The accelerating urbanization in the world is bringing an increasing degradation in air quality (http://www.scgh.com/green-news/the-cleanest-and-the-most-polluted-cities/). While this is not reflected in life expectancy (http://en.m.wikipedia.org/wiki/List_of_countries_by_life_expectancy) yet, it is definitely causing emerging new health threats to the world population, as 70% will be living in cities by 2050. The life expectancy of humans on the Earth is likely to gradually decrease because of such threats and can very well show up in future statistics of the coming decades. What regards consumption of natural resources there are no definite answer on What is Enough? or How enough is enough? (http://www.mnn.com/leaderboard/stories/meet-the-woman-who-elevated-conservation-photography-to-a-whole-new-level). Facts on the role of urbanization on air quality, i.e. sources/types of pollution, are given in a document by United Nations Environment Program (http://www.unep.org/urban_environment/PDFs/handbook.pdf).

What is interesting and many of us may not know is that the world cleanest air is indeed mostly available in the Southern Hemi-sphere because of three reasons: (1) most of the land in the Northern Hemi-sphere is very much populated; (2) major parts of emission of atmospheric pollution is produced in the Northern Hemi-sphere; and (3) the atmospheric mixing of air between the northern and southern hemi-spheres is quite limited. The cleanest areas in the Northern Hemi-sphere are either above the troposphere, i.e. at elevated altitudes, or far away from emission areas, i.e. quite near in the Arctic region and/or quite near to the Arctic.

 http://www.travelandleisure.com/slideshows/the-worlds-cleanest-air