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 PM_2.5 concentrations dominated by iron-rich particles from mechanical abrasion. These exposures raise concerns about long-term health effects.

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 PM_2.5 exposure and health outcomes. Exposure estimates combined Transport for London data, PM_2.5 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.

Most LU commuters were estimated to encounter hourly PM_2.5 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% PM_2.5 reduction produced minimal survival change.

LU commuters experience PM_2.5 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.