for a few uses), which have been used for such things as refrigeration, air conditioning and solvents. Lots of chemical substitutes have hit the market since then. Yet, due to the bad chemicals' persistence in the atmosphere, it's predicted that many decades will pass before the "ozone hole" no longer occurs annually.

Common misimpression: Some people conflate the terms "ozone hole" with "global warming." They're NOT the same thing.

For more cool info:

* Find out whether your community today has "bad ozone" levels here: http://airnow.gov/

* Get today's ultraviolet ray index forecast for your city or town here: http://www.epa.gov/sunwise/uvindex.html

* To see the current size of the hole over the South Pole, go to: ozonewatch.gsfc.nasa.gov/

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