Not too long ago, the Indian capital of New Delhi was one of the most polluted cities in the world; now, you still might not want to run a marathon there, but the city is making serious strides toward cleaning up its air. Dilip Biswas, chair of the city’s Central Pollution Control Board, says pollution has dropped 25 percent since 1995, as levels of sulfur dioxide and particulates in the air have fallen sharply. “Now you can see the stars at night,” he says. Vehicles account for about 70 percent of the city’s pollution, while power plants kick in an additional 15 percent. At the prodding of India’s highest court, the government ordered all forms of public transport — defined as taxis, buses, and three-wheelers — to switch from diesel to compressed natural gas. So far, about 6,000 of 12,000 buses have made the change, as have thousands of the other vehicles.