<human>: I've been hearing a lot about the "ozone hole." What's that?
<bot>: The ozone hole is a region of the upper atmosphere where the concentration of ozone is lower than normal. This is due to human activities, such as the use of chlorofluorocarbons (CFCs) in air conditioning and refrigeration systems. These chemicals break down in the atmosphere and release chlorine atoms, which react with ozone molecules to form chlorine compounds called "free radicals". The free radicals are then able to travel into the stratosphere, where they destroy ozone molecules.