UCL

Early life exposure to microbiota of the natural environment
Exposure to green space in early life appears to provide particularly large benefits. This topic is too large to review here but I give a few examples. The prevalence of essentially all psychiatric diseases is lower in adults who were exposed to green space during the first 10 years of their upbringing. The same is true for atopic sensitisation, and for inflammatory bowel diseases, and for stress responsiveness in a controlled laboratory stress test.
We know that exposure to green space changes the skin and gut microbiome. Therefore it is likely that at least some of the protective effect of exposure to green space during early life operates via the microbiome, and perhaps particularly by setting up efficient immunoregulation. Note that the conditions listed in this paragraph are mostly immunoregulatory disorders.

Hedges and pollution
Greenery can also exert another beneficial effect. Some types of hedge planted between school playgrounds and roads polluted by traffic usefully reduce pollutant levels, in addition to inevitably increasing the richness of the microbiota to which the children are exposed.

Clinical trials
Adding leaf and soil material from a forest to playgrounds built with concrete or asphalt altered the microbiota of 3-5 year old children and increased their levels of biomarkers of immunoregulation. The same group has now devised an even simpler protocol for formal clinical trials in which sandpits in asphalt/concrete playgrounds are supplemented with microbe-rich soil, or with a material of similar appearance, but without microbes. Again changes in the skin microbiota of the children are observed, and immunoregulatory biomarkers are modified. Similar experiments and clinical trials can be performed by exposing office workers to commercially available "green walls".

Conclusion
It is clear that much of the health benefit of exposure to nature in early life is mediated via exposure to microorganisms that set up immunoregulation and populate the microbiome. The result is suppression of chronic inflammatory disorders, and reduced prevalence of disorders associated with raised background inflammation. The methods outlined in the previous paragraph will allow more detailed analysis of the optimal microbial exposures though perhaps the crucial issue is biodiversity.

It is more difficult to know how much of the health benefit observed in nature-exposed adults is mediated via microbial inputs, but we suspect that it is a major mechanism, acting in concert with factors such as exercise and relaxation. Contact with soil does not need to be intimate for proximity of green space to exert significant effects on microbial exposures.