An exploration of issues and trends affecting science, medicine and the environment.

Q: Why doesn’t ozone in auto exhaust help to patch the hole in the ozone layer?

A: It is in the wrong place to help. The ozone layer is at the top of the stratosphere and only the lightest gases, such as chlorofluorocarbons, can rise to it. Ozone, which is much heavier than oxygen and other gases, is there only because sunlight creates it from oxygen on-site. Ozone produced by cars is too heavy to rise to the ozone layer and thus provides no help.