In December 2015 China issued a red-alert warning for their capital city, Beijing.
Smog and air particle levels were 10 times higher than the World Health Organisation safety standards. The alert shut down schools and stopped construction, reflecting China's ongoing issues with pollution.
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At the same time the rest of the world was meeting in Paris for the United Nations Climate Change Convention in an attempt to tackle growing environmental concerns.
So, while Beijing is covered in smog, we wanted to know where the cleanest cities in the world are.
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A 2012 Green City Index examined each major world region to determine which global factors contribute to cleanliness.
In Europe many of the top spots were taken by Nordic countries and Copenhagen in Denmark was named the cleanest city overall.
Zurich in Switzerland also received high marks for having the lowest CO2 emissions.
Both countries have strict limits on high-polluting vehicles but there is also push for people to use public transport, bicycles and walking.
In South America the clear and repeat winner is Curitiba in Brazil. This city was the first to introduce bus rapid transit in 1974 and has continued the trend of working with its citizens to provide environmental benefits.
A program called 'Garbage' has led to around three-quarters of the city's rubbish being recycled.
Another top recycler, this time in Asia, is the city state of Singapore. On average residents generate less trash per person than the rest of the continent but they also keep the city clean through high density planning and infrastructure investment.
With so many people living so close together it is very important to keep waste to an absolute minimum.
Over in America it's San Francisco that takes the lead. According to the GCI the city recycles as much as 77% of its waste, which is more than any European city apart from Leipzig in Germany.
Another factor is that the city requires commercial building owners to produce regular energy reports, leading to considerably lower usage overall.
Unfortunately no cities in Africa score well above average. The highest ranking is Accra in Ghana which is not necessarily the cleanest but they get extra points for having a direct bureaucratic link with the government concerning the environment, this helps not only in monitoring but also in action.
Overall results show that consciousness of environmental friendliness seems to be based on society's willingness to take part.