This indicator shows the frequency of exceeding of the hourly limit value for sulphur dioxide (SO₂) concentration (350 μg/m³), the frequency of exceeding of the 24-hour limit value for SO₂ concentration (125 μg/m³), the mean annual SO₂ concentration and exceeding of annual critical SO₂ concentration for vegetation protection (20 μg/m³) in Slovenia from 1992 onwards.

The health protection limit value is the limit concentration set on the basis of scientific knowledge. The objective of compliance with the limit value is to avoid, prevent or reduce harmful effects of SO₂ on human health and ecosystems. The limit value must be met within the set time limit determined by the legislation. Once the goal is achieved, the limit value may not be exceeded again.

The plant protection critical value is the concentration set on the basis of scientific knowledge. Its being exceeded may result in direct harmful effects on certain receptors, such as trees, other plants or natural ecosystems, but not on humans.

The main sources of SO₂ are large thermal power plants and heat production plants and, in some urban areas, small and medium-sized coal-burning boiler facilities. SO₂ is also generated in certain industrial processes (e.g. cellulose production) and in combustion of fuels containing sulphur. Due to its acidic properties, it causes the formation of acids that are deposited on waters and the ground (acid deposition). Acidification may adversely affect the water ecosystems of rivers and lakes as well as forests, crops and other vegetation. Due to the acidity of sulphur dioxide, damage to buildings and cultural monuments may also occur. Emissions of SO₂ contribute to the formation of particulate matter in the atmosphere.

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In urbanised areas in Slovenia that were heavily polluted by sulphur dioxide due to the large number of small and medium-sized boiler facilities using coal, limit values have not been exceeded since 2006. This is partially due to further expansion of district heating provided by large combined heat and power plants, a switch to gas as a heating source and the use of cleaner fuels for heating in general, and largely a result of the installation of a flue gas desulphurisation plant at the Trbovlje power plant and gradual connection of lower stacks to a flue gas desulphurisation plant at the Šoštanj power plant.

Occasionally, slightly elevated SO₂ concentrations still occur at higher altitudes within the impact areas of the Šoštanj and Trbovlje thermal power plants as a result of direct transmission of flue gases from the power plants and during periods of pronounced temperature inversion in the colder part of the year. Also, elevated SO₂ concentrations in the vicinity of the Šoštanj thermal power plant are a consequence of a limited capacity of the flue gas desulphurisation plant. Elevated SO₂ concentrations in the Zasavje region are partially a result of an unfavourable settlement pattern at the bottom of narrow valleys with low-wind conditions in wintertime when the impact of emissions from small heating units and industry is greater. The impact of SO₂ emissions from the Trbovlje power plant was greater in elevated areas of the Zasavje region during failures in the operation of the flue gas desulphurisation plant.

The critical annual SO₂ concentration for vegetation protection (20 μg/m³) that is relevant at measuring sites in rural areas was last exceeded in 2006 at the Veliki Vrh measuring site.