Mobility shapes heat exposure inequalities in cities

Segregation is a key driver of environmental inequalities, with disadvantaged groups often living in neighborhoods where heat-related risks are highest. Yet, it remains unclear how daily mobility patterns, embedded within heterogeneous urban heat fields, shape heat exposure inequalities across sociodemographic groups. Using mobile phone daily mobility flows and urban temperature fields across 23 Spanish cities, we develop a network-based framework to quantify how different sociodemographic groups experience heat through their daily movements. We further apply the framework to tract-level home-work commuting networks across 30 major US cities as an external validation, yielding comparable patterns. We find systematic income-related inequalities, with low-income groups consistently experiencing higher exposure than high-income groups, while age-related disparities are smaller in magnitude. These inequalities intensify during commuting trips, indicating that routine mobility amplifies spatial heat gradients more than non-routine movements. We further use parsimonious population-based mobility models to assess whether these observed inequalities can be reproduced using group-agnostic mobility rules based on population distributions and geographic distance. Although the gravity model more accurately reproduces exposure levels, the parameter-free radiation model better preserves the observed exposure disparities between groups. This suggests that a substantial component of inequality in mobility-based heat exposure emerges from the interplay between the unequal organization of daily activities across sociodemographic groups and urban heat gradients. Our findings provide a generalizable framework to characterize inequalities in mobility-based heat exposure across cities and inform climate-resilient urban planning and public health strategies in the context of intensifying climate-related risks.