While air quality varies dramatically from place to place and day to day, nearly the entire world — about 99% of the global population — is exposed to air at some point that doesn't meet the strict standards set by the World Health Organization. Polluted air, laden with noxious gasses or tiny, invisible particles that burrow into human bodies, kills 7 million people prematurely every year, the U.N. health agency estimates. And for the millions living in some of the world’s smoggiest cities — many of them in Asia like New Delhi; Dhaka, Bangladesh; Bangkok; and Jakarta, Indonesia — bad air might seem inescapable. Photo: Mahmud Hossain Opu/ AP