Putting fine particulate matter and dementia in the wider context of noncommunicable disease : where are we now and what should we do next : a systematic review

Introduction: A significant proportion of the global population regularly experience air quality poorer than that recommended by the World Health Organization. Air pollution, especially fine particulate matter (PM2.5), is a risk factor for various noncommunicable diseases (NCDs) and is emerging as a risk factor for dementia. To begin to understand the full impact of PM2.5, we review the longitudinal epidemiological evidence linking PM2.5 to both dementia and to other leading NCDs and highlight the evidence gaps. Our objective was to systematically review the current epidemiological evidence for PM2.5 as a risk factor for cognitive decline and incident dementia and to put this in context with a systematic overview of PM2.5 as a potential risk factor in other leading NCDs. Methods: We performed 2 systematic reviews. A high-level review of reviews examining the relationship between PM2.5 and leading NCDs and an in-depth review of the longitudinal epidemiological data examining relationships between PM2.5 incident dementia and cognitive decline. Results: There were robust associations between PM2.5 and NCDs although in some cases the evidence was concentrated on short rather than longer term exposure. For those articles reporting on incident dementia, all reported on longer term exposure and 5 of the 7 eligible articles found PM2.5 to be associated with increased risk. Conclusion: The evidence base for PM2.5 as a risk factor for dementia is growing. It is not yet as strong as that for other NCDs. However, varied measurement/methodology hampers clarity across the field. We propose next steps.