Ozone is a gas occurring in the upper levels of the atmosphere - 30 - 50 km above the earth surface and in the ground air layer. The highly situated ozone layer performs protective functions - protection from the ultraviolet light, whereas at the ground layer it may have adverse effect.
Ozone is a powerful oxidizing agent. It is not directly emitted into the atmosphere. It is formed as a result of the interaction of nitrogen oxides with the volatile organic compounds under the influence of high temperatures and sunlight. There are no anthropogenous emissions in the air. The natural background values of ozone in the air are about 30 mg/m3, but much higher values may be reached (such as 120 µg/m3).

Impact on human health
Ozone penetrates and produces toxic effect through the respiratory system. The health effects consist of respiratory organ inflammation and lung function deterioration, accompanied with tachypnea. It affects the immune system and reduces the resistance to respiratory diseases. Most often those who work in the open air and have asthmatic diseases are exposed to the ozone risk effect. It is recommended that people with higher sensitivity should avoid continuous stay in the open air in the cases of ozone content above the LV.
The toxic effect of ozone is expressed in oxidation of the sulfhidrile and amino groups of enzymes, co-enzymes, proteins and peptides. It oxidizes as well the unsaturated fatty acids to fatty peroxides.
Ozone toxicity depends on the exposure level. Short-term acute effects start with eye irritation at about 200 µg/m3 ozone, and at higher concentrations the lungs are affected.
Epidemiological surveys establish lung damage as a result of child exposure to concentrations of 220 µg/m3. Disorders of the lung function are as well witnessed with asthmatic patients, when exposed to 160 - 340 µg/m3.
Based on the ozone health effect studies the WHO recommends admissible one hour concentration of 150 - 200 µg/m3, and for eight-hour exposure - 100 - 120 µg/m3.

The following limit values for ozone content in the air are set by the Bulgarian legislation with Regulation No. 4 (State Gazette, issue 64/05.07.2004): - health protection LV, defined as 8-hour average value - 110 µg/m3;
- population information threshold, defined as an average value for a period of one hour (1 hour LV) - 180 µg/m3;
- population alarm threshold defined as an average value for a period of one hour (1 hour LV) - 240 µg/m3, measured during three consecutive hours by the monitoring stations, which are representative for the air quality in a certain whole region or agglomeration.