999 must be the highest the scale goes.
Those places in China with the 999 rating MUST be beautiful places to live(SARC).
It also looks like there is a Indonesia city pegging the needle too.
Mexico City looks to be the worst in the western hemisphere followed by a few places in Chile hemmed in by the Andes Mountains.
Looks like most of the rest of meh-eek-O is a barren waste land. Or nirvana with ZERO pollution. ;-)