Greater exposure to plants, particularly of diverse and native species, may support immune health. In a study of nearly 50,000 children in New Zealand, children who were exposed to more vegetation were 6% less likely to develop asthma, and children who were exposed to greater plant diversity were 6.7% less likely to develop asthma.
Geoffrey Donovan, Demetrios Gatziolis, and Jeroen Douwes. “Vegetation diversity protects against childhood asthma: results from a large New Zealand birth cohort.” Nature Plants. 4(6): 358-364 (2018). https://doi.org/10.1038/s41477-018-0151-8