As the prevalence of asthma and allergies continues to climb in most Western societies, the causes of these chronic conditions are still not known. Though, with a collection of news stories, research pieces, and press releases coming in over the last couple weeks, the 'Hygiene Hypothesis' on allergies is only looking stronger. Of these, the most prominent were studies that examined Finnish children and another that took a look at Amish children in Indiana and the link between growing up in more ‘natural environments’ and a predisposition for allergies and asthma.
_In the Finnish study, 14-18 year olds were tested to compare how many and what kinds of bacteria, particularly gammaproteobacteria, could be found in and on their bodies. This data, when compared to the type of environment they grew up in, showed that children who had 
grown up and lived in more natural environments (read, less urban; more trees, less asphalt), showed a greater diversity of these tiny bacteria in their skin. They were also LESS likely to suffer from allergies.
_Though the second study was very general in nature, in comparing Amish farm children with Swiss farm children and Swiss children who did not live on farms, they found that children not on farms had the highest rates of asthma and allergen sensitivity. Swiss children on farms had lower rates of asthma but the same sensitivities to allergies; and Amish children on farms had the lowest rates of asthma and sensitivities to allergies.
_While neither study is a smoking gun, solidly proving the Hygiene Hypothesis, both lend at least some measure of support to the idea that the more sanitized the environment we grow up in, the more likely our immune systems are going to go haywire around harmless substances, like pollen, pet dander and dust.
As someone who grew up in a very rural part of Ohio, my siblings and I have experiences that fit this theory. Having spent much of my childhood on our family farm, playing in the woods, or chasing chickens, there are few instances outside of school where any of my sibling and I were ‘clean.’ My father literally nicknamed my one brother, ‘Dirt’. To this day, none of us three older boys have any problems with allergies or asthma, despite a family history of the latter.
_While surely it is anecdotal, my two youngest siblings both suffered from asthma. Between us three elder boys and the younger two, there is nearly a 13 year gap. My youngest brother and sister did not grow up as we had. The amount of time they spent inside as children was 
inconceivable for us older three. In the summer, it was fairly common for the doors to the house to be locked. And why not? We had eaten breakfast or lunch. We lived off a dirt road and generally saw two vehicles pass by every day. We had a spigot outside, and air conditioning isn't cheap when you have three kids running in and out all day long.
_So while the two youngest children in my family enjoyed the convenience of air conditioning, Playstation and Wii, the dirt, germs, and farm life likely served our immune systems better…. which I think is only fair considering the hundreds of rows of carrots, corn, potatoes, and peppers in our family garden that three older boys weeded throughout our childhood. To date, the youngest two siblings pulled weeds out of exactly zero rows of crops in the garden.