This page explains what ozone is and why it is harmful.
Ozone is a gas that is formed in the atmosphere when three atoms of oxygen combine.
Ground-level Ozone One way that ozone forms low in the atmosphere is when certain substances emitted by trees and other vegetation, soil microorganisms, and lightning react together to form low, background concentrations of ozone. If ground-level ozone were produced only from natural sources of emissions, it would be of no concern. Both animal and plant life tolerate natural background concentrations of ozone. Ozone pollution is a key component of smog. It is mainly a daytime problem during the summer months.