The issue of transboundary air pollution is back in the spotlight with the return of hazy conditions in several parts of Singapore this week.
Transboundary air pollution is air pollution which originates partly or in whole from one state but adversely affects another state.
The problem first made global news in the 1960s, when Swedish and Norwegian scientists noted that increased acidity levels in large numbers of their lakes resulted in fish and other aquatic life dying. Some species were even wiped out.
Acid rain was the culprit and the source was located outside the Scandinavia region: from the United Kingdom, Ruhr Valley of then West Germany, and the heavily polluting countries of Eastern Europe.
Acid rain is produced by sulphur dioxide and nitrogen oxides in the atmosphere reacting with the water droplets in clouds. The chemical produced returns to Earth as sulphuric or nitric acid in rain.
The early solution of building taller chimneys for factories emitting industrial pollutants like sulphur dioxide and nitrogen oxides only dispersed the pollution over wider and longer distances.
Sweden then took the issue to the United Nations. In 1979, a convention on long-range transboundary air pollution was adopted in Geneva. It entered into force in 1983 and has since been ratified by 51 UN Economic Commission for Europe members.
This first international legally binding instrument to deal with transboundary air pollution has been hailed as a success.
Between 1980 and 2005, sulphur dioxide emissions declined by more than 75 per cent, nitrogen oxides by 30 per cent and volatile organic compounds by 40 per cent. Countries like Sweden and Norway no longer worry that acid rain will harm their lakes or forests.