The majority of people now live in towns and cities and the global urban population is growing by an estimated one million people each week. The total could exceed six billion people by the end of the century. However, to do so, they face a number of health risks, such as air pollution. This environment separates them from rural areas and any contact with animals, birds and the trees which purify the air.
A spokesman told BBC News: "When we take an overview, urban areas are very important determinates of health.
"The way we live on a day-to-day basis in cities affects our health in so many ways, whether it is the air that we breath, or the fact that many people in cities around the world are very sedentary (sitting at a desk in an office or sitting in a car to get to and from work) - and many people are also eating very unhealthy foods.
"We are facing global epidemics of non-communicable diseases - heart disease, diabetes and so on - and mental health problems. We are also seeing the health impact of climate change, such as heat-stress in cities and changes in the distribution of infectious vector-borne diseases."
How many health risks are people willing to take for the benefits that come with urban lives?
pic: The world’s largest eco-city rising from wastelands in China. http://www.bbc.com/future/story/20120503-sustainable-cities-on-the-rise
But not everyone lives amongst nature. They might live in a tower block with no parks in sight. Or perhaps they're surrounded by slums. An occasional weed poking from between the hard-packed ground could be their only reminder of a natural life.
Are your surroundings pleasant?