Ozone is a component of smog.
Ozone is one of several air pollutants subject to national ambient air quality standards, set by the EPA, which counties must meet. When the ozone standard was established, the EPA believed that only high spikes of ozone were harmful. It is in nonattainment if this reading is exceeded on more than one day. A success, it would seem at first. The distribution of peak and average ozone concentrations thus has become more compact.