Millions of people need to move around this crowded and sprawling metropolis of 11 million each day, and many do so in an auto-rickshaw that can dart around larger or slower forms of transport

Auto-rickshaws are cheap, convenient, and in use all over Asia. Up until recently, the ones here in Bangladesh all were powered by two-stroke gasoline engines; (also found in many motorcycles and larger three-wheelers), which alone were responsible for 60 percent of all vehicle-related pollution here

two-stroke engines don’t use gasoline efficiently. Up to 40 percent of the fuel emerges from the tailpipe unburned, as smoke and soot. Burning gasoline and diesel produces carbon dioxide, carbon monoxide, sulfur dioxide, nitrogen oxides, hydrocarbons, and soot that warm the climate, threaten human health, and pollute water, air, and soil

Natural gas is 95 percent methane and releases significantly fewer tailpipe emissions than does gasoline or diesel fuel