Our atmosphere is made up of many gases, but one in particular - ozone - has to be the most misunderstood of all. Sometimes ozone is hailed as a good thing, necessary for life on earth. At other times it's portrayed negatively as a pollutant that we could do without.

There's actually some truth to both identities - it just depends on where the ozone is. Most is 10 to 30 miles up, in the layer called the stratosphere. There, ozone absorbs some of the sun's harmful ultraviolet radiation. This radiation can increase the incidence of skin cancer, among other harmful effects. So acting in this role, ozone is clearly something we want in the atmosphere. But ozone near the ground is the primary ingredient in smog. Down here, ozone is a pollutant that's unhealthy to breathe and harmful to plants and some materials. Here's a catchy way to remember ozone's double identity: "Ozone is good up high, but bad nearby."

Ozone smog is at its worst in summer, so this is a good week to focus on ozone as a pollutant. And that's what weíll do the next few days.