A systematic review and meta analysis published in Lancet Planetary Health looks at long-term air pollution exposure and dementia incidence.
Dr Mark Dallas, Associate Professor in Cellular Neuroscience, University of Reading, said:
“While air pollution joined dementia’s 14 modifiable risk factors in 2024, the specific culprits remain unclear. This new research examined existing data and identified three main culprits: tiny particles from car exhaust, nitrogen dioxide from vehicles and power plants, and black carbon from diesel engines. These findings strengthen the evidence that we can protect brain health through cleaner policies targeting diesel pollution and better city planning. However, we still need to understand exactly how these pollutants damage the brain and increase the diversity in dementia research participants. This will help us learn more about how air pollution affects different types of dementia and whether some communities face higher risks than others.”
Dr Tom Russ, Reader in Old Age Psychiatry, University of Edinburgh, said:
“This high quality article summarises the evidence in this rapidly-expanding area up to October 2023. This article improves on many previous reviews but is subject to similar limitations because of the way this research is often conducted; this reflects the quality of the studies it summarises rather than any shortcomings of this specific article. The review includes articles which examine the association of exposure to air pollution for at least one year (described as ‘long-term’ exposure) with the emergence of dementia diagnosed by a doctor. It includes more studies than any previous article and because of the large number of studies included, the authors can be more accurate in their estimate of the size of the effect of dementia – for instance, their data suggest that the risk of dementia resulting from exposure to air pollution would be 9% lower in Edinburgh compared to London.
“It is helpful to see the effects of different pollutants examined – though the authors acknowledges that these pollutants may, in fact, interact with each other in having their harmful effects. This speaks to an area this article cannot deal with – if exposure to air pollution does indeed increase the risk of someone developing dementia, what is the mechanism by which this happens? This question has not yet been addressed – in contrast to air pollution and the cardiovascular system where we have a clear mechanistic understanding of the effects of air pollution exposure on the body through experiments where people are exposed to controlled levels of air pollution. We need a similar body of research focused on the brain.
“The authors try to examine air pollution in relation to different subtypes of dementia – an important area – but because this is often poorly recorded in medical records, they were not able to really tackle this. Most of the time, dementia is simply recorded as ‘dementia’ rather than the specific diagnosis (e.g., Alzheimer dementia, vascular dementia, dementia with Lewy bodies). A further complication is that around half of people with dementia never receive a diagnosis and so don’t appear in medical records.
“One limitation of all the studies included in the review is that they estimate the amount of air pollution exposure based on someone’s home address. This is not the most accurate measure of air pollution exposure but I am not aware of any studies which have done this any other way, though a better approach is sorely needed.
“Finally, since we know that many conditions which result in dementia have their origins decades before the emergence of symptoms, studies really need to look at truly long-term air pollution exposure – much longer than one year. Researching this is challenging because few long-term studies have people’s home addresses from their whole lives and measurement or modelling of air pollution levels is rare before the 1990s.
“This article answers the question of whether air pollution exposure is associated with dementia better than previous work, but we still need better research to clarify how and why air pollution might be bad for the brain. Dementia remains a public health priority but air pollution is just one of several important risk factors and stopping smoking, controlling diabetes, controlling blood pressure and cholesterol in mid-life (amongst other things) are crucial for individuals who want to reduce their own risk of dementia, as well as minimising exposure to air pollution.”
Dr Ian Mudway, Associate Professor of Environmental Toxicology and Visiting Professor for Environmental Health, Gresham College, Imperial College London, said:
“This aligns very closely with previous attempts to examine the association between air pollution and dementia. I worked on this back in 2019, and at that time, given the available evidence, we concluded it was too premature to perform a meta-analysis. There were simply too many inconsistencies between studies, particularly concerning exposure assessment.
“While I believe the evidence base has improved since then, inherent challenges remain in linking long-term air pollution changes to dementia incidence due to the decades-long prodromal period of the disease. It raises the crucial question: “How far back must we look to capture the relevant long-term exposures impacting brain health?”
“Additionally, as the authors acknowledge, distinguishing between vascular dementia and Alzheimer’s disease purely from medical records remains quite difficult, despite their efforts.
“The robust associations observed for NO2, black carbon/PM2.5 absorbance, and PM2.5 itself suggest that the effect is related to both local-scale traffic emissions and more regional particulate matter sources. Overall, this paper strongly supports the contention outlined in the Lancet Commission’s dementia reviews that air pollution is a significant and modifiable risk factor for dementia, and addressing it would substantially improve brain health.”
Prof Roy Harrison FRS, Professor of Environmental Health, University of Birmingham, said:
“This combined analysis of 51 previously conducted independent studies gives a clear signal that the risk of developing dementia is strongly influenced by air pollution exposure. This finding is consistent with other research showing associations between a number of measures of brain function and air pollution, and is particularly important given the devastating impacts of dementia both upon individuals and their families, and society as a whole. It adds to our ever-increasing knowledge of the many diverse harmful effects of air pollution upon health and strengthens the case for firm action to further improve air quality.
Dr Samuel Cai, Lecturer in Environmental Epidemiology, University of Leicester, said:
“The press release is accurate, although it could also be mentioned that studies included in this meta-analysis are quite heterogeneous.
