This would help in the development of effective public health policy interventions (Box 3.1). An ADB study compared actual and recommended numbers of air quality monitoring stations in Asian cities based on population and pollution level, as implied by an EU directive though terrain and other factors also affect the number of sites needed (ADB, 2014). Generally, cities in developed Asian economies had enough monitoring sites for their population; Singapore, for example, has 15, while 13 are recommended for a city of its size (Table 3.1).