Air pollution linked to dementia and cognitive decline

F
or decades, scientists have established a connection between air pollutants and circulatory issues. These pollutants narrow and thicken blood vessels, contributing to cardiovascular disease, strokes and other circulatory problems as a result, which can eventually lead to vascular dementia, a condition caused by damage to the blood vessels in the brain. Now, a new study has reinforced that connection, with the Committee on the Medical Effects of Air Pollutants (COMEAP) finding that air pollution is likely to increase the risk of developing dementia.

The Committee published its findings after reviewing almost 70 studies that analysed how exposure to emissions affects the brain over time, concluding that air pollution is likely to increase the risk of accelerated “cognitive decline” and of “developing dementia” in older people. Experts presume that this is likely caused by tiny toxic particles seeping into the bloodstream from the lungs, which then irritate blood vessels and disrupt circulation to the brain. In addition, in rare cases, extremely small air pollution particles can pass the blood-brain barrier and damage neurons directly.

air pollution is likely to increase the risk of accelerated “cognitive decline” and of “developing dementia” in older people.

The authors of the COMEAP report said: “We think there is a strong case for the effects of air pollutants on the cardiovascular system having a secondary effect on the brain. That such an effect might well lead to damage to the brain seems, to us, likely. We therefore regard the association between exposure to air pollutants and effects on cognitive decline and dementia as likely to be causal with respect to this mechanism.”

They continued: “The epidemiological evidence reviewed fairly consistently reports associations between chronic exposure to air pollution and reduced global cognition and impairment in visuospatial abilities as well as cognitive decline and increased risk of dementia. Results are heterogeneous as regards to other cognitive domains such as executive function, attention, memory, language and mild cognitive impairment. The identified neuroimaging studies consistently report associations between exposure to air pollution and white matter atrophy.”

It adds that the studies are split over which pollutant is most associated with these effects. They suspect it is likely to be PM2.5, particles with a diameter of less than 2.5 micrometers (about 3% of the width of a human hair) – only a small proportion is able to pass the blood-brain barrier directly, but their prolonged indirect effects can still lead to health issues. It is also the case that air pollution might stimulate the immune cells in the brain, leading to further neurological damage. Once these particles enter the brain, they take a long time to dissipate (if at all).

This existing research alongside increased investigations will help develop international air quality guidelines and policy on particulate matter targets.

The evidence base for dementia studies has grown substantially over the last 15 to 20 years, as the number of people living with dementia in the UK has increased, reaching over than 900,000 cases. The authors wrote: “Clearly, an understanding of the magnitude of the effect of air pollution on neurodegenerative conditions is critical.” They have called for further research into the topic, and while it is not currently possible to directly measure the impact of air pollution on cognitive decline or dementia, it may be possible to develop an indirect method to quantify the effects on the brain. This existing research alongside increased investigations will help develop international air quality guidelines and policy on particulate matter targets.

Under the Environment Act 2021, ministers must set new targets for cutting air pollutants by the 31st of October of 2022. Their proposed goal for cutting annual levels of PM2.5 to 10 micrograms per cubic metre across England by 2040 has attracted criticism from campaigners, and it is notably twice as high as the guidelines set by the World Health Organisation. However, the release of this report is likely to call for more focused efforts to further cutting these pollutants. The report calls dementia “one of the greatest, if not the greatest, global challenge for health and social care in the 21st century”, and dealing with air pollution may be one flank in the battle against it.