Children who grow up in rural areas are less likely to develop allergies than those who grow up in urban towns and cities, according to a new study from Finland.
The possible reason: Children raised in the country are exposed to a broader array of friendly microbes, which may protect their bodies against allergies, asthma and other inflammatory diseases.
The authors of the study, which was published online this week in the Proceedings of the National Academy of Sciences (PNAS), say their findings may help explain the rising prevalence of chronic inflammatory and autoimmune diseases and may provide yet another argument to support the creation of green spaces — particularly biodiverse ones — in urban areas.
The study also appears to add support to the hygiene hypothesis — the idea that our homes and other living spaces are often kept too clean to help our children’s bodies develop a strong immune system, thus putting them at greater risk of developing allergies and asthma.
Methodology and results
The study involved 118 randomly selected Finnish teenagers who were already participating in an allergy study. Each child had lived in a single location — an urban town, a country village or a stand-alone farmhouse — all their lives. A team of researchers, led by ecologist Ilkka Hanski of the University of Helsinki, swabbed the teenagers’ skin and then analyzed the swabs for the presence of microbes. They also took blood samples from the children.
After analyzing all the data and controlling for factors known to affect children’s allergies, such as parents who smoke and family pets, the researchers found that teenagers who grew up in the most rural environments were the least likely to have allergies. They also had the most bacterial diversity on their skin, particularly of gammaproteobacteria, a type of bacteria commonly found in soil and on plants.
Some types of gammaproteobacteria have been linked to anti-inflammatory responses in the body. And, indeed, those teenagers in the study with more gammaproteobacteria on their skin also tended to have more of an anti-inflammatory molecule called interleukin-10 (IL-10) in their blood.
As a final step in the study, Hanski and his colleagues surveyed the plants surrounding each teenager’s home. Not surprisingly, they found a greater biodiversity around the rural homes. One group of plants — an uncommon flowering species native to eastern Finland, where all the children in the study lived — was 25 percent more likely to be found in the yards of healthy teenagers than in the yards of ones who had allergies, the researchers report.
This study has its limitations, of course. It involved only a small number of teenagers, for example, and its findings need to be tested in other locations. In addition, the discovery of more diverse bacteria on the skin of the rural teens might have been a reflection of a difference in washing habits between them and the urban teens (although Hanski and his co-authors think this unlikely). The study’s findings are provocative, nonetheless.
“We are proposing that contact of people, particularly children, with the natural environment and biodiversity could be really important for the development of the immune system,” Hanski told ScienceNOW.
You can read the study in full through the PNAS website.