megacities

Sometime in the first century A.D., Rome’s population passed 1 million. It took more than 18 centuries for a city to surpass the 10 million mark, which both New York City and Tokyo did by 1950. Just six decades later, the world now has about 20 such “megacities” with populations of 10 million or more, including the largest, Tokyo, with a population of 35 million.

Megacities — cities with populations equaling or greater than 10 million people — are producing an unprecedented amount of air pollution, according to scientists at the American Chemical Society's annual meeting in Washington, D.C., on Tuesday.