Theme: URBAN AREAS Issue: Urban air quality
Indicator: Concentrations of sulphur dioxide and nitrogen oxides
Current trends in urbanization, private transport and industrial development in many parts of the world continue to raise concerns
about urban air quality and the impacts on human health. Although comprehensive international data on urban concentrations of
sulphur dioxide (SO2), nitrogen oxides (NOx), particulate matter (PM), ground-level ozone and toxic chemicals are not available, recent information for selected cities from the OECD indicates that levels of SO2 have decreased substantially since the early 1990s, and are remaining below guideline values set by WHO. The levels of NO2 show a more variable picture. Many cities still have concentrations above the WHO guideline, and some have been rising again in recent years (OECD 2005).