A University of Cambridge study reveals that prolonged outdoor air pollution exposure is associated with a significantly increased risk of developing dementia
New research from the University of Cambridge has found a clear link between long-term exposure to outdoor air pollution and a heightened risk of developing dementia. The findings, detailed in The Lancet Planetary Health, contribute to growing concerns about how environmental factors may contribute to cognitive decline over time.
Dementia is expected to triple by 2050
Dementias such as Alzheimer’s disease are estimated to affect more than 57.4 million people worldwide, and this figure is expected to almost triple by 2050. There are some indications that the cases of dementia are decreasing in Europe and North America, suggesting it may be possible to reduce the risk of the disease at a population level.
Air pollution has recently been identified as a risk factor for dementia, with studies pointing towards the number of pollutants. However, the strength of evidence and ability to determine a causal link varied.
Researchers at the Medical Research Council (MRC) Epidemiology Unit, University of Cambridge, conducted a systematic review and meta-analysis of existing scientific literature to examine the link between pollution and dementia. They reviewed 51 studies, including data from more than 29 million participants who had been exposed to air pollutants for at least one year, mostly from high-income countries. Of these, 34 papers were included in the meta-analysis: 15 originated in North America, 10 in Europe, seven in Asia, and two in Australia.
The researchers found a significant link between dementia and three air pollutants: particulate matter (PM2.5), nitrogen dioxide (NO₂), and soot. PM2.5 originates from sources such as vehicle emissions, industry, and wood burning, and can penetrate deep into the lungs. NO₂, primarily from fossil fuel combustion, can exacerbate asthma and impair lung function. Soot, also from exhaust and burning wood, affects both health and the climate, increasing the risk of respiratory and heart problems.
According to the researchers, for every 10 micrograms per cubic meter (μg/m³) of PM2.5, an individual’s relative risk of dementia would increase by 17%. The average roadside measurement for PM2.5 in Central London in 2023 was 10 μg/m³.
For every 10 μg/m3 of NO2, the relative risk increased by 3%. The average roadside measurement for NO2 in Central London in 2023 was 33 µg/m³.
For each 1 μg/m³ of soot as found in PM2.5, the relative risk increased by 13%. Across the UK, annual mean soot concentrations measured at select roadside locations in 2023 were 0.93 μg/m³ in London, 1.51 μg/m³ in Birmingham and 0.65 μg/m³ in Glasgow.
Senior author Dr Haneen Khreis from the MRC Epidemiology Unit said: “Epidemiological evidence plays a crucial role in allowing us to determine whether or not air pollution increases the risk of dementia and by how much. Our work provides further evidence to support the observation that long-term exposure to outdoor air pollution is a risk factor for the onset of dementia in previously healthy adults.
“Tackling air pollution can deliver long-term health, social, climate, and economic benefits. It can reduce the immense burden on patients, families, and caregivers, while easing pressure on overstretched healthcare systems.”
How does air pollution potentially cause dementia?
Several mechanisms may explain how air pollution contributes to the development of dementia, primarily involving brain inflammation and oxidative stress. Both oxidative stress and inflammation play an established role in the onset and progression of dementia.
Air pollution is believed to trigger oxidative stress and inflammation through direct entry to the brain or via the exact mechanisms underlying lung and cardiovascular diseases. Air pollution can also enter circulation from the lungs and travel to solid organs, initiating local and widespread inflammation.
The researchers note that the majority of people included in the studies were white and lived in high-income countries, even though marginalised groups tend to have a higher exposure to air pollution. They call for future work to urgently ensure better and more adequate representation across ethnicities and in low- and middle-income countries and communities.
Further analysis revealed that while exposure to these pollutants increased the risk of Alzheimer’s disease, the effect seemed stronger for vascular dementia, a type of dementia caused by reduced blood flow to the brain.
Joint first author Clare Rogowski, also from the MRC Epidemiology Unit, said: “Efforts to reduce exposure to these key pollutants are likely to help reduce the burden of dementia on society. Stricter limits for several pollutants are likely to be necessary, targeting major contributors such as the transport and industry sectors. Given the extent of air pollution, there is an urgent need for regional, national, and international policy interventions to combat air pollution equitably.”
Joint first author Dr Christiaan Bredell from the University of Cambridge and North West Anglia NHS Foundation Trust said: “These findings underscore the need for an interdisciplinary approach to dementia prevention. Preventing dementia is not just the responsibility of healthcare: this study strengthens the case that urban planning, transport policy, and environmental regulation all have a significant role to play.”
