The sun rarely shines in Linfen. It rises in the morning, of course, but its rays barely make it past the thick cloud of smog that hangs over this northern Chinese city.

At street level, residents regularly wear masks to avoid inhaling too much of the putrid air.

Linfen has the dubious distinction of being one of the world's most polluted cities, a place where taking a simple breath can make you ill. Nearby coal mines are a major culprit, but the city is also overrun with automobiles. Cars and motorcycles are the main mode of transport there, as public transit is almost nonexistent. The city averages roughly one bus for every 2,000 people - one of the lowest rates in China.

Linfen's transit woes are typical of the developing world. Urban areas from Argentina to Angola struggle with extremely poor transit service, perpetuating poverty and pollution. Without sufficient bus/train routes, some cities barely manage to sustain themselves.

"There's no way to overestimate the value of a public transit system," says Nancy Kete, director of the World Resources Institute Centre for Sustainable Transport. "Without it, people are so disconnected that the city doesn't function as a single unit."

Many cities in Africa, Asia and South America had public transit systems in the 1950s and 1960s but couldn't afford to keep them. Buses fell into disrepair, services suffered and eventually collapsed. By the 1990s, poor funding ended government-run transit in urban areas of Peru, Venezuela, Chile, and a number of other countries.