Ground-level ozone, also known as surface-level ozone and tropospheric ozone, is a trace gas in the troposphere, with an average concentration of 20–30 parts per billion by volume, with close to 100 ppbv in polluted areas. Wikipedia

Tropospheric ozone is a secondary pollutant formed from emissions of nitrogen oxides, non-methane volatile organic compounds and carbon monoxide. Ozone scars lung tissue, makes eyes sting and throats itch. It has been implicated as a contributor to forest dieback, damage to agricultural crops, etc. OmegaWiki