Conclusion

The present systematic review and meta-analysis provides evidence that exposure to higher environmental biodiversity has a protective effect on the development of asthma, allergic sensitization, and wheezing. Although the evidence on the effect of inner layer (human microbiota) biodiversity suggested that bacterial diversity was slightly higher among individuals with asthma, there was no consistent evidence of an association between inner layer biodiversity and asthma, wheezing or allergic sensitization.