Quiet, clean, green, and active: A Navigation Guide systematic review of the impacts of spatially correlated urban exposures on a range of physical health outcomes

Background: Recent epidemiologic analyses have considered impacts of multiple spatially correlated urban exposures, but this literature has not been systematically evaluated. Objectives: To characterize the long-term impacts of four distinct spatially correlated urban environmental exposures traffic-related air pollution (TRAP), noise, natural spaces, and neighborhood walkability - by evaluating studies including measures of at least two such exposures in relationship to mortality, cardiovascular disease, chronic respiratory disease, allergy, type 2 diabetes, or reproductive outcomes. Methods: Following the Navigation Guide framework, the literature was searched for studies published since 2003 and meeting predefined inclusion criteria. Identified studies were scored individually for risk of bias and all studies related to an exposure-group set were appraised for overall quality and strength of evidence. Results: A total of 51 individual studies (TRAP and noise: n = 29; TRAP and natural spaces: n = 10; noise and natural spaces: n = 2; TRAP, noise, and natural spaces: n = 7; TRAP, noise, natural spaces, and walkability: n = 3) were included. When TRAP and noise were considered jointly, evidence was sufficient for increased cardiovascular morbidity with higher noise exposures; sufficient for no effect of TRAP on CVD morbidity; sufficient for increased mortality with higher TRAP exposures, but limited for noise; and limited for increased adverse reproductive outcomes with higher TRAP exposures and no effect of noise. Looking at natural spaces and TRAP, there was limited evidence for lower risk of chronic respiratory disease and small increases in birthweight with greater natural space; this relationship with birthweight persisted after adjustment for noise as well. Evidence was inadequate for all other exposure groups and outcomes. Discussion: Studies that properly account for the complexity of relationships between urban form and physical health are limited but suggest that even highly correlated exposures may have distinct effects. Review registration: PROSPERO 2018 CRD42018106050.