Commuting on the London Underground and Long-term Health Outcomes: A Cohort Study Using Externally Linked Census Data.
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BACKGROUND: Metro systems are essential for urban mobility but often expose commuters to particulate matter (PM) at levels far above roadside air. The London Underground (LU), the world's oldest metro, supports ∼4 million daily journeys and has elevated PM2.5 concentrations dominated by iron-rich particles from mechanical abrasion. These exposures raise concerns about long-term health effects. METHODS: Using the Office for National Statistics Longitudinal Study (ONS-LS), we analysed two retrospective cohorts of economically active adults (≥16 years) living and working in London and recorded in the 1991, 2001, or 2011 Censuses. Cohort 1 (n = 7,343) compared LU commuters with above-ground rail users. Cohort 2 (n = 4,094) examined dose-response patterns between externally linked cumulative LU PM2.5 exposure and health outcomes. Exposure estimates combined Transport for London data, PM2.5 measurements, and geospatial modelling. Outcomes were all-cause mortality (1991 to 2017) and cancer incidence (1991 to 2015). Analyses used inverse probability weighting and the parametric g-formula. RESULTS: Most LU commuters were estimated to encounter hourly PM2.5 levels >100 μg/m3. LU users were 1.93% (95% CI: 0.58-3.66) less likely to survive the follow-up period than above-ground rail users, while cancer risk differences were negligible. No dose-response pattern emerged, and a hypothetical 50% PM2.5 reduction produced minimal survival change. CONCLUSIONS: LU commuters experience PM2.5 concentrations roughly 8-10 times higher than above-ground levels. Mortality risk appeared slightly elevated, though uncertainty remains. Findings highlight the need for better exposure assessment and further research on metro-related PM.