Changes in land use and climate change have been reported to reduce biodiversity of the environment and human microbiota. This may lead to inadequate and unbalanced stimulation of immunoregulatory circuits and ultimately, to clinical diseases, such as asthma and allergies. We summarised available empirical evidence on the role of inner and outer layers of biodiversity in the development of asthma, asthma-like symptoms, and allergic sensitization. We conducted a systematic search in SciVerse Scopus, PubMed MEDLINE, and Web of Science up to 5 December 2022 to identify relevant studies assessing the relations between inner and outer layers of biodiversity and the risk of asthma, wheezing and/or allergic sensitization. We applied random-effects models to calculate summary effect estimates. The protocol was registered in PROSPERO (CRD42022381725). Of 75 studies, 20 provided effect estimates for the meta-analysis showing an association between high outer layer biodiversity and a low risk of asthma development (Shannon diversity index: OR (95% CI) = 0.77 (0.55; 1.06); bacterial richness: OR (95% CI) = 0.74 (0.57; 0.96)). Although the evidence on the effect of inner layer biodiversity suggested that bacterial diversity was slightly higher among individuals with asthma, there was no clear evidence of a significant association between inner layer biodiversity and the risk of asthma, wheezing or allergic sensitization. The weight of evidence suggests that environmental exposure to high biodiversity may protect from the development of asthma, whereas there was no consistent evidence on any association between inner layer biodiversity and asthma, wheezing or allergic sensitization.

Background: Previous inconsistent evidence on effects of green space on the development of allergic rhinitis could be explained by the season of exposure. We explored whether the season and timing of exposure to green space play a role in the development of allergic rhinitis during the first 27 years of life. Methods: In a longitudinal study of 2568 participants from the Espoo Cohort Study, green space was assessed using the mean Normalized Difference Vegetation Index (NDVI) within 300 m of the participant’s residence during pregnancy and the first two years after birth during spring and summer seasons. We applied Cox proportional hazards regression to estimate adjusted hazard ratios (aHRs) and 95% confidence intervals (95% CI) for the associations between cumulative exposure to NDVI (1-unit increase) and allergic rhinitis. Results: Early-life exposure to abundant vegetation during the spring was associated with an increased risk of allergic rhinitis at 12 years of age [aHR (95% CI) = 1.726 (1.078; 2.765)] and 27 years of age [1.703 (1.139; 2.545)]. However, abundant vegetation during the summer was associated with a decreased risk of allergic rhinitis at 12 years of age [0.754 (0.585; 0.972)] and 27 years of age [0.801 (0.649; 0.989)]. Perinatal exposure to green spaces had no effect on allergic rhinitis. Conclusions: Green space has opposite effects on the development of allergic rhinitis in the spring and summer: early-life exposure to green spaces during the spring increases the risk of developing allergic rhinitis, whereas exposure to greenness in the summer decreases this risk.

This is a Letter and does not include an abstract.