Association between exposure to air pollution and hippocampal volume in adults in the UK Biobank

Background: The hippocampus is important for memory processing. Several neuropsychiatric diseases including Alzheimer's disease are associated with reduced hippocampal volume, and further the hippocampus appears vulnerable to environmental insult. Air pollution has been associated with cardiovascular disease, abnormal brain structure, and cognitive deficits. Objective: Because of hippocampal vulnerability to environmental insults and based on the association between exposure to air pollution and cognitive function and brain structure, we evaluated the association between exposure to toxins in air pollution and left and right hippocampal volume using brain-imaging and air-pollution data from the UK Biobank, a large community-based dataset. Methods: We used regression modelling to evaluate the association between exposure to nitrogen dioxide, nitrogen oxides, PM2.5, PM2.5-10, and PM10, and left and right hippocampal volume controlling for age, sex, body mass index, overall health, alcohol use, smoking, educational attainment, socioeconomic status, inverse distance from the nearest major road, and a measure of total brain volume. Results: In these models, PM2.5 concentration was associated with smaller left hippocampal volume. None of the other measures of air pollution was associated with either left or right hippocampal volume, although interaction models provided some evidence that sex might moderate the relationship between air pollution and hippocampal volume. In adjusted models, age, sex, educational attainment, income, overall health, current smoking, alcohol intake, and body-mass index were associated with hippocampal volume. Conclusions: PM2.5 at levels found in the United Kingdom was associated with smaller left hippocampal volume. Additional associations between several covariates and hippocampal volumes indicate that hippocampal volume might be associated with several potentially modifiable variables.