Your grandma was probably right: rural, farm-like air is better for your health, at least asthma-wise. The key? Microbes.

Farm air might ward off asthma, the new study reports.

In recent years, the incidence of asthma in children has been increasing. Although the causes are not exactly clear, the phenomenon has been linked with urbanization, and particularly the lack of certain microbes. The relationship between the microbial world and asthma is not well understood, but there does seem to be a connection between the two. Researchers also figured out something strange: children on farms don’t really get asthma. Now, they’re starting to understand why this is happening.

Previous research has suggested that exposure to rural house dust can reduce the incidence of asthma, although it’s not exactly clear what particular species of the microbiota contribute to this.

Researchers from Finland led by Pirkka Kirjavainen studied the indoor dust microbiota from the rural and suburban homes of 395 Finnish children. They discovered a very distinct pattern associated with farm homes — a pattern which was not present in urban areas.

“The microbial composition in farm homes was clearly distinct from that in non-farm homes,” the researchers write.

The lower incidence of asthma in rural areas is well known, but the team wanted to see if this can be traced to or compared with the microbiota. So they replicated the microbe patterns found in farms and applied it to the homes of 1,031 German children, showing a reduced risk of asthma in children living in non-farm homes where the microbiota resembled that of the Finnish farm homes. This strongly supports the idea that the microbes themselves were providing the asthma-protective effect.

“The indoor dust microbiota composition appears to be a definable, reproducible predictor of asthma risk and a potential modifiable target for asthma prevention,” the team continues.

It may seem counterintuitive that microbes have a protective effect, but by this point, it’s not exactly surprising. Although there are some substantial knowledge gaps on this issue, it’s become increasingly clear that the microbiota plays a much more important role than we thought, affecting our bodies in a number of ways. Since the dawn of our species, humans have adapted to live in rich microbial communities, and urbanization is changing that faster than the microbiota can adapt.

This isn’t the first study to find that farm air wards asthma. Future research will attempt to better define the species responsible for this phenomenon and better study how their protective effect can be replicated to reduce the asthma risk for children.