Background: High ambient temperatures lead to increased mortality, especially in older adults. Climate change will increase the frequency and severity of heatwaves globally. Most of the UK population lives in urban areas, which often have higher temperatures than rural areas (the urban heat island [UHI] effect) and higher rates of heat-related mortality. We estimated the mortality burden in terms of attributable mortality and years of life lost (YLLs), and social costs attributed to the UHI effect in summer 2018 in Greater London.

Methods: We estimated the UHI effect using advanced urban climate modelling. We applied a quantitative health impact assessment to estimate mortality and YLLs attributable to high air temperature. We estimated social costs using value of statistical life (VSL) and value of statistical life-years (VOLY) methods.

Findings: We attribute 785 (95% CI 655-919) deaths in summer 2018 in Greater London to high air temperature. Half of these (399 [350-446]) are attributable to the UHI effect, or approximately 5·0 (4·1-5·9) thousand YLLs. Social costs of the summer UHI effect due to mortality are estimated at £987 million (866 million-1·10 billion) using VSL or £453 million (367-533 million) using VOLY (2023 prices).

Interpretation: Monetised costs attributed to the UHI effect remain high using either VSL or VOLY approaches. The findings demonstrate the seriousness of heat as a public health risk, set a scale at which society may be willing to pay for urban heat mitigation, and give tangible support for large-scale urban heat mitigation and adaptation policies.

Funding: Wellcome Trust.

Copyright © 2025 The Author(s). Published by Elsevier Ltd. This is an Open Access article under the CC BY 4.0 license. Published by Elsevier Ltd.. All rights reserved.