Spatial social polarisation: using the Index of Concentration at the Extremes jointly for income and race/ethnicity to analyse risk of hypertension

Background Growing spatial social and economic polarisation may be an important societal determinant of health, but only a few studies have used the recently developed Index of Concentration at the Extremes (ICE) to analyse the impact of joint concentrations of privilege and privation on health outcomes. We explore use of the ICE to investigate risk of hypertension in an urban, multiracial/ethnic, and predominantly working-class study population of US adults. Methods We generated novel ICE measures at the census tract level that jointly assess extreme concentrations of both income and racial/ethnic composition. We then linked the ICE measures to data from two observational, cross-sectional studies conducted in the Boston metropolitan area (2003-2004; 2008-2010; N=2145). Results The ICE measure for extreme concentrations of white compared with black residents was independently associated with lower odds of hypertension (OR=0.76; 95% CI 0.62 to 0.93), controlling for race/ethnicity, age, gender, smoking, body mass index, household income, education and self-reported exposure to racial discrimination. Even stronger associations were observed for the ICE measures that compared concentrations of high-income white residents versus low-income residents of colour (OR=0.61; 95% CI 0.40 to 0.96) and high-income white versus low-income black residents (OR=0.48; 95% CI 0.29 to 0.81). Conclusions Results suggest public health studies should explore the joint impact of racial/ethnic and economic spatial polarisation on population health.