Air pollution is typically linked to respiratory conditions. But evidence is mounting that it could be associated with another equally concerning condition: dementia.
A recent study published, in JAMA Neurology, is the latest to find that higher exposure to fine particulate matter may exacerbate the neurological changes associated with Alzheimer’s disease and dementia.
The study’s authors emphasise that more research needs to be done to fully understand the connection, but the evidence for a link is mounting.
A meta-analysis published in July 2025 by The Lancet Planetary Health examined data pertaining to 29 million people gathered between the late 1980s and early 1990s from multiple countries to investigate the impact of PM2.5 (fine particulate air pollution), nitrogen dioxide (NO2 ) and soot on brain health.
It found “dementia diagnosis to be significantly associated with long-term exposure to [fine particulate matter air pollution]”.
But these are only the latest examples. Evidence for a link has been accumulating for a few years now. A study published in The Lancet in 2017 was among the first to show that living near major roads was associated with a higher incidence of dementia.
So, what is the problem that air pollution poses and what can we do about it?
The role of particulate matter
Air pollution comes in various forms. Particulate matter (PM) is just one. It describes the tiny bits of solid or liquid matter floating in the air – dust, smoke, droplets that are so small you usually can’t see them.
PM is measured in sizes from ultra-fine (PM0.1), through to fine and all the way up to coarse (PM10).
PM2.5 is very fine – less than a 30th of the width of a human hair. And because it’s so fine and light, it can remain airborne for a long time, which makes it very easy for people to inhale.
According to Dr Holly Elser, an epidemiologist and coauthor of the most recent study published in JAMA Neurology, “[PM2.5 pollution is] relevant for wide-ranging health outcomes.” Those outcomes include asthma, lung cancer, heart disease and, as increasing evidence finds, dementia.
What complicates matters is that PM2.5 isn’t just one thing that has a single origin. “[PM2.5] has many sources and traffic is just one of them,” says Dr Haneen Khreis, who researches the health impacts of urban mobility at the University of Cambridge and was a co-author of the study in The Lancet Planetary Health.
Power plants, factories, construction activity, wildfires, wood burning, biomass burning, (burning just about anything, in fact), as well as dust storms and other natural sources all generate PM2.5.
“The chemistry of the particles, and therefore their toxicity, will partly depend on the sources of PM2.5,” says Khreis.
Because it is composed of so many different things and originates from such a wide variety of sources, this makes it very difficult to address.
There are two proposed routes of entry for PM2.5 into the central nervous system, according to Khreis: “direct translocation via the olfactory nerve (that is, through the nose) and via peripheral circulation across the blood-brain barrier”.
How it reaches and affects the brain
Because PM2.5 is so small, it can enter the smallest air spaces of the lungs where gas exchange occurs. From there, it can get into the circulatory system and, once it’s in the blood stream, it can eventually reach the brain, where, as Khreis explains, “it triggers inflammation and oxidative stress, damaging neurons and blood vessels over time”.
Other experts pose additional hypotheses for how pollution impacts the brain and jeopardises cognition: that it causes white matter damage and myelin disruption; that it enters the brain via the nose, travels along olfactory pathways to the hippocampus, the brain’s memory centre, promoting the build-up of toxic amyloid and tau proteins (the signature markers of Alzheimer’s disease).
Also that it reduces cerebral blood flow, causing microvascular damage, which exacerbates the risk of vascular dementia.
Unsurprisingly, air pollution is very high next to busy roads, but studies find it falls away steeply the further away from traffic you are.
A 2017 study published in The Lancet based on findings from over six million residents in Ontario, found that people living within 50m (165ft) of a major road had about a 7–12-percent higher risk of dementia than those living more than 200m (approx 650ft) away.
Just as distance from a major road might impact dementia risk, so too does the burden of PM2.5.
According to Khreis, for every 10 micrograms per cubic metre (μg/m³) of PM2.5, a person’s relative risk of dementia increases by 17 per cent.
To put that in context, the average roadside measurement for PM2.5 in Central London in 2023 was 10μg/m³.
For every 10μg/m³ of NO2 , a gas made of one nitrogen atom and two oxygen atoms that forms when fossil fuels (petrol, diesel, gas, coal and oil) are burned, the relative risk rose by three per cent.
The average roadside measurement for NO2 in Central London in 2023 was 33 µg/m³.
Burning of fossil fuels is by far the biggest culprit when it comes to air pollution, though – particularly PM2.5.
Lower your exposure
If you live or work by a road, it might not be easy or practical to significantly reduce your exposure to air pollution. But given how many of us live in urban locations, it is a priority – one that Khreis says is only “largely modifiable by targeted policy measures and switching away from our reliance on fossil fuels to clean energy”. In other words, something that individual actions can’t solve.
That said, it is helpful for people to understand more about air quality (which tends to be poorer on warm summer afternoons and better after rain).
Elser explains that on days where the Air Quality Index exceeds 100, “it means the air is unhealthy to breathe and time spent outdoors should be minimised if possible.” If you can’t avoid staying outside, a fit-tested N95 or a KN95 mask can help protect you from PM2.5 exposure.
If you’re inside on days when the air quality is poor, but the heat is forcing you to open your windows, “an air purifier, or even a fan, can improve indoor air quality,” says Elser. You can pick up decent models of either appliance for around £100, making it a relatively affordable solution.
There are also things you can do to reduce your exposure when moving around busy cities. Khreis, for example, cycles on roads with less traffic and more greenery – you could do the same if you run or walk. Fewer vehicles means lower levels of air pollution, but the foliage also plays a part.
If there’s enough of it, the vegetation can absorb a significant amount of air pollution, reducing its concentration by as much as 50 per cent, according to a 2018 study. To have an effect to this extent, however, the vegetation needs to be tall and thick, and placed like a barrier between the road and pedestrians.
PM2.5 measurements have also tested high on the London Underground and New York subway. Some deeper London Underground stations measured 18 times higher than at street level, according to one paper, so wearing a mask down there is something some doctors advocate.
You could also close car windows when you’re in traffic and turn your engine off instead of letting it idle. When you’re at home, always ventilate your kitchen well when cooking.
Just being aware is a good start, but as Elser says, it’s important to acknowledge there are numerous risks for dementia. Air pollution exposure is only one of them.
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