Socioeconomic position and outdoor nitrogen dioxide (NO2) exposure in Western Europe: A multi-city analysis

Abstract

Background: Inconsistent associations between socioeconomic position (SEP) and outdoor air pollution have been reported in Europe, but methodological differences prevent any direct between-study comparison.

Objectives: Assess and compare the association between SEP and outdoor nitrogen dioxide (NO₂) exposure as a marker of traffic exhaust, in 16 cities from eight Western European countries.

Methods: Three SEP indicators, two defined at individual-level (education and occupation) and one at neighborhood-level (unemployment rate) were assessed in three European multicenter cohorts. NO₂ annual concentration exposure was estimated at participants' addresses with land use regression models developed within the European Study of Cohorts for Air Pollution Effects (ESCAPE; http://www.escapeproject.eu/). Pooled and city-specific linear regressions were used to analyze associations between each SEP indicator and NO₂. Heterogeneity across cities was assessed using the Higgins' I-squared test (I²).

Results: The study population included 5692 participants. Pooled analysis showed that participants with lower individual-SEP were less exposed to NO₂. Conversely, participants living in neighborhoods with higher unemployment rate were more exposed. City-specific results exhibited strong heterogeneity (I²>76% for the three SEP indicators) resulting in variation of the individual- and neighborhood-SEP patterns of NO₂ exposure across cities. The coefficients from a model that included both individual- and neighborhood-SEP indicators were similar to the unadjusted coefficients, suggesting independent associations.

Conclusions: Our study showed for the first time using homogenized measures of outcome and exposure across 16 cities the important heterogeneity regarding the association between SEP and NO₂ in Western Europe. Importantly, our results showed that individual- and neighborhood-SEP indicators capture different aspects of the association between SEP and exposure to air pollution, stressing the importance of considering both in air pollution health effects studies.

Keywords: Air pollution; Environmental inequality; Europe; Socioeconomic position.