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“The present results demonstrate that biodiversity can be surprisingly strongly associated with atopy.”

Also, those who lived on farms were much less likely to have atopy — an umbrella term for certain types of hyperallergic sensitivity, including hay fever, asthma and eczema.

The researchers also found that the teenagers' risk of atopy -- a tendency to have allergic reactions -- inched up the closer they lived to the main road.

But the drop in atopy was linked to a certain group of bacteria, not to the range of microbes.