Could air pollution be a risk factor for dementia?

5543835085_fb0b017685_oDr Ruth Peters works at the Imperial Clinical Trials Unit of Imperial College London. In this blog, she discusses her recent Age & Ageing paper on the links between air pollution and dementia.

A few years ago, a chance conversation with a cardiologist about the adverse impact of air pollution on cardiovascular health set me thinking would such exposure also be a risk factor for dementia?

Looking into the literature, and joining forces with respiratory and air pollution experts, I discovered that whilst air borne pollutants reach the brain via the lungs, inhaled particulates are also thought to reach the brain directly via the olfactory bulb. Both routes potentially induce inflammatory responses, increased microglial activation and production of reactive oxygen species, and possibly also promote production and deposition of amyloid beta. Given the evidence base linking both cardiovascular risk factors and inflammation to an increased risk of dementia, it seems likely that exposure to air pollution would indeed increase risk of cognitive decline or dementia.

Adding an informatician to our team, we decided to investigate further via a systematic review (1). We found that surprisingly few longitudinal studies have published in this area and still fewer report on incident cognitive decline and none on dementia. At present there is a hint that exposure may increase risk of cognitive decline but the evidence base needs strengthening with further analyses.

Particularly rich sources may be existing longitudinal studies where cognitive data is already being collected at multiple visits and data on various air borne pollutants may be available via local monitoring stations or from routine (current and past) records. Meanwhile the air pollution cardiovascular link grows stronger with the same cardiologist Professor David Newby (British Heart Foundation Professor of Cardiology at the University of Edinburgh) publishing a systematic review and meta-analysis in March this year, linking short-term exposure to increased risk of stroke (2).

Following the completion of our review, I have had further discussions with an expert in risk factors for cognitive decline, dementia and ageing, namely Professor Kaarin Anstey, who is Director of the Centre for Research on Ageing, Health and Wellbeing at the Australian National University.

Prof Anstey notes that ‘the epidemiological evidence is supported by neuropathological evidence that has linked exposure to air pollution with Alzheimer’s neuropathology in teenagers in Mexico City (3)’and emphasized that ‘air pollution is a threat to healthy ageing… the potential global impact of air pollution on brain health is enormous and indicates a need for greater intervention to regulate or remove the causes of air pollution’.

Overall, it seems that this area is one to watch over the coming months and years. Hopefully in the not too distant future we will be able to redo our review and report on a more robust evidence base for this hugely important issue!

  1. Is air pollution associated with increased risk of cognitive decline? A systematic review – Age & Ageing, first published online July 18, 2015doi:10.1093/ageing/afv087
  2. Shah et al. Short term exposure to air pollution and stroke Short term exposure to air pollution and stroke: systematic review and meta-analysis. BMJ 2015;350:h1295
  3. Calderon-Garciduenas L et al. Neuroinflammation, hyperphosphorylated tau, diffuse amyloid plaques, and down-regulation of the cellular prion protein in air pollution exposed children and young adults. J Alzheimers Dis 2012;28:93-107.

Image credit: Jean-Etienne Poirier via flickr.

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