“This is a comprehensive and timely review, including latest primary studies published over the last few years. The conclusion was generally backed by the data presented.
“Air pollution was only recently identified as a new risk factor for dementia in a Lancet-commissioned research. At the time, evidence for the harmful effects of PM2.5 on dementia seems to be more certain, but evidence for other pollutants is less conclusive. This review has significantly strengthened the current knowledge base, reporting that PM2.5, NO2 and soot are all adversely linked to dementia development, based on some of most recent publications.
“This is a systematic review and meta-analysis, and therefore consideration of confounders are usually not applicable in this type of articles. There are two more limitations which may worth further investigation. First, in the studies included in this review, did the effects of air pollution on dementia incidence have been adjusted for other environmental exposures such as greenspace and traffic noise? These two exposures may interact with air pollution in a complex way, and therefore may affect the risk posed by air pollution leading to dementia onset?
“Second, it is not very clear, at which life stage that air pollution exposure is relatively more important in triggering dementia? There is some evidence that late-life air pollution exposures seem to be more relevant to dementia incidence, as compared to mid-life or early-life. I think the current evidence pool is still weak on this question, but certainly a direction warranting more research.
“The implications mentioned by the authors are correct. Air pollution needs to be formally recognised as a risk factor for dementia in clinical practices, and that societal-wide policy actions are needed to tackle air pollution, particularly that from traffic in UK cities and towns, to protect brain health as UK population is ageing.”
Prof Barbara Maher FRS, Professor of Environmental Magnetism, Lancaster University, said:
“This is another meticulous and large study (~30 million people over 4 continents), which reviews and analyses other painstaking studies, attesting to the damage being done to our brains by breathing in air pollution particles. While this study links outdoor PM2.5 (fine particles less than 2.5 micrometres diameter) with increased dementia incidence, this might represent just the tip of the iceberg. Air pollution contains huge numbers of ultrafine particles (< 100 nanometres) which are so small and dangerous they can be transported inside brain cells, causing inflammation, DNA changes, and eventually cell death.
“It’s now 9 years since our discovery of huge numbers of traffic-derived, metal-rich nanoparticles inside the frontal cortex of human brains…anywhere between 900 million and 40 billion particles in a gramme of brain tissue. Similar particles have been found directly associated with the amyloid plaques typical of Alzheimer’s disease. And the likely health impacts of exposure to such small, toxic particles don’t end with the brain. They have now been found in human blood, heart, placenta, kidney, bone joints…the body has no effective defense against the ultrafine particle cocktails we generate outdoors, especially from traffic, and indoors, for example, in heating our homes using stoves.
“What’s more, of course, the nanoparticle ‘mix’ varies from place to place and city to city, so the full scale of the dementia/air pollution pandemic will only become more obvious when epidemiological studies take particle composition, as well as ultrafine size, into account.”
Dr Isolde Radford, Senior Policy Manager at Alzheimer’s Research UK, said:
“Air pollution is not just an environmental issue – it’s a serious and growing threat to our brain health. If no one were exposed to air pollution, there would be three fewer cases of dementia for every 100 people who develop it now. This rigorous review adds to mounting evidence that exposure to air pollution – from traffic fumes to wood burners – increases the risk of developing dementia.
“But poor air quality doesn’t affect all communities equally. As this analysis highlights, marginalised groups are often exposed to higher levels of pollution, yet remain underrepresented in research. Future studies must reflect the full diversity of society – because those most at risk could stand to benefit the most from action.
“What’s still unclear is exactly how air pollution affects the brain. There are several biological pathways that could explain the link, and to prevent dementia in the future, we need to deepen our understanding of these mechanisms.
“Air pollution is one of the major modifiable risk factors for dementia – but it’s not something individuals can solve alone. That’s where government leadership is vital. While the 10-Year Health Plan acknowledges the health harms of air pollution, far more needs to be done to tackle this invisible threat. Alzheimer’s Research UK is calling for a bold, cross-government approach to health prevention — one that brings together departments beyond health, including DEFRA, to take coordinated action on the drivers of dementia risk.
“The UK is still working to meet the World Health Organization’s air pollution limits by 2040 – but that timeline simply isn’t good enough. We have the evidence and the means to reach these targets by 2030. Doing so could help prevent thousands more people from developing dementia. The Government must act now to set stronger, health-based air quality targets – ones that protect our brains as well as our lungs.”
‘Long-term air pollution exposure and incident dementia: a systematic review and meta-analysis’ by Clare B Best Rogowski et al. was published in The Lancet Planetary Health at 23:30 UK time on Thursday 24th July.
DOI: https://doi.org/10.1016/S2542-5196(25)00118-4
Declared interests
Dr Mark Dallas: Dr Dallas receives research funding from the Medical Research Council and Carbon Monoxide Research Trust.
Dr Tom Russ: I don’t have any conflicts as such but am active in research in this area.
Prof Roy Harrison: Roy Harrison is a member of the Defra Air Quality Expert Group and the DHSC Committee on the Medical Effects of Air Pollutants. He has research funding from UKRI, Defra and the European Union Horizon Programme.
Dr Samuel Cai: I do not have any conflict of interest to declare.
Prof Barbara Maher: None to declare
For all other experts, no reply to our request for DOIs was received.