Does childhood exposure to biodiverse greenspace reduce the risk of developing Asthma?

The prevalence of inflammatory diseases is increasing in populations throughout the industrialized world. An increasing proportion of human populations grow up and live in urban areas, probably with reduced exposure to diverse soil biotas and other biodiversity. Decreased exposure to microorganisms from natural environments, in particular in early childhood, has been hypothesized to hamper development of the human immune system and lead to increasing risks of inflammatory diseases, such as asthma.We investigated 40,249 Danish individuals born 1995-2015, spending their first two years of life in one of six contrasted municipalities (urban vs rural and low vs high Natural Capital Index). Percentage greenspace was assessed in a 2 km buffer around home addresses of individuals’. The Danish Biodiversity Map, charting occurrence density of red-listed animals, plants and macrofungi, was used as a proxy for multi-taxon biodiversity, including soil biodiversity.We found no evidence of decreasing risk of developing asthma with higher levels of biodiversity. In contrast, greenspace exposure was associated with higher risk of asthma. Exposure to farmland was also associated with elevated risk of developing asthma, even at relatively low agricultural landcover. In the subset of children growing up in highly urbanized settings, we found high exposures to urban greenspace to be associated with reduced risk of developing asthma.Our results lend no support to the hypothesis that early childhood exposure to biodiverse environments reduces the risk of acquiring inflammatory diseases later in life. However, access to urban greenspace, such as parks, which typically harbour low levels of biodiversity, seems to reduce asthma risk, potentially through exposure to common soil microbiota. Our results suggest that biodiversity conservation must be motivated with other arguments than human health benefits.