Taken from The Guardian
According to the British newspaper, more than 95% of the world's population breathes polluted air, especially the ever-growing population in cities. Scissors between air quality in developed and developing countries are further open. Since 1990, the gap between the least and most polluted areas has increased from six times to eleven times.
The World Health Organization considers air pollution to be the biggest global environmental problem (more here) with more than six million deaths a year caused by polluted air.