Markets 
Back away from the broom.
New research suggests that bringing more grime into your home might actually help your kids’ asthma and allergies, especially if you live in the city.
But it’s not just any dust or dirt — you’ll have to venture into the great outdoors for this one.
In the study, a team of Finnish researchers dug up soil from beneath more than two feet of snow in a forest, sifted it and froze it to preserve the tiny living organisms inside.
They then pressed about a tablespoon of the forest soil into flat-woven commercial rugs, embedding the dirt deep into the fibers.
Next, the researchers recruited six households in eastern Finland, ranging from an eighth-floor apartment to a single-family home. Each had between two and five residents, and some had pets while others didn’t.
They gave five of the homes a “forest rug” to place just inside their front door, leaving the sixth home as a control for comparison.
The team wanted to see if a rug packed with forest soil could change the home’s microbiome — or the invisible community of bacteria and fungi that fill our indoor spaces.
Because young children spend most of their time inside, the microbes they encounter at home are thought to play a key role in training their immune systems.
Previous research has shown that urban homes often host a less diverse mix of environmental microorganisms than rural ones, a pattern some studies suggest may be linked to higher rates of asthma and allergies in city kids.
The researchers figured that placing the forest-soil rugs by the front doors would let foot traffic stir up tiny particles, sending them into the air and giving the home’s microbiome a boost.
They replaced the rugs with fresh soil every four weeks for eight weeks. Dust samples were collected before the rugs were added, then every two weeks afterward, with monitoring continuing for six weeks after the final rug was placed.
The samples were taken at two heights: 12 inches, about infant breathing level, and five feet, closer to adult breathing height.
The team then analyzed the dust in a lab for bacterial and fungal DNA, tracking how the indoor microbiome responded to the forest soil.
Over the 20 week period, they observed a clear shift.
“Applying forest soil onto a rug led to a clear rise in forest soil‑associated bacteria in the air,” Martin Täubel, a microbiologist and lead author of the study, said in a press release.
“The effect was most pronounced at infant breathing height for the first two weeks after application, and the signal was also detectable in other areas of the home.”
By four weeks, the team found that bacterial signal had largely returned to baseline, suggesting the forest soil would need to be refreshed regularly to maintain a more diverse microbiome.
The biggest changes showed up in the eighth-floor apartment, home to one adult, one part-time child, no pets and fully mechanical ventilation.
There, forest bacteria at infant breathing height near the rug jumped by nearly 10 percentage points after the soil was added just one time compared to the control home.
While the study didn’t measure health outcomes directly, it did show a rise in FaRMI — a marker researchers have previously linked to lower asthma risk in children.
“It was promising to see that microbial exposure signatures associated with a lower asthma risk may be increased in urban homes with such a simple, low‑cost intervention,” said Pirkka Kirjavainen, senior author of the paper.
“The next step is to see whether this type of intervention translates into the health benefits we expect.”
Still, the findings come with some caveats. The study was small and limited to eastern Finland, meaning larger studies in other settings will be needed to confirm whether the results hold up.
But if that proves to be the case, the findings could be helpful in the US.
Asthma is the most common chronic childhood disease in the country, affecting about 5 million kids nationwide.
The consequences are wide reaching. Each year, poorly managed asthma leads to more than 10 million missed school days, 74,000 hospital stays and 767,000 trips to the emergency room, according to the National Heart, Lung and Blood Institute.
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