Within the framework of the National Scientific Program " Environmental Protection and Reduction of Risks of Adverse Events and Natural Disasters" funded by the Ministry of Education and Science, the System for Chemical Weather Forecast (air pollution) was created. The System present the 72 hours forecast for surface air pollution in real-time, starting from 00:00 of the current day and extending two more days ahead for the territory of Bulgaria, with a spatial resolution of 3 km and for three selected cities - Sofia, Plovdiv, and Varna, with a resolution of 1 km each.

For each of the mentioned domeins, one of the five main pollutants can be selected - Nitrogen Dioxide (NO₂), Ozone (O₃), Sulfur Dioxide (SO₂), Fine Particulate Matter with a diameter below 10 µm (PM₁₀), Fine Particulate Matter with a diameter below 2.5 µm (PM_2.5), from which the Air Quality Index (AQI) is calculated and visualized. The system also forecasts the Universal Thermal Comfort Index (UTCI), indicating the thermal comfort/discomfort of the human body.

Ozone (O₃)

Sources

Ozone is a gas found in the upper atmosphere - 30 to 50 km above the Earth's surface and in the lower atmospheric layer. The ozone layer located high up has protective functions, expressed in shielding against ultraviolet rays, while in the lower layer, it can have an adverse effect and mostly can be found in and around big cities.
Ozone is a potent oxidant. It is not emitted directly into the atmosphere but is formed by the interaction of nitrogen oxides and volatile organic compounds under the influence of high temperatures and sunlight. There are no anthropogenic emissions into the air. Natural background ozone levels in the air are around 30 µg/m3, but they can reach much higher levels (e.g., 120 µg/m³).

Impact on Human Health

Ozone penetrates and has its toxic effects through the respiratory system. Health effects consist of inflammation of the respiratory organs, reduced lung function, accompanied by accelerated breathing. It affects the immune system and reduces resistance to respiratory diseases. Those who work outdoors and have asthma conditions are most commonly exposed to the risk effects of Ozone. It is recommended that people with increased sensitivity avoid prolonged outdoor exposure when ozone levels exceed the Maximum Permissible Concentration (MPC).
The toxic effect of Ozone is expressed in the oxidation of thiol and amino groups of enzymes, coenzymes, proteins, and peptides. It also oxidizes unsaturated fatty acids to fatty peroxides.
The toxicity of ozone depends on the level of exposure. Short-term acute effects begin with eye irritation at around 200 µg/m³ ozone, and at higher concentrations, they affect the lungs. Epidemiological studies have identified lung impairments in children exposed to concentrations of 220 µg/m³. Changes in lung function are also observed in asthmatics with exposure to 160 - 340 µg/m³.
Based on observations of the health effects of ozone, the WHO recommends a permissible one-hour concentration of 150 - 200 µg/m³, and for an eight-hour exposure, 100 - 120 µg/m³.

Legislation

Information on the impact of atmospheric pollutants on human health is coordinated with the Ministry of Health (MH) and the National Center for Public Health and Analyses under Art. 44, para. 2 of Regulation No. 12 of July 15, 2010, and Order No. RD-09-159/14.04.2003 of the MH.
Regulation No. 12 of July 15, 2010 (prom. SG, No. 58 of July 30, 2010) establishes norms for Maximum Permissible Concentrations (MPC) for nitrogen dioxide, ozone, sulfur dioxide, and fine particulate matter. The introduced MPCs aim to protect against their harmful effects on human health and the environment. The following norms are regulated for different types of pollutants:

Health Protection Threshold (8-hour Rolling Average) - 120 µg/m³;
Information Threshold (Average Hourly Value) - 180 µg/m³;
Warning Threshold (Average Hourly Value) - 240 µg/m³ (measured over three consecutive hours);
Alarm Threshold * - 360 µg/m³, determined as the average value for a period of one hour (average hourly value).
*Exceeding the alarm concentrations requires the application of measures to limit the harmful effects of the pollutant, including:

• avoiding strenuous outdoor physical activities. If necessary, it is recommended to perform them early in the morning or late in the evening, if the control authorities have warned of lower air pollution levels. Certain pollutants (Ozone) reach very high levels during midday and afternoon hours;
• teachers should cancel outdoor physical education activities such as running, soccer, basketball, or other outdoor sports with non-competitive activities or those held indoors or in classrooms;
• continue to stay indoors until the intense atmospheric pollution episode subsides by tightly closing room windows;
• avoid smoking and avoid areas where smoking is allowed;
• avoid staying in areas with intense traffic, where vehicle emissions are massive, as well as presence in workplaces with additional exposure to aerosols, dust, and other irritating substances;
• limit activities that produce irritating substances to the nose, eyes, and lungs such as cooking, cleaning, or engaging in inappropriate hobbies;
• patients with coronary heart disease should avoid outdoor physical exertion on days with alarm levels of atmospheric pollutants due to the risk of acute myocardial ischemia or myocardial infarction;
• specific therapeutic preparations for the treatment of respiratory and cardiovascular diseases related to air pollution do not exist.

https://eea.government.bg/bg/output/daily/2004-air/november/30-11.html