Fact file

Sulphur dioxide (SO2) is a colourless gas that is very soluble in water and has an acidic character. With a pungent odour, it is irritating to the respiratory tract.

It is produced during the combustion of fossil elements, such as coal or petroleum. The sulphur contained in the fuel as an impurity oxidises to form sulphur dioxide (SO2) and, to a lesser extent, sulphur trioxide (SO3).


Origin of pollution

Sulphur dioxide emissions are closely linked to the use of fuels containing sulphur, but there are also other sources, such as the production of sulphuric acid or the metallurgy of metals whose ore comes in the form of sulphide. Volcanism can also be a significant natural source of sulphur dioxide.

In Wallonia, most of these emissions (61% in 2014) come from the industrial sector. Then comes the residential sector, with 25% of emissions, and the energy sector with 7%. The share of the transport sector (1.6%) is low compared to other pollutants, such as nitrogen oxides, for example.

Sulphur dioxide emissions are constantly decreasing: -91% between 1990 and 2014. This improvement is due to several factors: gradual abandonment of fuels rich in sulphur (coal, fuel oil) in favour of cleaner fuels (natural gas); limitation of the sulphur content of fuels; large share of nuclear in energy production; more rational use of energy; development of waste treatment; and, also, it must be admitted, the decline of heavy industry in the Walloon industrial landscape.


Health effects

Sulphur dioxide is irritating to mucous membranes and eyes. It causes respiratory discomfort in sensitive individuals (people with asthma or suffering from chronic respiratory diseases, etc.) which can lead to an increase in hospital admissions. In high doses, it is responsible for bronchoconstriction, bronchitis and tracheitis and, in the long term, chronic bronchitis. In association with high concentrations of particles, we can even observe an increase in mortality, as was the case for episodes in Engis in 1930 and in London in 1952.

In the atmosphere, sulphur dioxide is liable to transform into sulphates and thus indirectly participates in particulate pollution.


Environmental impact

Sulphur dioxide can alter plant growth and disturb ecosystems. In the atmosphere, it is transformed into sulphuric acid, which is deposited on the ground and on vegetation and thus contributes to the acidification and exhaustion of natural environments. Thus, in the past, it could be considered the main cause of the phenomenon of acid rain and of forest decline. Finally, it also contributes to the deterioration of materials used in construction and, in particular, it accelerates the degradation of buildings that are part of our historical heritage (limestone is particularly sensitive).


The situation in Wallonia

Following the sharp decrease in emissions, we can now safely say that, in our regions, sulphur dioxide pollution is a problem under control and that the European standards in force, as well as the recommendations of the World Health Organization, are widely respected.


Measurement methods

Sulphur dioxide is measured continuously and in real time using monitors using the principle of UV fluorescence.

Find out more :