Ozone is a trivalent oxygen molecule (O3), which occurs both in the stratosphere, about 20-30 km above the ground (the so-called ozone layer) and in the lower troposphere, just below its surface. Stratospheric ozone protects us from harmful UV-C radiation, while tropospheric ozone is harmful to humans and vegetation.

Because ozone is a strong oxidant, getting into the respiratory tract of a person causes irritation and discomfort in breathing. Short-term exposure to high concentrations may cause inflammation of the respiratory tract. Prolonged or repeated exposure may aggravate symptoms. Exposure to ozone is particularly dangerous for people with respiratory disorders and for the ones with reduced body immunity, especially older people and children.

Ozone is a secondary pollution, which means that it is not emitted into the atmosphere, but is formed in the air by the reaction of nitrogen oxides (NO and NO2)and volatile organic compounds (VOCs) under the influence of the sun. These compounds are called precursors of ozone. Nitric oxide (NO) and nitrogen dioxide (NO2)are mainly emitted by wheel transport and industry, while the anthropogenic emission of volatile organic compounds is also associated with industrial processes. High concentrations of these substances allow the formation of ozone, so its highest concentrations are observed in urban and suburban areas.

Due to the nature of the ozone formation reaction, high concentrations occur in sunny and warm days with poor winds, with the highest concentrations in the afternoon. High concentrations of ozone in Poland occur in the period from March to September, most often in the high tide. Exceeding the permissible level of 120 μg / m3 8-hour rolling average) and public information (180 μg /m3,1 hour average) are most commonly observed in July and August. Due to the strong dependence on weather conditions, the frequency of high concentrations of ozone in the following years may vary considerably. Below is an example of a concentration chart for a city station (Wroclaw, Korzeniowskiego Street) in August 2015, when the public information threshold was exceeded.

Fig. Average 1 hour ozone concentrations at Wroclaw - ul. Korzeniowski on 7-8.08.2015. The red line represents the level of public information. (source: WIOŚ)

Elaborated by Kinga Wałaszek