Forest soil in doormats may reduce asthma and allergy risks, study finds

A doormat treated with forest soil can introduce beneficial microbes into urban homes, potentially lowering the risk of asthma and allergies, according to a new study by the Finnish Institute for Health and Welfare and the University of Eastern Finland.

The research, published in Microbiome, found that sprinkling forest soil onto an entryway mat increased the presence of natural bacteria in indoor air while reducing human-associated bacteria. The effect was strongest at toddler breathing height and near the mat during the first two weeks after treatment.

Six homes in Kuopio participated in the study, with five receiving soil-treated mats three times at monthly intervals. Researchers collected dust samples before and after treatment, comparing them to a control home. The amount of soil used was described as comparable to “bringing a little extra forest soil home on your shoes after a walk,” said Pirkka Kirjavainen, a lead researcher from the University of Eastern Finland.

The study suggests that urban living reduces exposure to environmental microbes, which may increase the risk of asthma, allergies, and inflammatory diseases. Early childhood exposure is particularly important, as it influences immune system development. Previous research has shown that growing up on a farm with animals can halve the risk of these conditions.

While the findings are promising, Kirjavainen advises waiting for further studies before bringing forest soil indoors. Future research will explore whether this method can effectively reduce asthma and allergy risks in children.

The Finnish Institute for Health and Welfare has previously warned about disease risks from commercial potting soils and composts, noting that forest soil contains fewer pathogens.