Ozone - O₃, is the most important natural trace ingredient in the Earth’s atmosphere. It is generally known that ozone vertical distribution in the atmosphere is not homogenic. The highest amount occurs in the height between 20 and 25 km, in the so called stratospheric ozone layer - the ozonosphere. It is also known that sequential ozone decline occurs in this layer and ozone holes are formed so the intrusion of the lethal ultraviolet radiation with wave-length of 210 - 290 nm, dangerous for the Earth life existence, is more severe. 
Ozone behaves in the opposite way in the ground layer (0 - 2 km). It is the so called tropospheric ozone. Unlike the ozonosphere, the ozone concentration increases and that has a negative impact mainly on the living organisms health. 

The reasons of ozone occurrence in the troposphere
Negative effect of increased photochemical smog concentration during ozone episodes is manifested mainly by lungs function failure, increased irritability of breathing ways, possible negative effect on immune system and deteriorating of chronic problems and symptoms. Whereas the ozone concentration below 100 µ g per m³ in the ground layer has no impact on humans health at the maximum individual effect, the concentration of 200 µ g per m³ presents slight impact on health (chest discomfort, cough, decrease in lung function by 5 - 10 %), 300 µ g per m³ has a moderate impact on health (decrease in lung function by 15 - 30 %, the eyes, nose and throat irritation) and 400 µ g per m³ has serious impact on health (decrease in lung function by 25 - 50 % and more, significantly worsened symptoms of the respiratory system).