The natural environment, plant diversity, and adult asthma: A retrospective observational study using the CDC's 500...

A wealth of evidence links microbial exposure to better human immune function. However, few studies have examined whether exposure to plant diversity is protective of immune diseases, despite the fact that plant leaves support ~10²⁶ bacterial cells. Using the Centers for Disease Control and Prevention's 500 cities project data, we found that a 1-SD increase in exposure to taxonomic plant diversity is associated with a 5.3 (95% CI: 4.2-6.4; p < 0.001) percentage-point decline in Census-tract level adult-asthma rate. In contrast, A 1-SD increase in overall greenness exposure (measured using the normalized difference vegetation index) was associated with a 3.8 (95% CI: 2.9-4.8; p < 0.001) percentage-point increase in adult-asthma rate. Interactions between air pollution and both overall greenness and plant diversity were positive, suggesting that air pollution may potentiate the allergic effects of plant pollen. Results show that the relationship between the natural environment and asthma may be more complex than previously thought, and the combination of air pollution and plant pollen may be a particular risk factor for asthma in adults.