 A systematic review of previous literature suggests that greenness may modify the impact of air pollution on health outcomes, but only a limited number of studies have been conducted and results are inconclusive. The review summarizes findings on study characteristics and evaluates evidence for effect modification by greenness based on strength and direction of evidence for each study and overall evidence of effect modification by several study characteristics. The findings suggest that areas with high greenness may have lower impacts of air pollution on health, although some studies reported inconsistent findings. This article was authored by Ji Young-sun, Haeyon Michelle Choi, Kelvin C. Fong and others.