Can dirt and soil microbes improve immunity and mood?

It may, but it is unproven: small trials show soil microbes shift children's immune markers, while mood effects rest mostly on mouse studies.

Some scientists think losing contact with soil and plant microbes helped drive allergies and chronic inflammation¹. In small trials, children whose daycare play areas were enriched with microbe-rich soil and plants showed shifts in skin microbes and immune markers^2,3. Mouse studies hint at mood effects^4,5. But those trials measured markers, not illness, so this is early science⁶.

What you can do

Let children dig, plant and play in clean, natural green space, and have them wash hands before eating; one daycare study added forest-floor vegetation and sod to play yards⁶.
Spend time in green and natural places yourself; in Finnish and Estonian children, forest and farmland within a few kilometers of home went with less allergic sensitization⁷.

Be careful if

Soil carries risks too: 1 in 5 people worldwide have antibodies to Toxocara, a roundworm spread in dog and cat feces⁸; test garden soil for lead (see garden soil).
Toddlers who eat soil, pregnant people and the immunocompromised should not seek out dirt; ask a doctor first, since Toxocara exposure is more common at young ages⁸.
Do not use dirt, soil products or injections to treat asthma, eczema, depression or anxiety: killed Mycobacterium vaccae injections did no better than placebo in 166 children with eczema⁹.

Go deeper

How it works What it does in your body, in plain words 2 min

The biodiversity hypothesis

The idea is that contact with natural environments enriches the human microbiome and helps the immune system stay balanced, protecting against allergy and inflammatory disease. We are colonized by microbes from an outer layer (soil, water, plants, animals) that seeds an inner layer on our skin, gut and airways¹. It builds on the "old friends" idea in Too Clean? The Hygiene Hypothesis.

Clues from homes and neighborhoods

Among adolescents in one 100 by 150 km region, those with allergic sensitization lived in areas with less environmental biodiversity and had less diverse Gammaproteobacteria on their skin¹⁰. In four cohorts totaling 1,044 children and adolescents in Finland and Estonia, more forest and farmland within 2-5 km of home went with less allergic sensitization in children aged 6 and older⁷.

Daycare yard trials

In a 28-day trial in which urban daycare environments were enriched with biodiversity, children showed more diverse skin microbes, along with higher levels of a regulatory immune signal (TGF-beta1), more regulatory T cells and a higher IL-10 to IL-17A ratio, a shift toward calmer, less inflammatory signaling². In a double-blind, placebo-controlled trial of 26 children aged 3-5, 14 days of sandbox play in sand enriched with microbe-rich soil shifted skin bacteria and raised IL-10 compared with plain sand³. A two-year study found lasting changes in children's skin and gut bacteria⁶.

Mood: mostly mouse data

Heat-killed Mycobacterium vaccae, a harmless environmental bacterium, activated serotonin-producing neurons in mice and reduced a depression-like behavior⁴. Repeated immunization protected stressed mice from stress-induced colitis and anxiety-like behavior⁵. In 54 preschool children, a 10-week structured nature program, not soil specifically, changed gut bacteria and fecal serotonin and lowered perceived stress, especially anger¹¹.

What is missing

The daycare trials measured microbes and blood markers, not whether children had fewer infections or allergies, and the one trial that measured stress used a general nature program rather than soil¹¹. A 2016 review concluded that significant human research is still required¹². And in a randomized trial of injected killed M. vaccae in 166 children with eczema, the treatment did no better than placebo⁹.

Trying it safely

There is no proven dose of dirt. Gardening and outdoor play are worth doing for other reasons (see Is Gardening Good for Your Health?). Use clean soil, wash hands before eating, and keep sandpits free of dog and cat feces⁸.

The evidence The studies, what they found, and how much to trust them 4 min · 12 sources

Observational links

Hanski and colleagues analyzed atopic sensitization in a random sample of adolescents living across a 100 x 150 km region¹⁰. Atopic individuals had lower environmental biodiversity around their homes and significantly lower diversity of skin Gammaproteobacteria; in healthy but not atopic participants, IL-10 expression correlated with skin Acinetobacter¹⁰. Ruokolainen and colleagues combined four cohorts (1,044 participants aged 0.5-20) from Finland and Estonia and found forest and agricultural land within 2-5 km of home inversely and significantly associated with atopic sensitization from age 6, with land use explaining 20% of the variation in skin Proteobacteria in healthy individuals⁷. Limitations: both are cross-sectional, land cover is a crude proxy for what a child actually touches, and families differ in other ways.

Intervention trials

The first human intervention trial manipulated urban biodiversity for 28 days and compared intervention children with children in standard urban and nature-oriented daycares². Environmental and skin Gammaproteobacterial communities diversified, and this was associated with higher plasma TGF-beta1, a higher proportion of regulatory T cells and an increased IL-10 to IL-17A ratio². A later trial was described as the first double-blind, placebo-controlled test: 13 children aged 3-5 per group played in a sandbox for 20 minutes twice a day for 14 days, in sand enriched with microbially diverse soil or visually similar, microbially poor sand colored with peat³. Richness (P<0.001) and diversity (P<0.05) were higher in the enriched sand, skin communities shifted only in the intervention group (P<0.01), IL-10 and the IL-10 to IL-17A ratio changed more than with placebo (P=0.02), and the skin shift persisted at 28 days (P<0.02)³. A two-year study found that yards amended with forest-floor vegetation and sod raised environmental mycobacteria in surface soil; in year one the children's skin showed higher richness of Alpha-, Beta- and Gammaproteobacteria and lower abundance of potentially pathogenic taxa such as Haemophilus parainfluenzae, and gut Clostridium sensu stricto decreased⁶. Limitations: the placebo trial had 26 children and 14 days, outcomes were microbial and blood markers rather than illness, and the three studies come from the same research group, so independent replication is needed; the authors themselves recommend that future studies examine immune responses and disease incidence⁶.

