A United Nations report has found that the number of megacities in the world has increased almost three-fold in the past 24 years. A megacity is defined as an urban area with over 10 million inhabitants—New York City formed the world’s first in the 1950s—and as of 1990, ten such cities had grown up around the globe. But as of this year, there are now a staggering 28 megacities. And while dense urban environments may provide certain environmental benefits, they also create significant hazards.
