Associations between Greenness and Mortality by Ecoregion in the United States

Abstract

Background: Several epidemiological studies report that residential proximity to greenness is associated with positive health benefits. However, it is unclear whether such associations vary with ecology and the type of vegetation. Therefore, we investigated whether the associations between greenness and mortality vary by ecology, as categorized by EPA ecoregions. Methods: County level data from 2010 in the contiguous United States (n=2,733) were analyzed using linear regression to determine whether greenness was associated with log-transformed age-adjusted all-cause and cardiovascular mortality rates (CDC WONDER). In specific EPA level I ecoregions, greenness was estimated using the normalized difference vegetation index (NDVI) and enhanced vegetation index (EVI), which corrects for dense vegetation and distortions caused by airborne particles. Rural-urban continuum codes (RUCC) were used to categorize urbanicity. All models were adjusted for % male, % white, % smoking, median household income, population density, and fine particulate matter. Results: All-cause and cardiovascular mortality were inversely associated with EVI, and to a lesser extent, NDVI. Upon stratification by ecoregion, we observed that EVI was negatively associated with all-cause mortality (-4.5% per 0.1 EVI; 95% CI: -5.4, -3.5) and cardiovascular mortality (-4.6% per 0.1 EVI; 95% CI: -6.1, -3.0) in the Great Plains Region. In Eastern Temperate Forests, all-cause mortality was negatively associated with EVI (-1.8% per 0.1 EVI; 95% CI: -2.7, -1.0), but not NDVI (-0.1% per 0.1 NDVI; 95% CI: -1.1, 0.9). Stratifying by RUCC, we observed the strongest associations between greenness and mortality in rural counties with low populations. In cardiovascular mortality models, we observed stronger negative interactions with particulate matter and EVI than NDVI. Conclusions: The association between greenness and mortality varies by the surrounding ecology, such as the Great Plains region, an ecologically diverse region dominated by grasslands.