Could Air Pollution be a Cause of Dementia?

It is widely known that air pollution is harmful to our heart and lungs but further research has also linked air pollution exposure to increased dementia risk.

It is estimated that about 900,000 people in the UK suffer from dementia, with a projection of over 1 million by 2025. Additionally, a study in The Lancet in 2018 (1) suggested that about 60,000 of the 209,600 new cases of dementia diagnosed in the country each year could be due to poor air quality. If research can demonstrate that air pollution does directly cause damage or changes to the brain that result in dementia it could be possible to develop new ways to limit these effects, which could potentially result in a significant reduction in the number of new cases of dementia.

This week's article discusses the link between air pollution on cognitive impairment and the risk of dementia as a landmark report released last week by the government's committee on the medical effects of air pollutants (COMEAP) concluded that air pollution is likely to increase the risk of accelerated cognitive decline and of developing dementia in older people.

What is the link between air pollution and dementia?

Dementia is a progressive condition that leads to deterioration in cognitive function and affects memory, thinking, learning capacity, and judgement. 

Air pollution is made up of both gases and tiny particles called particle matter (PM) that are caused as a side effect of burning fuels. PM2.5 includes particles under 2.5 micrometres (which is about 3% of the width of a human hair) and PM10 is particles around 10 micrometres. 

Research so far has suggested that nitrogen oxides, PM2.5 and PM10 show the strongest link to cognitive decline and potentially dementia. 

A number of mechanisms have been suggested by which air pollutants could have direct effects on the brain. These include:

The translocation of these mentioned small particles from the lung to the bloodstream and hence to the brain.

Evidence suggests:

• that a small proportion of very small particles that are inhaled can enter the brain, both from the blood and via the olfactory nerves leading from the nasal passages to the olfactory bulbs (2)

• particles which enter the brain are cleared from the brain only slowly, if at all (2)

• Animal and in vitro studies of very small particulate material, diesel engine exhaust or ozone have all shown effects on the brain or brain cells

• Mechanisms involved include the generation and release of free radicals within the brain and the induction of an inflammatory response(2); these two mechanisms seem likely to be linked.

Is the epidemiological evidence sufficient?

COMEAP reviewed around 70 studies that analysed how exposure to air pollutants affects the brain over time before publishing its findings. However, a limitation of these epidemiological studies that were analysed is that many base their findings on where the person they had studied is currently living, without considering other factors such as how much time they spend at home, where they lived before or their working environment. 

The COMEAP report, therefore, stated that the evidence base is ‘currently inadequate to allow direct quantification using a meta-analysis of epidemiological studies linking air pollution with cognitive decline or dementia, and hence is subject to unknown uncertainty.’
What actions can be taken?

In the UK, the government’s most recent Clean Air Strategy proposes to reduce PM2.5 by 46 per cent, nitric oxide by 73 per cent as well as other harmful compounds by 2030. This would be achieved through legislation enforcement covering both national and local sources of pollution.
In terms of research, there needs to be a greater investment into the effect of air pollution on the brain- this would require long-term epidemiology studies and greater biomedical research. Future research should explore how air pollution affects different types of dementia as well.

Hence, whilst research to date has not yet definitively shown a direct mechanism by which air pollution could cause dementia, the evidence does point towards an indirect cause. This means that the effect of air pollution on our respiratory and cardiac health is likely to have a knock-on effect on brain health, increasing the risk of dementia.

References

(1) Carey et al. (2018). “Are noise and air pollution related to the incidence of dementia? A cohort study in London, England.” BMJ open vol. 8(9) doi:10.1136/bmjopen-2018-022404
(2) Cognitive decline, dementia and air pollution: A report by the Committee on the Medical Effects of Air Pollutants: Available at: https://www.gov.uk/government/publications/air-pollution-cognitive-decline-and-dementia

Further information and articles on dementia and air pollution link:
https://www.alzheimers.org.uk/about-dementia/risk-factors-and-prevention/air-pollution-and-dementia
Guzmán, Patricia et al.(2022). “Effects of air pollution on dementia over Europe for present and future climate change scenarios.” Environmental research. vol. 204,Pt A doi:10.1016/j.envres.2021.112012
https://www.alzheimers.org.uk/for-researchers/report-on-link-between-air-pollution-and-dementia