The contribution of air pollution to the socioeconomic inequalities in dementia risk in later life: The Whitehall II cohort study

<ns3:p> Lower socioeconomic position (SEP) is linked to increased dementia risk. This may be through higher rates of risk behaviours such as smoking but may also determine our living environment. Deprived neighbourhoods have been associated with higher exposure to hazardous factors such as air pollution. However, the impact of individual-level SEP (employment grade) and area-level deprivation (Index of Multiple Deprivation, IMD) on dementia risk, along with the potential mediating role of air pollution, remains poorly understood. This study aims to investigate the separate associations of employment grade and area deprivation with dementia risk, and to assess whether these associations are mediated by exposure to particulate matter ≤2.5μm (PM <ns3:sub>2.5</ns3:sub> ) and nitrogen dioxide (NO <ns3:sub>2</ns3:sub> ). We will use longitudinal data from the Whitehall II Study, a longitudinal study of civil servants in the UK, focusing on participants who were dementia-free at baseline. To maintain a prospective study design, we will define baseline as the 2002-2004 data collection wave. Employment grade (6 categories) was assessed during this period and area-level deprivation will be measured using the average IMD scores from 2004 and 2007. Air pollution exposure will be estimated using average PM <ns3:sub>2.5</ns3:sub> and NO <ns3:sub>2</ns3:sub> levels for the period 2002/2004 – 2009. Dementia incidence over 20 years will be ascertained through electronic health record linkage, with follow-up extending to 2023. Baseline covariates will include demographic characteristics such as age, sex, education level, and ethnicity. We will use Cox proportional hazards models to estimate the associations between SEP indicators and incident dementia. Causal mediation analysis will be conducted to estimate if air pollution is a mediator and the proportion of the association mediated by air pollution. </ns3:p>