What is – Ozone is a highly reactive gas, of pungent odor and to high concentrations of blue color, equipped with a high oxidizing power. Ozone is concentrated in the stratosphere at a height between the 30 and the 50 kilometers above the ground, its presence protects the Earth's surface from ultraviolet radiation from the sun that would be harmful to the life of living beings. The absence of this compound in the stratosphere is generally called "ozone hole". Ozone in the immediate area of the Earth's surface is instead a component of "photochemical smog" that originates mainly in the summer months in conjunction with an intense solar radiation and high temperature. Ozone has no direct sources, but is formed within a cycle of photochemical reactions involving in particular nitrogen oxides and VOCs.

Method of measurement - Ozone is measured by a method based on the absorption characteristic, the part of the molecules of Ozone, of ultraviolet radiation (UV) at a wavelength of 254 nm. The variation of the light intensity is directly related to the concentration of ozone and is measured by a detector. The unit of measure by which they are measured concentrations of ozone is the microgram per cubic meter (µg/m3).

Damage caused - Relatively low concentrations of ozone causing effects such as irritation of the throat and respiratory tract and burning eyes; higher concentrations can lead alterations in respiratory function and increase the frequency of asthma attacks. Ozone is also responsible for damage to vegetation on the crops, with the disappearance of some tree species from urban areas.

Evolution - Over the past decade the concentration of ozone has remained fairly constant ; this trend is mainly due to the stability of the concentrations of oxides of nitrogen present in the atmosphere that have not shown significant decreases. The oscillations of concentrations of ozone are therefore related to the variability of weather conditions.