Sulphur dioxide: key to air quality and environmental health

Sulphur dioxide (SO2) is a colourless gas with a pungent odour that causes an irritating sensation similar to shortness of breath. Its origin is anthropogenic, mainly industrial emissions from the use of fossil fuels and electricity generation, but also natural phenomena such as volcanic activity, which can release significant amounts of this gas.

It is a stable gas and its release during the usual processes of combustion of fossil fuels containing sulphur (natural gas, diesel, oil and coal) results in the emission of sulphur compounds into the air. These are highly hygroscopic, which means they react readily with atmospheric moisture to form aerosolized chemical products such as sulphuric and sulphurous acid.

Sulphur dioxide acts as a precursor for the formation of ammonium sulphate, which increases the levels of PM10 and PM2.5, air pollutants with serious health effects.

SO2 can remain in the air for up to five days and travel long distances, depending on meteorological conditions. This makes SO2 one of the most important pollutants in the atmosphere. It also has a serious impact on the environment, affecting the air, water, soil and living organisms. When it enters the respiratory tract and circulatory system, it also causes serious damage to human health.

In the air, it forms sulphuric acid on contact with atmospheric moisture, the cause of acid rain. Acid rain directly affects the vegetation of terrestrial ecosystems. It acidifies soil and water and generally alters their biodiversity.

SO2 is used as an indicator of air quality. High levels of sulphur dioxide indicate the presence of pollutants that are harmful to health and the environment. It is essential to monitor SO2 levels and work towards the reduction of its emissions in order to minimise its impact on the environment and human health.

Sulphur dioxide, an ally of industry

SO2 is an essential component in the production of sulphuric acid. This is one of the most widely used chemicals in the world. In fact, it is used as a unit of measurement for the industrial development of a country. This is due to the remarkable solubility of SO2 which, once combined as an acid, becomes an excellent solvent, decolouriser, fumigant and detergent precursor.

SO2 is widely used in the manufacture of synthetic fertilisers. It is also used in the petroleum refining industry and as a precursor for other chemical products such as sulphates and acids.

Sulphur dioxide has technical applications in the food industry where it is used as a preservative. It also has antioxidant and preservative properties and is used in juices, jams, dried fruits and nuts. It is widely used in the wine industry as an inhibitor of grape enzymes that cause wine oxidation.

It is also used in the paper industry as a bleaching agent for pulp. However, emissions from sulphur dioxide-related industrial activities contribute significantly to air pollution and thus to climate change, such as the paper industry, metallurgy, thermal power plants, industrial combustion of fossil fuels or electricity generation.

Another major contributor to air pollution and thus to climate change are emissions from locomotives, ships and other vehicles that use fuels with a high sulphur content.

At a technical level, SO2 production involves the combustion of sulphur in the presence of air, releasing SO2 along with small amounts of other gases. This gas is toxic and can have adverse effects on both the environment and human health. Its levels must therefore be monitored regularly.

The importance of sulphur dioxide and its role in the environment

When SO2 mixes with water in the air, it forms sulphurous acid, a compound that contributes to the formation of acid rain. In addition, SO2 can react with other compounds in the atmosphere to form ammonium sulphates. Its presence in the atmosphere is one of the main causes of the formation of smog, or haze mixed with smoke and particulate matter, characteristic of some industrial areas and large cities.

Volcanic SO2 is a major component of emissions from active volcanoes and its study is essential to understand the dynamics and effects of these geological phenomena. The climate and air quality can be significantly affected by volcanic SO2 emitted during eruptions. It is harmful in high concentrations. However, its presence influences cloud formation and moderates global temperatures.

An illustrative example of the effects of volcanic SO2 is the case of the emissions from the Miyake volcano, studied by Shinkuro et al. Their simulations showed that SO2 emissions can have a significant and long-term impact on air quality, human health and the climate.

It is important to continue to study and monitor volcanic SO2 in order to better understand the extent of its effects. At the same time, we need to develop effective strategies to mitigate it.