Mouse and nature-program data

In mice, heat-killed M. vaccae given into the airway or under the skin activated a subset of serotonin neurons in the dorsal raphe nucleus, increased serotonin metabolism in the prefrontal cortex and reduced immobility in the forced-swim test, a standard model of depression-like behavior⁴. In a chronic social stress model, repeated immunization reduced submissive, flight and avoidance responses to a dominant aggressor, prevented stress-induced colitis and had anxiety-reducing effects; depleting regulatory T cells erased these benefits⁵. Limitations: injected, heat-killed preparations are not garden soil, mice are not people, and doses cannot be translated. In people, Sobko and colleagues enrolled 54 preschoolers in a 10-week nature program¹¹. Afterwards the children felt more connected to nature, their gut microbiota and fecal serotonin changed, notably Roseburia, and perceived stress fell, particularly anger; the authors say mechanistic studies are needed¹¹.

Negative and conflicting evidence

Berth-Jones and colleagues followed up an earlier small trial by randomizing 166 children aged 5-16 with moderate-to-severe atopic dermatitis to intradermal killed M. vaccae (0.1 or 1 mg) or placebo⁹. Severity scores fell by similar amounts at 12 weeks: 26% with the high dose, 25% with the low dose and 24% with placebo (P>0.05), and 32 patients had injection-site reactions⁹. Biological plausibility did not translate into clinical benefit. A review of the mental-health literature concluded that, although findings are encouraging, significant human research is required¹². Haahtela notes that Finland's allergy program (2008-2018) emphasized tolerance instead of avoidance and that allergy burden has begun to fall, but that is an uncontrolled, population-level observation¹.

Soil also carries hazards

A meta-analysis of 250 studies (265,327 participants, 71 countries) estimated the global Toxocara seroprevalence at 19.0% (95% CI 16.6-21.4)⁸. Seropositivity was associated with rural living, young age and close contact with dogs, cats or soil⁸. Antibodies show past exposure or infection, not necessarily disease, but the finding is a reminder that soil is not simply benign.

Bottom line (confidence 2 of 5)

The idea is plausible and the early trials are intriguing, but they are small, short and measure markers rather than illness; the mood evidence is mostly from mice; and one human trial of an environmental bacterium was negative. Treat gardening and outdoor play as good for many reasons, not as a proven immune therapy.

Sources

A biodiversity hypothesis. Haahtela T. Allergy, 2019. PubMed 30835837
Biodiversity intervention enhances immune regulation and health-associated commensal microbiota among daycare children. Roslund MI, Puhakka R, Grönroos M, et al.. Sci Adv, 2020. PubMed 33055153
A Placebo-controlled double-blinded test of the biodiversity hypothesis of immune-mediated diseases: Environmental microbial diversity elicits changes in cytokines and increase in T regulatory cells in young children. Roslund MI, Parajuli A, Hui N, et al.. Ecotoxicol Environ Saf, 2022. PubMed 35930838
Identification of an immune-responsive mesolimbocortical serotonergic system: potential role in regulation of emotional behavior. Lowry CA, Hollis JH, de Vries A, et al.. Neuroscience, 2007. PubMed 17367941
Immunization with a heat-killed preparation of the environmental bacterium Mycobacterium vaccae promotes stress resilience in mice. Reber SO, Siebler PH, Donner NC, et al.. Proc Natl Acad Sci U S A, 2016. PubMed 27185913
Long-term biodiversity intervention shapes health-associated commensal microbiota among urban day-care children. Roslund MI, Puhakka R, Nurminen N, et al.. Environ Int, 2021. PubMed 34403882
Green areas around homes reduce atopic sensitization in children. Ruokolainen L, von Hertzen L, Fyhrquist N, et al.. Allergy, 2015. PubMed 25388016
Seroprevalence estimates for toxocariasis in people worldwide: A systematic review and meta-analysis. Rostami A, Riahi SM, Holland CV, et al.. PLoS Negl Trop Dis, 2019. PubMed 31856156
Killed Mycobacterium vaccae suspension in children with moderate-to-severe atopic dermatitis: a randomized, double-blind, placebo-controlled trial. Berth-Jones J, Arkwright PD, Marasovic D, et al.. Clin Exp Allergy, 2006. PubMed 16961710
Environmental biodiversity, human microbiota, and allergy are interrelated. Hanski I, von Hertzen L, Fyhrquist N, et al.. Proc Natl Acad Sci U S A, 2012. PubMed 22566627
Impact of outdoor nature-related activities on gut microbiota, fecal serotonin, and perceived stress in preschool children: the Play&Grow randomized controlled trial. Sobko T, Liang S, Cheng WHG, et al.. Sci Rep, 2020. PubMed 33319792
The Microbiota, Immunoregulation, and Mental Health: Implications for Public Health. Lowry CA, Smith DG, Siebler PH, et al.. Curr Environ Health Rep, 2016. PubMed 27436048