· Precipitation that is polluted by sulphur dioxide (SO2) and nitrogen oxides (NOX) is known as acid rain.

· Acidity is measured in pH units: the lower the pH, the more acidic the substance. Unpolluted rain has a pH of roughly 5.6, but there are natural variations. Acid rain and snow, with a pH of about 4.4, has fallen in southern Norway for many years, causing serious environmental damage in many areas. The severity of the damage depends more on the total input of sulphur and nitrogen than on the acidity of each raindrop.

· More than 90 per cent of the acid rain that falls over Norway originates in other countries.

· Cuts in emissions have reduced the acidity of rain and snow recently. Total emissions of sulphur in Europe were reduced by more than 40 per cent from 1980 to 1993. In the same period, Norway's emissions were reduced by more than 70 per cent.

· As a result of higher precipitation in recent years, sulphur deposition has not dropped as much as might be expected from the reductio

· Nitrogen oxides (NOX) react to form nitric acid, another important component of acid rain, in the atmosphere. Until now, nitrogen compounds in precipitation have mainly been absorbed by plants or stored in the soil. Scientists fear that the amount of nitrogen that can be taken up in this way will be exceeded, thus intensifying acidification in some areas. Road traffic is responsible for more than half of all nitrogen emissions in Europe

Fish mortality caused by acid rain is not a new phenomenon. Fish stocks probably died out in a number of Norwegian lakes as early as the turn of the century, but it was not until the 1950s and 1960s that this phenomenon was associated with acid rain. The connection was not generally accepted by scientists until the 1970s, at about the same time as the area affected began to increase dramatically.

In recent years, it has been shown that serious damage caused by acid rain is no longer confined to the southernmost counties of Norway. There is growing evidence of damage to fish stocks in Vestfold, Buskerud, Oppland, Akershus, Hedmark and Østfold as well. Eastern Norway is becoming acidic, and thousands of lakes are in danger.

Acid rain is affecting parts of Western Norway too. In 1989, it was found that 8.5 per cent of fish stocks in Hordaland county had been lost and 29 per cent depleted. The corresponding figures for Sogn og Fjordane were 5.5 per cent and 16 per cent, respectively. Further north, acid rain does not appear to be a problem as yet, except in eastern parts of Finnmark county. The large refineries on the Kola Peninsula in Russia spew out huge amounts of sulphur dioxide. The prevailing north-easterly winds carry most of this away from Norway, but despite this, more and more signs of damage are being discovered in rivers and lakes. Scientists estimate that critical loads have been exceeded in about 70 per cent of Sør-Varanger municipality.

The southernmost counties of Norway have been hardest hit. This region has received large amounts of airb

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