Socioeconomic inequalities in the urban external exposome: A population-based study from Bulgaria

The urban external exposome comprises multiple co-occurring environmental exposures that collectively shape population health. These exposures often cluster geographically and socially, potentially leading to environmental disadvantages across socioeconomic groups. However, most environmental justice studies remain single-exposure focused, providing limited insight into exposome-driven socioeconomic inequalities.

We assessed (1) the associations between high-risk external exposome profiles and individuals' socioeconomic status (SES) in urban Bulgaria and (2) compared results between two external exposome operationalizations.

We analyzed survey data from 4,640 adults across the five largest Bulgarian cities and estimated residential exposures to nitrogen dioxide, particulate matter, traffic noise, green space availability, distance to blue space, impervious surfaces, walkability, and nighttime light. We conducted city-specific latent profile analyses and constructed composite indices to identify high-risk external exposome profiles, followed by city-specific regressions linking these profiles to individual- and area-level SES indicators. Average marginal SES effects were compared across the high-risk exposome profile operationalizations, and city-specific results were pooled with random-effects regressions.

Latent profile analyses yielded two or three external exposome profiles per city. High-risk profiles combined multiple disadvantages, including low green space availability, high air pollution, noise, impervious surfaces, and nighttime light, alongside higher walkability and moderate distance to blue space. Associations between SES and high-risk exposome profiles varied markedly across cities. Higher area-level SES was positively associated with high-risk profiles in most cities. Individual-level indicators showed heterogeneous, and in some cases reversed, associations, with educational attainment, income difficulties, ethnicity, and employment status exhibiting context-specific patterns. Composite indices only partially mirrored findings from latent profile analyses.

Socioeconomic inequalities in the urban external exposome differed across Bulgarian cities, emphasizing the pivotal role of local urban contexts in shaping cumulative environmental disadvantage. Our findings further demonstrated that conclusions about socioeconomic patterns depend on how high-risk external exposome profiles are operationalized.