WHO OFFSET PUBLICATION nr 57 197,385 Air quality in selected urban areas 1977— -1978. GEMS: Global Environmental Monitoring System. Prep, in cooperation with the WHO Colloborating Centre on Environmental Pollu­ tion Control, United Stated Environmental Protection Agency Washington, DC, USA. Geneva 1980 World Health Organization 4 s. 71 WHO Offset Publ. nr 57A