A longitudinal, population-based, record-linked natural experiment on the effects of extreme heat events on mental health in urban communities: a study protocol.

INTRODUCTION: Studies suggest that extreme heat events can have negative effects on mental health. However, characterisation of these effects in urban communities remains limited, and few studies have investigated the potential modifying effects of demographic, clinical and environmental characteristics. The aim of this study is to address this knowledge gap and quantify the impacts of extreme heat on mental health, health service use and mental well-being in vulnerable urban populations. METHODS AND ANALYSIS: In this multidisciplinary project, we will assess mental health outcomes in different populations by bringing together two distinct datasets: electronic health record (EHR) data on mental health service users and data from general public participants of Urban Mind, a citizen science project. We will use EHRs from the South London and Maudsley NHS Foundation Trust (SLaM) and the North London NHS Foundation Trust (NLFT), from six boroughs which collectively cover more than 1.8 million residents in Greater London, to capture mental health service use and mortality among people with existing diagnoses of mental illness across 2008-2023. We will use smartphone-based ecological momentary assessment data from Urban Mind to measure mental well-being in the general population (2018-2023). These datasets will be linked to high-resolution spatiotemporal data on temperature, fine and coarse particulate matter (PM2.5, PM10), nitrogen dioxide (NO2), Normalised Difference Vegetation Index (NDVI) and density of large mature tree canopy. We will employ novel quasi-experimental designs, including case time series and case-crossover analysis, to examine the impact of extreme heat on mental health and explore effect modification by sociodemographic, clinical and environmental factors, including air pollution and types of green space coverage. We will also develop a microsimulation model combined with the InVEST urban cooling model to assess and forecast the mental health and social care impacts of extreme heat events and the mitigation of these impacts by different green space coverage and pollution-reduction policies. With a core team composed of researchers, community organisations, industry partners and specialist policy experts, this project will consider lived experience, benefit from broad stakeholder engagement and address gaps in policy and practice. ETHICS AND DISSEMINATION: Each component of this project has been approved by the relevant ethics committee (ref RESCM-22/23-6905 for Urban Mind, LRS/DP-23/24-41409 for the co-development of a screening tool, 23/SC/0257 for the SLaM EHRs, and 24/EE/0178 for the NLFT EHRs). Our dissemination plan includes peer-reviewed scientific articles, policy briefs, a practical guide on fostering ecological and human resilience at the neighbourhood level, and a technical guide for planting and improving the growing conditions of large canopy trees.