Dirt in the doormat: a new way to change your home’s microbiome
For years at HypoAir we’ve advocated for taking off your shoes at the door and using a doormat to avoid bringing dust, mold, bacteria and viruses into your home. That’s still a good thing to do. A new study, though, gave us a good reason not to shake out the doormat if you live near a wooded area or forest…it may help yours or your kids immune systems!
Did you ever hear that “a little dirt is good for you”? There is some truth to that saying. There are a number of studies that show growing up on a farm can be good for kids, and conversely, urbanization is not good for our immune systems. Here are just a few specific ones:
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A prior Finnish study showed that with increasing “built” or urbanized area around homes, diversity of bacteria in doormats decreased, the relative abundance of potentially pathogenic bacterial families and genera increased, and having pets minimally changed the results. (Urbanization Reduces Transfer of Diverse Environmental Microbiota Indoors)
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A Belgian study showed how nearby “high-growing” (vegetation greater than approximately 3 meters, or 10 feet) green space had a significant positive impact on fungal diversity, while nearby “low-growing” (less than 3 meters or 10 feet tall), had a negative impact on fungal diversity. Bacteria were positively associated with “nature” of both heights. (Residential green space can shape the indoor microbial environment)
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There’s a difference in the gut microbiome of infants raised in urban areas versus those raised in rural areas, and the urban microbiome is associated with increased risk for asthma, aeroallergen sensitization and eczema. (Urbanized microbiota in infants, immune constitution, and later risk of atopic diseases)
To counter the effects of urbanization, Finnish researchers decided to investigate a new method of “bringing the forest indoors”. They added forest soil dust to synthetic, rubber-backed rugs and placed them in the entryway of homes (five “intervention” homes received rugs with soil and 1 “control” home received a clean rug). They “seeded” the intervention rugs a total of 3 times over 22 weeks and monitored the change in indoor microbiota. Dust samples were collected from the homes every two weeks for 22 weeks, starting with baseline for 8 weeks and then during rug seedings. Settled dust samples indoors were collected from the entrance area, next to where the intervention rug was placed, and from the living room area in each home, at heights representing infant (IBZs) and adult breathing zones (ABZs), respectively. (Environmental microbiota transfer from forest soil into urban homes: a proof-of-principle study)
To be clear, they didn’t just scoop soil up from the forest and dump it onto a rug. The forest soil samples were taken in wintertime in a recreational area in the subarctic, in a forest dominated by spruce and birch. Only 15 grams (a little more than a tablespoon) was sifted onto the rugs, and the dust was “compacted” into the rug. This amount of soil dust is quite small, but it made a difference in the settled dust microbial characteristics, and therefore the air microbe characteristics, of the home.
Placing forest soil on entryway rugs caused the airborne microbial communities to look more like those found in natural environments. This is important, because the first 2 years of life, in addition to the last trimester of pregnancy, are often referred to as “the window of opportunity,” during which exposure to diverse, environmental microbes has the greatest potential to affect immune system development and human microbiome maturation in health-promoting ways. (Environmental microbiota transfer from forest soil into urban homes: a proof-of-principle study)
We already talked about the benefits of “probiotics for the air” in this article, and “forest bathing” (the practice of spending time in a forest) in this article. This new way of “seeding” the air with forest soil in the doormat seems to us to be a combination of the two! Instead of purchasing a device that sprays liquid probiotics into the air, you could try placing small particles of soil from a forested area in your doormat, or better yet, walking in forested soil and wiping your shoes on the indoor mat. An “organic” garden will work too; we note from the Belgian study that the green spaces that alter the fungal diversity the most had vegetation over 10 feet tall, meaning trees, but all heights of vegetation contributed to increased bacteria. You should, however, definitely avoid areas where pesticides have been applied recently; this is probably why the Finnish study chose to use soil from a boreal forest in the middle of winter. According to the study, doing this will naturally change your home’s biome little by little. If you have a baby or small child in your home, it could even help them resist development of allergic sensitivities and asthma.
Photo by Irina Iriser on Unsplash
