Air connects everything
Fortunately cities can control air quality through better choices of energy and transport systems, land development, etc. Since the problems with smog of the 20th century, many cities in the developed world have done a lot to improve air quality, mainly through relocation of polluting industries and regulations of fuel, vehicles and traffic. The last two decades have seen the same measures enacted in major cities in the developing world. Now some cities are taking the next step, becoming more walkable and livable through further transformation of the transport system, regulation of the use of hazardous substances, and air quality monitoring.