On the other hand, ambient SO2, which is present in the air we breathe, can cause serious respiratory and circulatory problems. It also promotes acid rain, which damages soil, water and vegetation. It also damages the stone of buildings. It has a great potential to change the climate.

SO2 as a pollutant needs to be reduced as it is a gas that is very harmful to the quality of air, soil and water, and generally to the living conditions of all living things. Both for the environment and for people, it is advisable to avoid exposure to it by implementing early detection systems for its presence in industrial and urban environments.

Hazards and effects of sulphur dioxide

SO2, whether of volcanic or anthropogenic origin, is a toxic gas that, when present in high concentrations in the environment, alters air quality and has a significant impact on the respiratory and cardiovascular health of the population, even at great distances from the source of emissions.

Exposure to high concentrations of sulphur dioxide, even for short periods, irritates the mucous membranes of the eyes, nose and throat. When it enters the respiratory tract, it causes mucus production and coughing. It can irritate the airways and cause breathing difficulties, which in turn are the cause of chronic diseases such as bronchitis and aggravate asthmatic reactions through bronchial congestion.

At a technical level, the control and proper management of SO2 is a major concern for engineers and environmental scientists. It is necessary to implement technologies and strategies to measure and reduce emissions of this harmful compound.

Legislation and sulphur dioxide

Laws and regulations have been in place for many years to control and limit the emission of SO2 due to its harmful effects on human health and the environment, and its direct link to changes in air quality and climate change.

These regulations vary from country to country and region to region, but generally focus on the establishment of emission limits, the imposition of penalties for non-compliance, and the promotion of technologies and practices that reduce SO2 emissions.

Understanding Sulfur Dioxide Health Advisory Levels

Sulfur dioxide air quality index good condition icon (green) Good (0–0.1 ppm)
No cautionary statement.

Sulfur dioxide air quality index moderate condition icon (yellow) Moderate (0.1–0.2 ppm)
Unusually sensitive people should consider reducing prolonged or heavy exertion outdoors.

Sulfur dioxide air quality index unhealthy-for-sensitive-groups condition icon (orange) Unhealthy for Sensitive Groups (0.2–1.0 ppm)
Active children and adults, and people with lung disease, such as asthma, should reduce prolonged or heavy exertion outdoors.

Sulfur dioxide air quality index unhealthy condition icon (red) Unhealthy (1.0–3.0 ppm)
Active children and adults, and people with lung disease, such as asthma, should avoid prolonged or heavy exertion outdoors. Everyone else, especially children, should reduce prolonged or heavy exertion outdoors.

Sulfur dioxide air quality index very-unhealthy condition icon (purple) Very Unhealthy (3.0–5.0 ppm)
Active children and adults, and people with lung disease, such as asthma, should avoid all outdoor exertion. Everyone else, especially children, should avoid prolonged or heavy exertion outdoors.

Sulfur dioxide air quality index hazardous condition icon (maroon) Hazardous ( > 5.0 ppm)
Triggers health warnings of emergency conditions. Entire population is more likely to be affected. Avoid outdoor activities & remain indoors. Leave the area if directed by Civil Defense.

The World Health Organization (WHO) has established exposure limits to protect human health, with the range varying according to the duration of exposure.

Summary of the range of ambient SO2 levels:

Average time Min (ppm) Max (ppm)
10-15 min 0.102 0.175
1 hour 0.057 1
24 hours 0.019 0.153
Yearly 0.008 0.038

The average exposure times (according to WHO) vary from 10 minutes to an annual balance. The following is a summary of the range of permissible values for each average period of ambient SO2 levels:

Exposure limit (ppm) Health effects
1-5 Respiratory response threshold to exercise or deep breathing in healthy individuals
3-5 The gas is easily detectable. Fall in respiratory function at rest and resistance to airflow
5 Increased resistance in healthy individuals
6 Immediate irritation to eyes, nose and throat
10 Worsens irritation in eyes, nose and throat
10-15 Prolonged exposure toxicity threshold
20+ Paralysis or death after prolonged exposure
150 Maximum concentration that can be resisted for a few minutes by healthy individuals