Measures taken by many industrialized countries in the 1970s to control urban and industrial air pollution (i.e. high chimney stacks) quite unintentionally sent increasing amounts of pollution across national boundaries in industrialized countries. During transport in the atmosphere, emissions of sulphur and nitrogen oxides and volatile hydrocarbons are transformed into sulphuric and nitric acids, ammonium salts and ozone. They fall to the ground, sometimes hundreds of thousands of kilometres from their origins, as dry particles or in rain, snow, frost, fog, and dew.
The UK is the biggest source of lead pollution in the North Sea and of pollution causing acid rain in Norway. 93% of sulphur deposited in Austria (mostly as acid rain) has its origin outside the country, principally arising from the Czech and Slovak Federal Republic and former East Germany.