Ground level ozone (O3) is the primary component of smog. It forms mainly during the summertime when chemicals from fossil fuel combustion, namely nitrogen oxides, carbon monoxide, and volatile organic compounds, react in the presence of sunlight. O3 pollution originating from urban and industrial centers can be carried hundreds of miles in the atmosphere and pollute areas that are otherwise pristine. It should be noted that ground level ozone is different from stratospheric (or upper-atmosphere) ozone, and that the latter does not have direct health impacts. Information source: U.S. EPA.
Elevated concentrations of ground level ozone can damage lung tissue and cause respiratory illness. Common indicators of ozone exposure include coughing, wheezing, throat discomfort, and aggravated asthma symptoms. Information source: U.S. EPA.