A lack of exposure to a "natural environment" could be resulting in more urban dwellers developing allergies and asthma, research has suggested.
Finnish scientists say certain bacteria, shown to be beneficial for human health, are found in greater abundance in non-urban surroundings.
The microbiota play an important role in the development and maintenance of the immune system, they add.
The findings appear in the Proceedings of the National Academy of Sciences.
"There are microbes everywhere, including in the built environment, but the composition is different between natural environments and human-built areas," explained co-author Ilkka Hanski from the University of Helsinki.
"The microbiota in natural environments is more beneficial for us," he told BBC News.
'Special function'
The team collected samples from 118 teenagers in eastern Finland, and found that those living on farms or near forests had more diverse bacteria on their skin, and also displayed lower allergen sensitivity.
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"They are important for us because they promote microbiota… that are important for the normal development and maintenance of the immune system," Dr Hanski observed.
The study also allowed the team to identify one class of bacteria, known as gammaproteobacteria, which had a "special function".
"It demonstrates that there are different functions between different microbes," he said.
One type of gammaproteobacteria , called Acinetobacter, was singled out as being "strongly linked to the development of anti-inflammatory molecules".
"Basically, our study showed that the more you had of this particular gammaproteobacteria on your skin then you had a immunological response which is known to suppress inflammatory responses ( to pollen, animals etc)."
Dr Hanski said that there was a tendency for gammaproteobacteria to be more prevalent in vegetative environment, such as forests and agricultural land rather than built-up areas and water bodies.
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We know that if you live more near green spaces, and you are from a deprived urban population, you are more likely to be healthier”
Prof Catharine Ward Thompson
OPENspace Research Centre
"Urbanisation is a relatively recent phenomenon, and for most of our time we have been interacting in an area that resembles what we now call the natural environment," he said.
"Urbanisation can be seen as a lost opportunity for many people to interact with the natural environment and its biodiversity, including the microbial communities."
While it was not possible to reverse the global trend of urbanisation, he said that there were a number of options.
"Apart from reserving natural areas outside of urban areas, I think it is important to develop city planning that includes green spaces, green belts and green infrastructure," Dr Hanski suggested.
Stressful spaces
Another recent study also illustrated a link between the lack of green spaces and higher stress levels among people living in urban areas described as deprived.
The study published in the journal Landscape and Urban Planning measured levels of cortisol, a hormone released in response to stress, found in residents' saliva.
"The stress patterns revealed by these cortisol samples were related to the amount of green space around people's homes," explained co-author Catharine Ward Thompson, director of the OPENspace Research Centre, based in Scotland.
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"We were actually surprised by the strong relationship between the two," she told BBC News.
Prof Ward Thompson said that the study provided an objective measure of stress associated by the lack of green spaces in urban areas.
"We know that if you live near more green spaces, and you are from a deprived urban population, you are more likely to be healthier," she observed.
Researchers from OPENspace have also been involved in another study that looks at the wellbeing of people over the age of 65 and their ability to get out and about.
The Inclusive Design for Getting Outdoors consortium (I'dgo) - involving scientists from the universities of Edinburgh, Heriot-Watt, Salford and Warwick - identified a direct link between the ease of getting outdoors and health and quality of life.
The study, involving 4,350 older people across the UK, found that good walkable access to local shops, services and green spaces doubled the chances of an older person achieving the minimum recommended amount of walking - 2.5 hours each week.
"One of the interesting things from my point of view is how strongly the importance of the natural environment came out in that study," said Prof Ward Thompson, who led the research.
"If you lived within 10 minutes of a park, then you were twice as likely to achieve the recommended minimum amount of physical activity."
However, she added that the study also highlighted that people needed to have confidence in reaching the park or shops before they would leave their homes.
Some of the barriers that would discourage people included uneven pavements, and a lack of seats or public toilets.
Growing interest
The studies are the latest offerings in a growing body of research that looks at the relationship between human health and access to green spaces.
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The concept of "nature deficit disorder " - a phrase coined by Richard Louv, the US author of Last Child in the Wood - has gained traction on both sides of the Atlantic.
In London, child expert Tim Gill published a report in November 2011 that looked at whether children in inner-city London were disconnected from the natural world.
While he acknowledged that "nature deficit disorder" had no clinical basis, he pointed out that his research showed that access to a natural environment formed part of a "balanced diet" in a child's development.
He added that children that had this access tended to fare better than those that did not.
More recently, the National Trust published a report that concluded that UK children were losing contact with nature at a "dramatic" rate, and their health and education were suffering as a result.
Prof Ward Thompson said there was probably an underlying reason why researchers were reaching these sorts of conclusions.
"Some of the theories behind the green space and human health suggest that our whole neuroendocrine system has evolved over millennia to respond positively to environments that are seen as providing what we need to live and thrive," she suggested.
"There is something about the natural environment that is biologically part of our system. In a way, we are hard-wired to respond to it.
"Ecosystem services - even at a local, urban level - by giving people the opportunity to mentally, as well as physically, engage with the natural environment may just be tuning our bodies back into something, biologically, we have evolved to respond positively to."
