Links between respiratory and cardiovascular diseases and air pollution are well-recognised in epidemiological discourse. Most recently, in edition 316 of BC Disease News (here), we reported on the so-called ‘silent epidemic’ of pollution-induced heart disease.
Since then, researchers at the Harvard TH Chan School of Public Health have published an article in the Lancet Planetary Health, reviewing the first nationwide analysis of associations between long-term contact with fine particulate pollution (PM 2.5) and neurogenerative disease, thus pushing the boundaries of what is already known about the injurious impact of exposures on the body.
To facilitate this landmark longitudinal study, the paper focused on the development of brain disease in an American cohort of over 63 million people admitted to hospital over a 17-year period.
Of these 63 million patients, there were 1 million cases of Parkinson’s disease and 3.4 million cases of Alzheimer's disease and related dementias.
The researchers went on to discover that, for every increase in annual PM 2.5 concentration by 5 micrograms per cubic metre of air (μg/m3), hospital admissions relating to neurodegenerative disease rose by 13%. There was strong evidence of a linear relationship up to 16 μg/m3 exposure, beyond which the trend line plateaued (this could be due to the fact that no more than 5% of exposures exceeded 16 μg/m3).
What is more, women and Caucasians, as well as urban populations, appeared to be particularly susceptible to neurological damage, compared to any other sub-group observed. According to the experts, higher life expectancy could have been responsible for the former, with the abundance of metal-bearing nanoparticles being responsible for the latter.
Overall, prolonged exposure to airborne pollutants was ‘significantly associated’ with a higher risk of neurological health deterioration, even at concentrations below 12 μg/m3, which is half the legal annual mean air quality limit value prescribed by the European Union and the UK (25 μg/m3).
This finding goes some way to explain why there have been calls for the UK to adopt the World Health Organisation’s recommended guideline limit of 10 μg/m3 – see our analysis of the British Safety Council’s (BSC) White Paper: Impact of air pollution on the health of outdoor workers – The case for action, which featured in edition 275 of BC Disease News (here).
That being said, Research Manager at Parkinson’s UK, Dr Beckie Port, who believes that Harvard’s investigation provides the ‘most robust evidence to date’, also theorises that ‘there may be no safe level of exposure’.
Although there was no concrete determination as to cause and effect, i.e. the specific mechanism by which exposure impairs the brain, the Deputy Director of the Centre for Discovery Brain Sciences at the University of Edinburgh and Programme Lead at the UK Dementia Research Institute, Professor Tara Spries-Jones, reflected that:
‘It is possible that air pollution acts through damaging cardiovascular health, which is linked to risk of neurological diseases, but further studies are needed to understand the biology of this association’.
More evidence is required, not least because the present study could have been compromised by several limitations, owing to its design. For instance, the fact that cigarette smoking and other neurodegenerative risk factors were not taken into account. Given that dementia and Parkinson’s are typically latent conditions, it could be highly relevant that full residence histories were not explored by the research team.
The general feeling, however, is that weight has been added to prior assertions that air pollution is neurotoxic, with co-lead author Xiao Wu, having signalled that they have built on ‘the small but emerging evidence base’.
In fact, Dr. Sara Imarisio, who is Head of Research at Alzheimer’s Research UK, warned that, since only the most severe cases of dementia (hospitalised cases) were picked up by the academics:
‘… the true number of those developing dementia [is] likely to be even higher’.
It is unsurprising, therefore, that Dr. Stefan Reis, who is Head of Atmospheric Chemistry and Effects at the UK Centre for Ecology and Hydrology (UKCEH), deemed that:
‘… reducing exposure to PM2.5 in the whole population should become a key priority for air pollution control policies’.
 Shi L et al., Long-term effects of PM2·5 on neurological disorders in the American Medicare population: a longitudinal cohort study. Lancet Planetary Health (2020) <https://www.thelancet.com/action/showPdf?pii=S2542-5196%2820%2930227-8> accessed 4 November 2020.
April Roach, ‘“Significant link” between air pollution and neurological disorders, says study’ (19 October 2020 The Evening Standard) <https://www.standard.co.uk/news/world/air-pollution-link-neurological-disorders-a4572211.html> accessed 4 November 2020.
 Expert reaction to longitudinal cohort study on exposure to fine particulate matter (PM2.5) and neurological disorders’ (19 October 2020 Science Media Centre) <https://www.sciencemediacentre.org/expert-reaction-to-longitudinal-cohort-study-on-exposure-to-fine-particulate-matter-pm2-5-and-neurological-disorders/> accessed 4 November 2020.
 Comments of Dr. Ivan Koychev, Senior Clinical Researcher at the University of Oxford and Clinician Scientist at Dementias Platforms UK (DPUK).
 Prince M et al., Dementia UK: Update (2nd Edition) (November 2014 Alzheimer’s Society) <https://www.alzheimers.org.uk/sites/default/files/migrate/downloads/dementia_uk_update.pdf> accessed 4 November 2020.