Concentrations of sulphur dioxide (SO2) in the atmosphere are increasing, particularly in certain urban areas but also in regions remote from industrial emissions. It often occurs together with fine particulate matter, emission of which is also increasing. In the presence of water vapour and certain particulate matter in the atmosphere it is transformed into a fine mist of sulphuric acid.
Sulphur dioxide and its transformation products in the atmosphere may cause: haze conditions which reduce visibility; increased rate of corrosion of metals; damage to certain species of plants; and aggravate emphysema and chronic bronchitis in man. At the concentrations sometimes reached near oil refineries, the level (3 ppm) can be high enough to produce an irritating odour. At lower concentrations (0.3 ppm), and with 8-hour exposure, injury to certain species of plants can be caused. At even lower concentrations (0.12ppm) in humid conditions, corrosion rates have been shown to be 50% higher than in dry climates. Even at relatively low concentrations (0.10 ppm) there may be reduced visibility (8 kms) if the sulphur dioxide is produced over a large area causing a haze which is lightly scattered along the whole path between the object and viewer.
The main source of sulphur dioxide production is the burning of fossil fuel, particularly coal and oil. Both coal and oil vary in their sulphur content depending on the region of origin. Commonly used coal can vary from 0.5 to 6% sulphur content by weight, while crude oil varies from virtually zero to 4.5% sulphur content by weight. In the process of oil refining, most of the sulphur remains in the heavy fraction which is used in large scale heating installations. The other major sources are industrial and include smelting, sulphuric acid manufacture and petroleum refining.
Sulphur dioxide has a limited life in the atmosphere. It is oxidized within a few days, and the major process for removal is by precipitation in rain or snow. Nevertheless, the general expectation that requirements for energy will double in the next decade, means that despite the increasing use of low sulphur natural gas and nuclear energy, the use of oil and coal for combustion will increase and, hence, there will be increased pollution unless control measures are taken. Furthermore, depending on meteorological conditions, large masses of air with high concentrations may be carried over long distances (several hundred kilometres) to cause pollution conditions far from the source of the initial pollution. In this context, a particular concern is the phenomenon of acid rain which is causing widespread damage to forests, particularly in Sweden and Germany, as a result of sulphur dioxide emission in industrial areas of western Europe. This transfer of pollution across national boundaries is beyond the control of the individual countries affected.
Occupational exposure may occur in oil refineries, certain mines for sulphur or sulphur-containing ore, in smelters where sulphur-containing ore is roasted, in the paper and pulp industry, in factories manufacturing sulphuric acid, in some chemical plants where sulphur dioxide is used for organic synthesis, and in any work near chimneys or furnaces where coal, oil or other fossil fuel is burned. Sulphur dioxide acts as a powerful irritant to the mucous membranes of the eyes and the upper respiratory tract. It causes rapid, acute irritation of the eyes with tears and redness; its action on the upper respiratory tract causes cough, shortness of breath and spasm of the larynx. Acute injury by the gas is very rare.
Regarding chronic exposure to lower level pollution, there is no conclusive evidence of long-term effects on human health at the levels generally occurring in cities. Individuals respond differently – those with latent disposition to asthma and bronchial complaints and those with cardiac insufficiency, are much more susceptible to environmental stress from sulphur dioxide. Particulate sulphates and sulphites which are deposited on the tissues lining the respiratory organs also produce damage which may be more injurious than the sulphur dioxide itself.
Most countries in Europe have reduced sulphur emissions since 1980 (the 1985 protocol to reduce emissions by 30% was signed by 20 countries). For Europe as a whole there has been a 59% reduction in sulphur emissions between 1980 and 1993. Those which have increased emissions are (percentage of increase 1980-1993): Albania (31%); Greece (49); Portugal (11); Turkey (236). The formerly centrally planned economies of eastern Europe are notorious for their high energy intensities and emissions of pollutants, where by far the overwhelming majority of sulphur pollutants come from combustion of locally-produced coal. In the Czech and Slovak Republics, Hungary and Slovenia, coal use created more that 75% of sulphur dioxide emissions in 1988; in Poland 90%; and by contrast, in Austria and Italy less that 20%. Highest emissions of sulphur dioxide in 1988 (more that 100 tonnes per square kilometre) were in Northern Bohemia (Czech Republic) and Upper Silesia (Poland); by contrast they were below 2 tonnes per square kilometre in some regions of Austria. The largest emitter in the region, the power plant in Belchatow, Poland, emitted 342 kilotonnes of sulphur dioxide in 1988 – almost three times as much as all Austrian sources together (116 kt). There were at least nine other plants in central Europe (including Italy and Austria) which had emissions exceeding those of Austria in 1988. In total, the 25 largest of the 402 point sources of sulphur dioxide in the region were responsible for 30% of all sulphur dioxide emissions in 1988. Whilst sulphur emissions overall are expected to drop steadily in Europe and North America, they will double in Southeast Asia between 1990 and 2010. In 1989 the Japanese Environmental Protection Agency reported the incidence of acid rain (with pH's between 4.4 and 5.5) across the country. By the year 2020, preliminary modelling has predicted that depositions of sulphur in large areas of Asia would be many times critical loads, causing rapid degradation of the environment. Even if the best available control technology were used, it would cut emissions by only 70%, leaving them in 2020 higher than in 1990.
Some 600 million people live in urban areas where the average level of sulphur dioxide pollution endangers their lives.
In many areas of the world, atmospheric sulphur dioxide has been a major source of sulphur for vegetation, including agricultural crops. As pollution regulations restrict the amount of sulphur dioxide emitted to the atmosphere, it is becoming increasingly necessary in many countries to provide essential plant nutrient sulphur to crops by other means, usually by incorporating sulphur into fertilizers.