Many cities around the world have been found to have pollution levels well above recommended levels, increasing the likelihood of a range of ailments for their occupants.  The World Health Organisation (WHO) recommended level is below 10 micrograms per cubic metre.  The WHO has released data revealing the cities with the worst air quality, the 15 worst of which (in micrograms per cubic metre) are:

1) Zabol Iran  217

2) Gwalior India  176

3) Allahabad India  170

4) Riyadh Saudi Arabia  156

5) Al Jubail Saudi Arabia  152

6) Patna India  149

7) Raipur India  144

8) Bamenda Cameroon  132

9) Xingtai China  128

10) Baoding China  126

11) Delhi India  122

12) Ludhiana India  122

13) Dammam Saudi Arabia  121

14) Shijiazhuang China  121

15) Khanna India  114  
As can be seen above, India has the most highly polluted cities worldwide.  Turkey claims eight of the top ten spots amongst European cities, and the most polluted cities in the US and Canada are dominated by California.  In South America, Coyhaique in Chile ranks worst where wood burning and a lack of rainfall are thought to be contributing factors.  Bamenda in Cameroon tops the pollution list for Africa, but every city in New Zealand and Australia at least matches the WHO’s recommended 10µg/m³.
There is a worrying number of cities with high levels of pollution.  However, the way we live today is not necessarily the way things need to continue to be done.  It certainly isn't the way they were done only a century ago or for the expanse of human history before that.  Key causes of pollution are increased urban power demand driving up power plant emissions and the soaring use of private motor vehicle transport.  
While great thinkers are responsible for much modern industrial invention, people continue to come up with even better ways of doing things. For instance, the rise of eco-homes and emerging architects who imagine office blocks, homes and hotels decked from top to toe in a verdant blaze of shrubbery and plant life.  One example is architect Stefano Boeri, who has plans to create entire “forest cities” in China, a country that has sadly become synonymous with environmental degradation and smog despite its long-standing historical tradition of being at one with and respecting nature.
Boeri told The Guardian:

“We have been asked to design an entire city where you don’t only have one tall building but you have 100 or 200 buildings of different sizes, all with trees and plants on the facades.  We are working very seriously on designing all the different buildings. I think they will start to build at the end of this year. By 2020 we could imagine having the first forest city in China.

It is positive because the presence of such a large number of plants, trees and shrubs is contributing to the cleaning of the air, contributing to absorbing CO2 and producing oxygen.  And what is so important is that this large presence of plants is an amazing contribution in terms of absorbing the dust produced by urban traffic.” Let us hope that great human minds continue to think of new and innovative ways to allow humans to lead healthier lives, given the number of cities around the world that currently fall short.