In rapidly growing cities such as Beijing and New Delhi, air pollution has become so severe that respiratory disease is on the rise, flights have been grounded, and the public is often warned against letting children play outside.

Chokingly thick blankets of smog are often a byproduct of economic growth, which results in more vehicles on the road and more burning of fossil fuels, especially coal. The bad air can be deadly: Outdoor air pollution, both in cities and rural areas, prematurely killed 3.7 million people worldwide in 2012, according to the World Health Organization. With two-thirds of the population expected to live in cities by 2050, the need to ease urban pollution is particularly acute.

The problem