The health of as many as 1.6 bi l l ion people l iving in urban areas throughout the w o r l d may be at r isk from poor air quality. Without the effective implementation of pol lut ion control policies and measures this number may rise as pol lut ion emissions from motor vehicles, industry and households escalate and the population expands (due to urban migration as well as high b i r th rates in Southern countries). The composi t ion and concentrations of the cocktai l of pollutants enveloping urban areas and the severity of the health threat confronting residents v a r y between cit ies, but there are c o m m o n aspects that can be identified. Hundreds of thousands of motor vehicles used by commuters and for transporting goods stream in and out of the city, creating narrow belts of intensely polluted air w h i c h criss-cross the city. Pollutant emissions at road junctions, traffic lights and bottlenecks produce pol lut ion hot-spots. Emissions from power stations, oi l refineries, chemica l and metal process ing plants, waste inc inerators , and manufactur ing plants scattered across a c i ty or grouped into indust r ia l estates produce loca l i sed areas of toxic po l lu t ion , whi le the tal l stacks characterist ic of many industrial plants send narrow plumes of pollutants sweeping across the city. Each household is a t iny pinpoint of pol lutant emissions, but collectively these emission specks merge to blanket the city's res idential dis tr ic ts w i t h a ve i l of po l lu t ion . Emiss ions from h o u s e h o l d cookers and heaters not only pollute the urban atmosphere, but pollute the air inside people's homes as wel l .