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Question

During the summer I am always hearing about ozone warnings in my city. This ozone is "bad", but then you hear about the ozone layer and it is "good". How can ozone be both good and bad?

Answer

The thing that makes ozone both good and bad is its location. Ozone is "good" when it is in the stratosphere. The stratosphere is a layer of the atmosphere starting at the level of about 6 miles (about 10 kilometers) above sea level. The stratosphere naturally contains about 6 parts per million of ozone, and this ozone is very beneficial because it absorbs UV radiation and prevents it from reaching us (see How sun tans and sunburns work for a description of the damage that UV radiation can cause to skin). See this page for a good description of the chemical reaction that allows ozone to absorb UV light.

At ground level, ozone is "bad". Ozone is a very reactive gas that is hard on lung tissue. It also damages plants and buildings. Any ozone at ground level is a problem. Unfortunately, chemicals in car exhaust and chemicals produced by some industries react with light to produce lots of ozone at ground level. In cities the ozone level can rise to a point where it becomes hazardous to human health. That's when you hear about it as an ozone warning on the news.

Here are several interesting links to help you learn more about ozone:

• Ozone
• Ozone warning issued
• The Ozone Layer
• Measuring the Pollution in Our Air
• The new ozone standard
• Ozone myths
• Code red ozone warning - see also this page for descriptions of other warnings.
• The ozone hole over Antartica - a great article explaining why the ozone layer has been completely destroyed over Antartica
• Scientists Receive Nobel Prize for Ozone Research
• Ozone: Life-Threatening Pollutant or Powerful Healing Agent?
• Clean Air Action

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